Chapter 4: Museum Collections Environment

Chapter 4:  Museum Collections Environment
Chapter 4: Museum Collections Environment
A. Overview.................................................................................................................................. 4:1
What information will I find in this chapter? .................................................................................. 4:1
Who should read this chapter? ................................................................................................... 4:1
What are the agents of deterioration that affect the museum environment? ................................... 4:1
B. Developing the Critical Eye...................................................................................................... 4:3
What is the "critical eye?" ........................................................................................................... 4:3
What kinds of materials will I find in a museum collection?............................................................ 4:4
What is deterioration? ................................................................................................................ 4:5
What is chemical deterioration? .................................................................................................. 4:6
What is physical deterioration? ................................................................................................... 4:6
What is inherent vice? ................................................................................................................ 4:7
Why is it important to understand the environmental agents of deterioration
and how to monitor them? ....................................................................................................... 4:8
C. Temperature ............................................................................................................................ 4:9
What is temperature?................................................................................................................. 4:9
How does temperature affect museum collections?...................................................................... 4:9
D. Relative Humidity..................................................................................................................... 4:9
What is relative humidity (RH)?................................................................................................... 4:9
What is the psychrometric chart? .............................................................................................. 4:10
How do organic objects react with relative humidity?.................................................................. 4:13
What deterioration is caused by relative humidity?..................................................................... 4:13
What are the recommendations for relative humidity control? ..................................................... 4:13
E. Monitoring and Controlling Temperature and Relative Humidity .......................................... 4:14
Why should I monitor temperature and relative humidity?........................................................... 4:14
What kind of monitoring equipment should I have? .................................................................... 4:15
How do I maintain a hygrothermograph? ................................................................................... 4:18
How do I read a hygrothermograph chart or datalogger graph?................................................... 4:22
How do I use the hygrothermograph or datalogger data?............................................................ 4:23
How do I organize and summarize the data from my hygrothermograph charts
or datalogger graphs? ............................................................................................................ 4:23
How do I summarize long-term data?........................................................................................ 4:28
How do I control temperature and relative humidity? .................................................................. 4:29
What are humidistatically controlled heating and ventilation systems?......................................... 4:33
What is the time-weighted preservation index (TWPI)?............................................................... 4:33
F. Light ...................................................................................................................................... 4:33
What is light?........................................................................................................................... 4:34
What are the standards for visible light levels?........................................................................... 4:35
G. Monitoring and Controlling Light........................................................................................... 4:37
How do I monitor light levels? ................................................................................................... 4:37
How do I improve the lighting to minimize damage to objects on exhibit or in storage? ................. 4:38
How do I limit light damage from research use?......................................................................... 4:39
How do I fill out the Light and Heat Measurement Record? ......................................................... 4:40
Is there any way to directly monitor light damage? ..................................................................... 4:42
How do I control light levels? .................................................................................................... 4:42
H. Dust and Gaseous Air Pollution............................................................................................. 4:43
What are particulate air pollutants? ........................................................................................... 4:44
What are gaseous air pollutants?.............................................................................................. 4:44
I.
Monitoring and Controlling Particulate and Gaseous Air Pollution ....................................... 4:46
How do I monitor air pollution?.................................................................................................. 4:46
Are there ways to monitor for air pollution?................................................................................ 4:47
How do I control air pollution?................................................................................................... 4:48
J.
Selected Bibliography............................................................................................................ 4:51
K. Endnotes ............................................................................................................................... 4:53
List of Figures
Figure 4:1. How to Use a Psychrometric Chart .......................................................................... 4:12
Figure 4.2. Relative Humidity Optimum Ranges for Various Materials Housed in a
Park’s Museum Collection ..................................................................................................... 4:14
Figure 4.3. Example Hygrothermograph Calibration Record....................................................... 4:21
Figure 4.4. Hygrothermograph Chart that Illustrates the Relationship Between
Temperature and Relative Humidity ....................................................................................... 4:24
Figure 4.5. Hygrothermograph Chart that Indicates Operation
of Air Handling Equipment ..................................................................................................... 4:25
Figure 4.6. Hygrothermograph Chart that Illustrates the Results
of Turning off the Furnace ..................................................................................................... 4:26
Figure 4.7. Museum Environmental Monitoring Record.............................................................. 4:27
Figure 4.8. Example Light and Heat Measurement Record ........................................................ 4:41
Figure 4.9. Deterioration to Museum Objects Caused by Air Pollution ........................................ 4:46
Figure 4.10 Types of Materials that Can Harm Objects and Types of Materials that are
Considered Safe to Use with Museum Objects for Storage and Exhibit..................................... 4:50
CHAPTER 4: MUSEUM COLLECTIONS ENVIRONMENT
A. Overview
1.
What information will I find in
this chapter?
This chapter will give you information on how to protect your collection
from deterioration caused by interaction with the surrounding environment.
From the moment an object is created, it begins to deteriorate. The factors
that can cause deterioration are called “agents of deterioration.” See
Chapter 3 for a more complete discussion of the agents of deterioration.
This chapter will address four agents that can be grouped under the term
environment:
•
temperature
•
relative humidity
•
light
•
air pollution
Understanding how the environment affects your collection and how to
monitor and control these agents of deterioration is the most important part
of a preventive conservation program.
In order to understand how the agents of deterioration react with the objects
in your collection, you must develop a “critical eye.” This skill allows you
to identify active deterioration and its causes. How you do this is described
below.
2.
Who should read this
chapter?
You should read this chapter if you are responsible for the care of the
museum collections. Use this chapter to develop an understanding of how
objects deteriorate. This chapter will help you develop environmental
monitoring programs so you can identify and block these agents of
deterioration before they can damage your collections or so you can
mitigate the damage that has been caused. Every park should have an
environmental monitoring program.
3.
What are the agents of
deterioration that affect the
museum environment?
The agents of deterioration are forces that act upon objects causing
chemical and physical damage.
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•
Contaminants disintegrate, discolor or corrode all types of objects,
especially reactive and porous materials. Contaminants include:
−
gases (pollutants such as hydrogen sulfide, nitrogen dioxide, sulfur
dioxide and ozone; oxygen)
−
liquids (plasticizers that ooze from adhesives, grease from human
hands)
−
solids (dust that can abrade surfaces, salt that corrodes metals)
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−
•
•
•
Air pollution can include all these types of contaminants.
Radiation can be:
−
ultraviolet radiation that disintegrates, fades, darkens, and/or
yellows the outer layer of organic materials and some colored
inorganic materials
−
unnecessary visible light that fades or darkens the outer layer of
paints and wood
Incorrect temperature can be:
−
too high causing gradual disintegration or discoloration of organic
materials
−
too low causing desiccation, which results in fractures of paints,
adhesives, and other polymers
−
fluctuating causing fractures and delamination in brittle, solid
materials
Incorrect relative humidity can be:
−
damp (over 65% RH) causing mold and corrosion
−
above or below a critical value hydrating/dehydrating some
minerals
−
above 0% supports hydrolysis that gradually disintegrates and
discolors organic materials, especially materials that are
chemically unstable
−
fluctuating, which shrinks and swells unconstrained organic
materials, crushes or fractures constrained organic materials,
causes layered organic materials to delaminate and/or buckle,
loosens joints in organic components.
This chapter discusses protection from incorrect temperature, incorrect
relative humidity, light (electromagnetic radiation), and pollutants. Other
chapters cover the other agents of deterioration. Direct physical forces are
discussed in Chapter 7: Handling, Packing and Shipping. Thieves,
vandals, and fire are discussed in Chapter 9: Security and Fire Protection.
Pests are described in Chapter 5: Biological Infestations. Water and many
of the other agents are discussed in Chapter 10: Emergency Operation
Plans.
There are a variety of ways you can protect your collections from the agents
of deterioration. There are four steps to stop or minimize damage:
•
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Avoid the agents of deterioration. For example, choose a site for
your collection storage that is away from the flood plain of a river or
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stream. Build a storage facility that is properly insulated and does not
have windows in collections areas.
•
Block the agents when you cannot avoid them. This is probably the
main way most museums protect their collections in historic buildings.
For example, if your collection storage area has windows, cover them
with plywood. Place UV filters on fluorescent lights to block
damaging radiation. Fill cracks and gaps in a building structure to limit
entry to pests.
•
Test the methods you use to block agents of deterioration by
monitoring. For example, set up an Integrated Pest Management
(IPM) program to find out if you have pests. Monitor relative humidity
and temperature to find out if your HVAC system is working properly.
•
Respond to information you gather with your monitoring
programs. Monitoring is a waste of time if you do not review,
interpret, and use the information. This chapter will tell you how to
monitor temperature and relative humidity, light, and air pollution and
how to respond to the data that you collect.
Only if these first four approaches fail should you have to recover from
deterioration. Recovery usually means treating an object. While a treated
object may look the same, once damage has occurred, an object will never
be the same. Your aim in caring for your collection should be to carry out
preventive tasks so that treatment is not needed.
Many objects will come to your museum collections damaged and
deteriorated from use and exposure. Because of their history, even in the
best museum environment, some objects will need treatment. You should
develop a treatment plan for immediate problems in the collection. Your
primary goal, however, is to create a facility that will minimize damage and
maintain the collection through preventive measures.
B. Developing the Critical Eye
1.
What is the “critical eye?”
The “critical eye” is a way of looking at objects to evaluate their condition
and identify reasons for changes in the condition. You develop this skill
over a period of time through both training and experience. You must
continually ask yourself the questions:
•
What is occurring?
•
Why is it occurring?
•
What does it mean?
The critical eye is a trained eye.
Your trained eyes will focus on the materials and structure of the object and
look for visual clues to the agents of deterioration in the environment. A
person with a trained eye readily recognizes danger signs, records them and
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associates them with the condition of the museum collections, and
implements actions to slow down or stop deterioration. Examples of
problems that you will see with a trained eye include:
•
sunlight falling on a light sensitive surface
•
condensation forming on cold surfaces
•
water stains appearing on ceilings or walls
•
insect residues and mouse droppings
You must learn about the following topics to develop your critical eye:
•
types of materials that make up a museum collection
•
inherent characteristics of objects
•
types of deterioration
The success of a preventive conservation program relies on the gathering,
recording, and evaluating of all this information in order to implement
solutions and to mitigate environmental factors that are harmful to a park’s
museum collection.
2.
What kinds of materials will I
find in a museum collection?
Museum objects are often divided into three material-type categories:
organic, inorganic, and composite. You must understand the properties of
each of the materials in each of these categories.
Organic Objects: Organic objects are derived from things that were once
living — plants or animals. Materials are processed in a multitude of ways
to produce the objects that come into your collections. Various material
types include wood, paper, textiles, leather and skins, horn, bone and ivory,
grasses and bark, lacquers and waxes, plastics, some pigments, shell, and
biological natural history specimens.
All organic materials share some common characteristics. They:
4:4
•
contain the element carbon
•
are combustible
•
are made of complicated molecular structures that are susceptible to
deterioration from extremes and changes in relative humidity and
temperature
•
absorb water from and emit water to the surrounding air in an ongoing
attempt to reach an equilibrium (hygroscopic)
•
are sensitive to light
•
are a source of food for mold, insects, and vermin
NPS Museum Handbook, Part I (1999)
Inorganic Objects: Inorganic objects have a geological origin. Just like
organic objects, the materials are processed in a variety of ways to produce
objects found in your collections. Material types include: metals, ceramics,
glass, stone, minerals, and some pigments.
All inorganic objects share some common characteristics. They:
•
have undergone extreme pressure or heat
•
are usually not combustible at normal temperature
•
can react with the environment to change their chemical structure (for
example, corrosion or dissolution of constituents)
•
may be porous (ceramics and stone) and will absorb contaminants (for
example, water, salts, pollution, and acids)
•
are not sensitive to light, except for certain types of glass and pigments
Composite Objects: Composite or mixed media objects are made up of
two or more materials. For example, a painting may be made of a wood
frame and stretcher, a canvas support, a variety of pigments of organic and
inorganic origin, and a coating over the paint. A book is composed of
several materials such as paper, ink, leather, thread, and glue. Depending
on their materials, composite objects may have characteristics of both
organic and inorganic objects. The individual materials in the object will
react with the environment in different ways. Also, different materials may
react in opposition to each other, setting up physical stress and causing
chemical interactions that cause deterioration.
3.
What is deterioration?
Deterioration is any physical or chemical change in the condition of an
object. Deterioration is inevitable. It is a natural process by which an
object reaches a state of physical and chemical equilibrium with its
immediate environment.
The types of deterioration can be divided into two broad categories:
physical deterioration and chemical deterioration. Both types often
occur simultaneously.
4.
What is chemical
deterioration?
NPS Museum Handbook, Part I (1999)
Chemical deterioration is any change in an object that involves an
alteration of its chemical composition. It is a change at the atomic and
molecular level. Chemical change usually occurs because of reaction with
another chemical substance (pollution, water, pest waste) or radiation (light
and heat). Examples of chemical change include:
•
oxidation of metals (rusting)
•
corrosion of metals and stone caused by air pollution
•
damage to pigments by air pollution or reaction with other pigments
•
staining of paper documents by adjacent acidic materials
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5.
What is physical
deterioration?
•
fading of dyes and pigments
•
darkening of resins
•
darkening and embrittlement of pulp papers
•
burning or scorching of material in a fire
•
embrittlement of textile fibers
•
bleaching of many organic materials
•
cross-linking (development of additional chemical bonds) of plastics
•
rotting of wood by growing fungus
Physical deterioration is a change in the physical structure of an object. It
is any change in an object that does not involve a change in the chemical
composition. Physical deterioration is often caused by variation in
improper levels of temperature and relative humidity or interaction with
some mechanical force. Examples of physical deterioration include:
•
melting or softening of plastics, waxes, and resins caused by high
temperature
•
cracking or buckling of wood caused by fluctuations in relative
humidity
•
warping of organic materials caused by high relative humidity
•
warping or checking of organic materials caused by low relative
humidity
•
shattering, cracking, or chipping caused by impact
•
crushing or distortion caused by a harder material pressing against
flexible material
•
abrasion caused by a harder material rubbing against a softer material
•
structural failure (for example, metal fatigue, tears in paper, rips in
textiles)
•
loss of organic material due to feeding by insects and/or their larvae
•
staining of textiles and paper by mold
Physical deterioration and chemical deterioration are interrelated. For
example, chemical changes in textiles caused by interaction with light also
weaken the fabric so that physical damage such as rips and tears may occur.
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6.
What is inherent vice?
In addition to deterioration caused by the agents of deterioration, certain
types of objects will deteriorate because of their internal characteristics.
This mechanism of deterioration is often called inherent vice or inherent
fault. It occurs either because of the incompatibility of different materials
or because of poor quality or unstable materials.
In nature, materials often possess characteristics that protect them from
natural degradation. Their structure and composition may include features
such as protective layers, insect and mold resistant chemicals, and
photochemical protection. Processing during object manufacture can
remove these natural safeguards. Additives may be applied to give a
desired result, without concern for long-term preservation (for example, the
addition of metal oxides in the manufacture of weighted silk). This
processing results in inherently less stable materials or combinations of
mutually incompatible substances that have damaging interaction. There
are three kinds of inherent vice:
Short-lived materials: Short-lived materials are often the result of
manufacturing processes that do not consider the long-term stability of the
items that were produced. Many objects now in park museum collections
originally were made to fulfill temporary needs. Examples of impermanent
materials with inherent vice include:
•
cellulose nitrate and cellulose ester film
•
wood pulp paper
•
many 20th century plastics
•
magnetic media, including electronic records
Structural nature: Inherent vice can also be related to the structure of an
object. Poor design, poor construction, or poor application of materials
may cause structural failure. Examples of such damage include:
•
drying cracks in paint improperly applied
•
broken or lost attachments
•
loose joints
History: The way an object was used or where it was stored or deposited
before it comes into your collection may lead to inherent vice. Here,
damage and deterioration is caused by the original function of the object, its
maintenance, or its environment. Examples of inherent vice caused by
history include:
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•
accumulation of dissimilar paint layers, such as oil and latex
•
saturation in a wooden bowl that had been used as a container for oil
•
deposits of soluble salts in an archeological ceramic during burial
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You may have trouble identifying deterioration caused by inherent vice
because often there is little or no information on the selection and
processing of materials, manufacturing details, and previous use of an
object. Train your critical eye by reviewing similar objects and by
developing knowledge of object technology. Over time, you will become
more proficient at identifying inherent vice.
7.
Why is it important to
understand the environmental
agents of deterioration and
how to monitor them?
If you understand basic information about the chemistry and physics of
temperature, relative humidity, light, and pollution, you will be better able
to interpret how they are affecting your museum collections. This chapter
gives you a basic overview of these agents and describes how to monitor
them. You will be able to tell how good or bad the conditions in your
museum are and whether or not the decisions you make to improve the
environment are working the way you expect.
The rest of this chapter gives you guidelines for deciding on the best
environment that you can provide for your collections. However, because
of the huge variation in materials found in collections and the extremes in
geography where NPS collections are stored, no strict standards can be set.
In the past, simplified standards such as 50% RH and 65°F were promoted.
With research and experience, it is now understood that different materials
require different environments. You must understand the needs of your
collection for the long-term in order to make thoughtful decisions about
proper care.
You will want to develop microenvironments for storage of particularly
fragile objects. A microenvironment (microclimate) is a smaller area (box,
cabinet, or separate room) where temperature and/or humidity are
controlled to a different level than the general storage area. Common
microenvironments include:
•
freezer storage for cellulose nitrate film
•
dry environments for archeological metals
•
humidity-buffered exhibit cases for fragile organic materials
•
temperature-controlled vaults for manuscript collections
C. Temperature
1.
What is temperature?
Temperature is a measure of the motion of molecules in a material.
Molecules are the basic building blocks of everything. When the
temperature increases, molecules in an object move faster and spread out;
the material then expands. When the temperature decreases, molecules
slow down and come closer together; materials then contract. Temperature
and temperature variations can directly affect the preservation of park
collections in several ways.
2.
How does temperature affect
museum collections?
Temperature affects museum collections in a variety of ways.
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•
At higher temperatures, chemical reactions increase. For example,
high temperature leads to the increased deterioration of cellulose nitrate
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film. If this deterioration is not detected, it can lead to a fire. As a rule
of thumb, most chemical reactions double in rate with each increase of
10°C (18°F).
•
Biological activity also increases at warmer temperatures. Insects will
eat more and breed faster, and mold will grow faster within certain
temperature ranges.
•
At high temperatures materials can soften. Wax may sag or collect
dust more easily on soft surfaces, adhesives can fail, lacquers and
magnetic tape may become sticky.
In exhibit, storage and research spaces, where comfort of people is a factor,
the recommended temperature level is 18-20° C (64-68° F). Temperature
should not exceed 24° C (75° F). Try to keep temperatures as level as
possible.
In areas where comfort of people is not a concern, temperature can be kept
at much lower levels —but above freezing.
Avoid abrupt changes in temperature. It is often quick variations that cause
more problems than the specific level. Fluctuating temperatures can cause
materials to expand and contract rapidly, setting up destructive stresses in
the object. If objects are stored outside, repeated freezing and thawing can
cause damage.
Temperature is also a primary factor in determining relative humidity
levels. When temperature varies, RH will vary. This is discussed in more
detail in the next section.
D. Relative Humidity
1.
What is relative humidity
(RH)?
Relative humidity is a relationship between the volume of air and the
amount of water vapor it holds at a given temperature. Relative humidity is
important because water plays a role in various chemical and physical
forms of deterioration. There are many sources for excess water in a
museum: exterior humidity levels, rain, nearby bodies of water, wet ground,
broken gutters, leaking pipes, moisture in walls, human respiration and
perspiration, wet mopping, flooding, and cycles of condensation and
evaporation.
All organic materials and some inorganic materials absorb and give off
water depending on the relative humidity of the surrounding air. Metal
objects will corrode faster at higher relative humidity. Pests are more
active at higher relative humidities.
We use relative humidity to describe how saturated the air is with water
vapor. “50% RH” means that the air being measured has 50% of the total
amount of water vapor it could hold at a specific temperature. It is
important to understand that the temperature of the air determines
how much moisture the air can hold. Warmer air can hold more water
vapor. This is because an increase in the temperature causes the air
molecules to move faster and spread out, creating space for more water
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molecules. For example, warm air at 25°C (77°F) can hold a maximum of
about 24 grams/cubic meter (g/m3 ), whereas cooler air at 10°C (50°F) can
hold only about 9 g/m3 .
Relative humidity is directly related to temperature. In a closed volume of
air (such as a storage cabinet or exhibit case) where the amount of moisture
is constant, a rise in temperature results in a decrease in relative humidity
and a drop in temperature results in an increase in relative humidity. For
example, turning up the heat when you come into work in the morning will
decrease the RH; turning it down at night will increase the RH.
Relative humidity is inversely related to temperature. In a closed
system, when the temperature goes up, the RH goes down; when
temperature goes down, the RH goes up.
2.
What is the psychrometric
chart?
The relationships between relative humidity, temperature, and other factors
such as absolute humidity and dew point can be graphically displayed on a
psychrometric chart. Refer to Figure 4.1 for an explanation of how to use
this chart. The following definitions will help you understand the factors
displayed on the chart and how they affect the environment in your
museum.
•
Absolute humidity (AH) is the quantity of moisture present in a given
volume of air. It is not temperature dependent. It can be expressed as
grams of water per cubic meter of air (g/m3 ). A cubic meter of air in a
storage case might hold 10 g of water. The AH would be 10 g/m3 .
•
Dew point (or saturation temperature) is the temperature at which the
water vapor present saturates the air. If the temperature is lowered the
water will begin to condense forming dew. In a building, the water
vapor may condense on colder surfaces in a room, for example, walls
or window panes. If a shipping crate is allowed to stand outside on a
hot day, the air inside the box will heat up, and water will and
condense on the cooler objects.
•
Relative humidity relates the moisture content of the air you are
measuring (AH) to the amount of water vapor the air could hold at
saturation at a certain temperature. Relative humidity is expressed as a
percentage at a certain temperature. This can be expressed as the
equation:
RH =
Absolute Humidity of Sampled Air x 100
Absolute Humidity of Saturated Air at Same Temperature
Use the following example to understand how this concept relates to your
museum environment.
In many buildings it is common to turn the temperature down in the
evenings when people are not present. If you do this in your storage space,
you will be causing daily swings in the RH. Suppose you keep the air at
20°C (68°F) while people are working in the building. A cubic meter of air
in a closed space at 20°C (68°F) can hold a maximum of 17 grams of water
4:10
NPS Museum Handbook, Part I (1999)
vapor. If there are only 8.5 grams of water in this air, you can calculate the
relative humidity.
The AH of the air = 8.5 grams
The AH of saturated air at 20°C = 17.0 grams
Using the equation above
RH = 8.5 x 100% = 50%
17.0
50% RH may be a reasonable RH for your storage areas. But, if you turn
down the heat when you leave the building at night, the RH of the air in the
building will rise rapidly. You can figure out how much by using the same
equation. If the temperature is decreased to 15°C (59°F), the same cubic
meter of air can hold only about 13 grams of water vapor. Using the same
equation
The AH of the air = 8.5 grams
The AH of saturated air at 15°C (59°F) = 13.0 grams
RH = 8.5 x 100% = 65%
13.0
By turning down the heat each night and turning it up in the morning you
will cause a 15% daily rise and fall in RH.
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3.
How do organic objects react
with relative humidity?
Organic materials are hygroscopic. Hygroscopic materials absorb and
release moisture to the air. The RH of the surrounding air determines the
amount of water in organic materials. When RH increases they absorb
more water; when it decreases they release moisture to reach an equilibrium
with the surrounding environment. The amount of moisture in a material at
a certain RH is called the Equilibrium Moisture Content (EMC). Refer to
Appendix N: Curatorial Care of Wooden Objects, for a further explanation
of this concept. Over time, these reactions with water can cause
deterioration.
4.
What deterioration is caused
by relative humidity?
Deterioration can occur when RH is too high, variable, or too low.
5.
What are the
recommendations for relative
humidity control?
•
Too high: When relative humidity is high, chemical reactions may
increase, just as when temperature is elevated. Many chemical
reactions require water; if there is lots of it available, then chemical
deterioration can proceed more quickly. Examples include metal
corrosion or fading of dyes. High RH levels cause swelling and
warping of wood and ivory. High RH can make adhesives or sizing
softer or sticky. Paper may cockle, or buckle; stretched canvas
paintings may become too slack. High humidity also supports
biological activity. Mold growth is more likely as RH rises above
65%. Insect activity may increase.
•
Too low: Very low RH levels cause shrinkage, warping, and cracking
of wood and ivory; shrinkage, stiffening, cracking, and flaking of
photographic emulsions and leather; desiccation of paper and
adhesives; and dessication of basketry fibers.
•
Variable: Changes in the surrounding RH can affect the water content
of objects, which can result in dimensional changes in hygroscopic
materials. They swell or contract, constantly adjusting to the
environment until the rate or magnitude of change is too great and
deterioration occurs. Deterioration may occur in imperceptible
increments, and therefore go unnoticed for a long time (for example,
cracking paint layers). The damage may also occur suddenly (for
example, cracking of wood). Materials particularly at high risk due to
fluctuations are laminate and composite materials such as photographs,
magnetic media, veneered furniture, paintings, and other similar
objects.
You should monitor relative humidity and implement improvements to
stabilize the environment. There are many ways to limit fluctuations, not
all dependent on having an expensive mechanical system. For example,
good control is achievable simply by using well-designed and wellconstructed storage and exhibit cases.
Ideally, fluctuations should not exceed ±5% from a set point, each month.
You should decide on a set point based on an evaluation of your particular
regional environment. Consult your regional/SO curator, a conservator or
other expert in museum environments. Establish maximum and minimum
levels by assessing the nature and condition of the materials in the
collection and the space where they are housed. Establishing ranges is
discussed in more detail in Section B. For example, if you are in Ohio you
may decide on a set point of 50%±5%. The humidity could go as high as
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55% or as low as 45% within a month. If you are in the arid southwest you
might choose 35% as your set point as objects have equilibrated at much
lower RH levels. Be aware though, you should not allow your RH to go as
high as 65% because of the chance that mold might develop. Below 30%
some objects may become stiff and brittle.
Over the year you may want to allow drift. Drift means that your set point
varies in different seasons––usually higher RH in the summer and lower
RH in the winter. Allowing drift will often save you money over the longterm as mechanical systems work less to maintain the proper environment.
If your collections are housed in a historic structure, preservation of the
structure may require drift. It is important to understand that these
variations in RH and temperature should be slow and gradual variations
(over weeks and months), not brief and variable.
Archeological Materials
Negligible Climate-Sensitive Materials ......................................... 30% – 65%
Climate Sensitive Materials ..............................................................30% - 55%
Significantly Climate Sensitive Materials ......................................30% - 40%
Metals............................................................................................................. <35%
Natural History Materials
Biological specimens..........................................................................40% - 60%
Bone and teeth.....................................................................................45% - 60%
Paleontological specimens ...............................................................45% - 55%
Pyrite specimens .......................................................................................... <30%
Paintings..............................................................................................40% - 65%
Paper ....................................................................................................45% - 55%
Photographs/Film/Negatives ..........................................................30% - 40%
Other organics (wood, leather, textiles, ivory)..........................45% - 60%
Metals............................................................................................................ <35%
Ceramics, glass, stone ......................................................................40% - 60%
Figure 4.2. Relative Humidity Optimum Ranges for Various
Materials Housed in a Park’s Museum Collection.1
E. Monitoring and Controlling
Temperature and Relative
Humidity
1.
4:14
Why should I monitor
temperature and relative
humidity?
You must monitor temperature and relative humidity so that you know what
the environment in your storage and exhibit spaces is like over time.
Monitoring helps you:
•
set a baseline of temperature and humidity to see if the storage space is
adequate
•
identify variations in the temperature and humidity throughout
collections areas
NPS Museum Handbook, Part I (1999)
2.
What kind of monitoring
equipment should I have?
•
monitor equipment to be sure it is working right
•
help develop strategies to improve the environment
•
identify whether your strategies are working to improve the
environment
There are a variety of temperature and relative humidity monitoring tools
that are available for monitoring the environment in your museum. They
can be divided into two types: spot measuring devices and continuous
recording devices. Each type is most effective for different specific tasks
so you may need to purchase more than one of the following pieces of
equipment:
•
Psychrometers: All parks should have a psychrometer. There are two
types: sling psychrometer and aspirating psychrometer. Of the
two, an aspirating psychrometer is more accurate. You use a
psychrometer to record daily readings (if you don’t have a
hygrothermograph), to make spot readings, and to calibrate dial
hygrometers and hygrothermographs. See Conserve O Gram 3/1,
“Using a Psychrometer To Measure Relative Humidity.”
A psychrometer gives you the RH by comparing the temperature
between a “dry bulb” and “wet bulb.” The dry bulb is a mercury
thermometer. The wet bulb is an identical thermometer covered with a
wetted cotton wick. Because of the cooling effect of evaporating
water, the wet bulb reads lower than the dry bulb. The drier the air, the
faster the water evaporates and the lower the reading.
To take readings with a sling psychrometer, whirl it around for one
minute to pass air over the wet and dry bulbs. Read the wet bulb
immediately and record the results. Repeat the process until you get
the same readings two times in a row.
The aspirating psychrometer uses a battery powered fan to steadily
blow air over the bulb at a set speed. Both these instruments are
accurate to ±5%. The aspirating psychrometer is more reliable because
it minimizes possible errors by the operator and ensures a constant air
flow past the wick. Accuracy will also depend on the length of the
thermometer and how accurately you can read the temperature.
Before you use a psychrometer, be sure to read the manufacturer’s
instructions. To ensure you get an accurate reading keep the following
points in mind:
NPS Museum Handbook, Part I (1999)
−
keep wick closely fitted to the thermometer bulb
−
do not touch the wick
−
keep the wick clean
−
use only deionized water to wet the wick
−
be sure that the aspirating psychrometer has a good battery
4:15
Accuracy of aspirating and sling psychrometers can be affected by
altitude, especially at lower relative humidities. At lower atmospheric
pressure water evaporates faster, lowering the temperature of the dry
bulb more. If your collections are 900 meters or more above sea level,
you should obtain pressure-corrected charts, tables, or slide rules or use
a pressure correction formula. See the article by Hitchcock and Jacoby
(listed in the bibliography) for more information about the effects of
high altitude on psychrometers, and indirectly on the equipment that
you calibrate using a psychrometer.
•
Hygrometers: You can use a hygrometer to measure relative humidity
levels when you don’t have a hygrothermograph or datalogger or in
spaces that are too small for psychrometers (for example, inside an
exhibit or a storage case). When you use a hygrometer, also record the
temperature. There are three types of hygrometers: dial hygrometers,
electronic hygrometers, and humidity strips.
In a dial hygrometer, a hygroscopic material (often paper) is attached
to a hand on a dial. As the hygroscopic material absorbs or gives off
moisture, it expands and contracts, causing the hand to move across the
dial. Dial hygrometers can be accurate to +5%, but they are very
inaccurate at low (<40%) and high (>80%) RH levels. Often they are
hard to calibrate, so over time will drift and become inaccurate. See
Conserve O Gram 3/2, “Calibration of Hygrometers and
Hygrothermographs.”
Digital hygrometers often have a built-in temperature monitor. If you
purchase one of these tools be sure it can be calibrated. They are often
calibrated with saturated salt solutions provided in a kit by the
manufacturer. Electronic hygrometers can be used to calibrate
hygrothermographs if you are sure the hygrometer is in proper
calibration.
Humidity indicator strips are a special kind of hygrometer that use
paper impregnated with cobalt salts. A series of patches are labeled
with RH, usually in 10% increments. The color is blue at low RH
levels and pink at high RH levels. Read the RH at the point of change
between pink and blue. These strips are inexpensive and can give you
some basic understanding of your RH levels at a variety of spots
around your building. If used in a moist environment, they can become
inaccurate.
•
4:16
Hygrothermographs: Hygrothermographs have been the basic
monitoring tool in museums for some time. They give you a
continuous record of temperature and humidity variations over a period
of 1, 7, 31, or 62 days. The instrument consists of six major
components:
−
the housing
−
a temperature element, usually a bimetal strip
−
a relative humidity element, which may be a human hair bundle or
a polymer membrane
NPS Museum Handbook, Part I (1999)
−
linkage arms and recording pens
−
a drive mechanism, which may be spring wound or battery
operated, that rotates a chart
−
a chart, which may be wrapped around a cylindrical drum or be a
circular disk
The temperature-sensitive element (the bimetal strip) and the
hygroscopic material (for example, the human hair) are connected to
arms with pens at their tips. The pens rest on a revolving chart and
move up and down as the bimetal strip and the hair react to
environmental changes.
Hygrothermographs are accurate within ±3-5% when properly
calibrated. They are most accurate within the range of 30-60% RH.
Note: You must calibrate your hygrothermograph at least quarterly;
monthly is better. It is especially important to calibrate your machine if
you experience sudden extremes of humidity in your collection areas.
See Conserve O Gram 3/2, “Calibration of Hygrometers and
Hygrothermographs.”
•
Electronic datalogger: Electronic dataloggers have become common
in museums. There are a variety of types of dataloggers available at a
range of prices. A model that records temperature, relative humidity,
and light will meet typical museum needs. The data must be
downloaded onto a computer. All datalogger companies provide at
least basic software programs that allow you to manipulate the data to
produce graphs and tables of information. Most allow you to transfer
this information in ASCII format to a spreadsheet program. They
require less calibration than hygrothermographs, though they must
usually be sent back to the company for calibration.
Many dataloggers do not display data so you will not have any
indication of what is occurring in your environment until you
download the data. Some now include a liquid crystal display unit.
See Conserve O Gram 3/3, “Datalogger Applications in Monitoring the
Museum Environment” for a discussion of your options when choosing
a datalogger. See Conserve O Gram 14/6, “Caring for Color
Photographs,” for information about using the Preservation
Environment Monitor (PEM). The PEM is a datalogger that
automatically figures the time-weighted preservation index (TWPI).
The TWPI is an estimate of how long organic objects will last at a
given temperature and RH. See Section F.10 for a further explanation
of the TWPI.
Electronic dataloggers can be very useful instruments, but they are not
exact replacements for hygrothermographs. Before purchasing all new
equipment evaluate what information you need from your continuous
monitoring equipment, consider these questions:
NPS Museum Handbook, Part I (1999)
−
How much can you spend?
−
How many areas do you need to monitor?
4:17
3.
How do I maintain a
hygrothermograph?
−
Do you need a portable monitor or will it remain in the same place
all the time?
−
Do you have the computer equipment and knowledge to properly
use dataloggers?
−
How much time do you have available for changing charts,
downloading data, calibrating instruments, and manipulating data.
−
How much data manipulation do you require? Can you just
review charts or do you want do be able to look at and produce
graphs that reflect daily, monthly, and yearly trends?
−
Do you need immediate notification of the environment in an area
so you can respond to changes?
−
Information regarding some kinds of environmental equipment is
published in the NPS Tools of the Trade. It is important to
evaluate equipment available through a variety of other companies,
as well. Electronic hygrometers and dataloggers are changing
constantly and you should make yourself aware of new options
before making your final choice.
In order to get the best information possible from your hygrothermograph,
you must maintain and calibrate it on a regular basis. Make it a standard
maintenance chore that you do at the same time each quarter, or preferably
once a month when you change the recording paper.
Before using your instrument, read the manufacturer’s instruction for
operation and maintenance. Hygrothermographs are delicate instruments
and you can easily damage yours by improper handling. See Conserve O
Gram 3/2, “Calibration of Hygrometers and Hygrothermographs.” Keep
the following points in mind when changing the paper and calibrating:
4:18
•
Keep the instrument clean and free of dust.
•
Locate the instrument in an area that minimizes vibration, but reflects
the environment throughout the room.
•
Do not touch the relative humidity sensor.
•
Replace the relative humidity sensor when you find you are frequently
adjusting the RH calibration.
•
Keep the pens clean and free flowing.
•
If you have metal tipped pens, use only the glycerine based ink
supplied with your instrument. Other types of ink will not work. You
can also get felt-tip cartridge pens, which are are easier to use.
However, these pens have a shorter shelf life. If properly maintained,
the metal pen points with ink are more cost effective.
NPS Museum Handbook, Part I (1999)
When you calibrate your hygrothermograph, you will check the instrument
against known relative humidity and/or temperature levels and make
adjustments as necessary. Note: Temperature rarely goes out of calibration
because the bimetal element is very stable. Use either a sling psychrometer
or an aspirating psychrometer to determine the relative humidity. See
Conserve O Gram 3/1, “Using a Psychrometer to Measure Relative
Humidity.” Next adjust the hygrothermograph to match the known
conditions. Use the example chart in Figure 4.3, Hygrothermograph
Calibration Record, to document the calibration.
Do the calibration:
NPS Museum Handbook, Part I (1999)
•
Read and follow any suggestions made by the manufacturer concerning
calibration of the instrument.
•
Record the information requested on the chart: date, time of day,
relative humidity reading from the hygrothermograph, and temperature
reading from the hygrothermograph.
•
Immediately after recording readings obtained from the
hygrothermograph, operate the psychrometer at the same location,
following the manufacturer’s instructions. Record the relative
humidity and dry bulb temperature readings on the psychrometer in the
spaces provided on the record.
•
Adjust the hygrothermograph to match the psychrometer readings.
Follow the instructions for making adjustments provided by the
instrument’s manufacturer. If the average differences are found to be
greater than 1% relative humidity or 1° in temperature, adjust the
hygrothermograph up or down to match the calibrating instrument
reading. For example, if the hygrothermograph is recording high by
5%, adjust the recording arm so that it shows the proper reading. If the
hygrothermograph is recording temperature low by 4º, adjust the
recording arm to the actual reading.
•
If the hygrothermograph requires calibration, record the temperature
and humidity difference in the appropriate spaces. For example, if the
relative humidity reading on the hygrothermograph was 48% and the
reading from the psychrometer was 45%, record the difference between
them as “hygro high by 3% RH.” Always record differences in terms
of whether the hygrothermograph reading is higher or lower than
the psychrometer reading because you are calibrating the
hygrothermograph. If there is no difference between the two readings,
simply enter “0 difference” in the space.
•
Wait for 15 minutes and take another psychrometer reading. Check the
reading on the hygrothermograph again. You may need to adjust the
instrument because the linkages often require time to equalize.
•
If significant differences still exist (over 5%) after a third check, refer
to the instruction manual for the hygrothermograph to determine why
the instrument might be malfunctioning. Relative humidity readings
most often are erroneous because of a broken or dirty hair element.
Temperature readings can be in error because of dust or other fouling
of the bimetal strip. Read and follow the manufacturer’s
4:19
instructions for cleaning, maintaining, and repairing the
hygrothermograph.
After the calibration has been completed and the hygrothermograph has
been adjusted properly, file the calibration record form with the charts from
the hygrothermograph. It is important that the forms be kept so that they
can be compared to future calibration records on the same instrument in
order to determine if there is a pattern of incorrect readings. If it becomes
apparent that a hygrothermograph has consistently given incorrect readings,
return it to the manufacturer for repairs.
4:20
NPS Museum Handbook, Part I (1999)
NATIONAL PARK SERVICE
HYGROTHERMOGRAPH CALIBRATION RECORD
Hygrothermograph:
Psychrometer:
Brand Weathermeasure
Serial No. 008581
Brand Cole-Parmer Thermo-hygrometer
Serial No. 942803
Model 5020-A
Property No. NP0000105512
Model 3309-60
Property No. NP0000105409
Location of Hygrothermograph: Museum Storage, top of Cabinet 3A
Name and Title of Person: Rhonda Fleming, Curator
Relative Humidity Readings
Temperature Readings
TIME
HYGRO.
PSYCH.
±RH DIFF.
HYGRO.
PSYCH.
±° DIFF.
4/11/99
1300
55%
54%
+1%
65°F
65°F
0°F
5/11/99
800
53%
54%
-1%
65°F
65°F
0°F
6/10/99
1400
55%
55%
0%
65°F
62°F
+3°F
DATE
Figure 4.3. Example Hygrothermograph Calibration Record
NPS Museum Handbook, Part I (1999)
4:21
4.
How do I read a
hygrothermograph chart or
datalogger graph?
If you have spent any time inspecting hygrothermograph charts or
datalogger graphs you may have observed readings that defy simple
explanations. There are many variables that may account for unusual
readings. Some of them include:
•
the quality and condition of the building where your collection is
housed (the “envelope”)
•
staff activity
•
public visitation
•
HVAC equipment performance and failure
•
barometric pressure
•
weather
•
the condition and accuracy of the monitoring equipment
•
an unusual source for moisture such as curing concrete, underground
cisterns, clogged drains
It is impossible to explain all of the patterns you may encounter in a
monitoring program. However, some common patterns and causes can be
explained:
•
Examine the hygrothermograph chart in Figure 4.4. This pattern
clearly illustrates the relationship between temperature and relative
humidity. As the temperature goes down, the RH goes up. As the
temperature goes up, the RH goes down. You may see this pattern
most often in well-enclosed spaces with minimal human activity (for
example, a storage space). A large number of people gathering in a
room would probably cause an increase in both temperature (because
of body heat) and relative humidity (because of perspiration and
transpiration).
•
Examine the hygrothermograph chart in Figure 4.5. This pattern is
characteristic of changes caused by regulated air-handling equipment.
In this case, a thermostat is regulating a furnace. The temperature
changes are so small (2°F) and rapid that the RH does not vary enough
to show up clearly on the chart until a larger, longer swing occurs and
is mirrored in the relative humdity.
A similar sawtooth pattern could be seen in the RH if your building had
humidification or dehumidification equipment controlled by a
humidistat. Cycling is generally harmful to museum materials that
respond quickly to environmental change. It is also very difficult to
completely elimate cycling from most ordinary HVAC equipment.
•
4:22
Examine the hygrothermograph chart in Figure 4.6. You may find that
changes in activity on the weekends result in a different pattern on your
hygrothermograph charts. In this instance, the furnace was turned
NPS Museum Handbook, Part I (1999)
down or off. Note the resulting rise in RH over the course of the
weekend. Note too, the high temperatures and the resulting low RH
during the week. In this instance, lowering the thermostat setting and
keeping the same setting throughout the week would be much better for
the museum objects and would conserve energy.
You may need more than one hygrothermograph or datalogger for your
monitoring program, especially in a historic structure located in a temperate
zone where summers are hot and humid and winters are cold and very dry.
You may need to place a hygrothermograph or datalogger in different
spaces (for example, basement, first and second floors) to gather enough
data to evaluate conditions properly.
5.
How do I use the
hygrothermograph or
datalogger data?
Imagine that the record reveals that the conditions within the structure are
too damp for most environmentally sensitive objects (for example, furniture
and wooden objects, textile and paper objects). Probably the basement will
have consistently high RH levels, the first floor will be somewhat drier, and
the second floor might be drier than the first floor. If you do find that the
building is too damp, there may be problems in your collections. You will
need to look with a critical eye for evidence of mold and insect activity
and/or damage and for sources of moisture in the structure’s walls and
basement. For example, rainwater runoff from the roof may be entering the
basement through deep window wells and masonry cellar walls.
Once you identify the problem you must take action. While waiting for
modifications to correct the runoff problem, you could put a dehumidifier
and fans in the basement. Be sure to seek advice on correcting the problem
from others who can help including: your regional/SO curator,
conservators, historic architects, cultural resources specialists, and
maintenance staff.
6.
How do I organize and
summarize the data from my
hygrothermograph charts or
datalogger graphs?
NPS Museum Handbook, Part I (1999)
You must organize the data recorded by each hygrothermograph or
datalogger to make it useful in developing strategies. Keep a record of
daily observations, noting occurrences, such as, unusual exterior climatic
conditions, a leaky roof, re-calibration of the equipment, or an unusual
visitation pattern. At the end of each month when you remove the
hygrothermograph chart or download datalogger data, compare this
information to the daily record. It may help to record unusual occurrences
directly on the chart or graph so that it is easy to see how the environment
affected temperature and relative humidity.
4:23
NATIONAL PARK SERVICE
MUSEUM ENVIRONMENTAL MONITORING RECORD
Col. 1
DATE
Col. 2
TIME
Col. 3
EXTERNAL
TEMP RH
Col. 4
CONTROLS
TEMP RH
Col. 5
INTERNAL
TEMP RH
Col. 6
REMARKS
Figure 4.7. Museum Environmental Monitoring Record
NPS Museum Handbook, Part I (1999)
4:27
7.
How do I summarize longterm data?
You can use a table or graph to summarize relative humidity and
temperature data. One way is to prepare a table that records information
collected over a period of time (for example, four to six weeks). You can
put the following information in a table:
•
high temperature
•
low temperature
•
maximum diurnal (24 hour) temperature change
•
high relative humidity
•
low relative humidity
•
maximum diurnal relative humidity change
The following example shows this information recorded for two
hygrothermographs in separate museum storage spaces in the park.
Monthly Summary of Temperature and Relative Humidity
Chart Date
Data
Storage #1
Storage #2
5/18-6/15/99
high
29°C (84°F)
28°C (83°F)
temperature
low temperature 21°C (69°F)
19°C (66°F)
max. 24 hr.
4°F
4°F
temp. change
high RH
54%
56%
low RH
45%
46%
max. 24 hr. RH 4%
3%
change
You can also summarize the data using graphs. You can design your
graphs in a variety of ways. For example:
•
Record both temperature and relative humidity on the same graph.
•
Record temperature for several different floors of a historic structure.
•
Compare temperature or RH parameters set for a building against
recorded data.
You can also summarize data by preparing room-by-room records for a
year. Each week, for each room or space:
4:28
•
Record high/low readings for temperature and relative humidity.
•
Record fluctuation patterns of temperature and relative humidity by
correlating with the time of day.
•
Note maximum diurnal RH fluctations.
NPS Museum Handbook, Part I (1999)
For example:
Furnished Historic
Structure
Room A
Temperature: 18°-22°C (64°-71°F)
5/18-5/24
Gradual rise in relative humidity through week;
no rapid fluctuations
Gradual daily fluctuations from 18°(64°F) to
22°C (71°F); low about midnight, high around 3
p.m.
Relative Humidity: 22% -32% RH
Maximum diurnal fluctuations: 10% RH
You should summarize data gathered from instruments and recorded on
your monitoring record. This helps you evaluate long-term trends and
watch for problems. Summary information helps you develop new
environmental control measures.
You can summarize your data for each space by season in the same format
as above. A summary gives you an idea of the variation that you have
throughout the year.
Use the summary documents in a variety of ways:
8.
How do I control temperature
and relative humidity?
•
Identify problems with your environment.
•
Build an argument about the need to get environmental upgrades or a
new building.
•
Evaluate whether or not changes you have made really do improve the
environment.
General considerations: When you control the climate surrounding
museum objects, you provide a stable environment that eliminates rapid
fluctuations and extremes in temperature and RH. When you develop a
strategy to control the environment in your museum spaces, keep the
following points in mind:
•
NPS Museum Handbook, Part I (1999)
NPS units are located in many different climate zones, so you must
develop acceptable ranges and limits of relative humidity for your
individual park. There is no general solution to controlling your
relative humidity. Every situation presents different variables that you
must evaluate before setting standards. Base your standards on:
−
the local climate (for example, tropical, temperate, arid)
−
the nature and condition of the materials in your collection
−
the nature and condition of the structure housing the collection
4:29
−
the ability of HVAC equipment to maintain the standard
−
the ability of staff to maintain equipment
•
In order to develop an effective control program, you must have good
information. Collect data for one year before establishing acceptable
ranges and limits.
•
Use a team approach in controlling relative humidity. Once you have
gathered your data, discuss control strategies with your regional/SO
curator, and others, such as conservators, historic architects, and
mechanical engineers. Strategies for controlling levels of RH and
temperature should keep energy costs in mind. Refer to Director’s
Order #28: Cultural Resource Management Guideline for guidance.
•
You will need to develop both active and passive measures for
controlling the environment. When adapting a historic structure
explore the use of simple modifications to your structure or space and
employ portable mechanical equipment (humidifier, dehumidifier,
heater, and air conditioner) or passive storage controls.
•
Once you have implemented strategies to improve the environment,
continue monitoring to evaluate whether or not your strategies are
working.
Maintain building envelope: With the help of your maintenance division
examine the structure and/or museum space for possible sources of
moisture. You must eliminate sources of moisture by repairing the
structure or correcting drainage proble ms. Problems that may cause high
levels of relative humidity include:
•
leaking roof, ceiling, or windows
•
gaps in walls, floors, or foundation vapor barrier
•
leaking plumbing
•
damaged gutters and downspouts
•
wet walls and foundations from poor drainage
•
open water sources such as sinks or toilets
Passive methods of control: There are a variety of practices that you can
adopt to passively control the temperature and RH. Carefully develop a
plan to use passive controls. After adopting the practice, continue to
monitor to be sure that the action improves that environment the way you
expect it to.
•
4:30
Avoid turning HVAC equipment on during the day and off at night.
This practice causes daily fluctations in RH levels.
NPS Museum Handbook, Part I (1999)
•
Limit the number of people in a room. Large groups of people can
raise the relative humidity from moisture introduced by breathing and
perspiring. You may have to open doors within a building to change
the circulation of the air.
•
Locate sensitive objects away from spotlights, windows, exterior walls,
air vents, and entrance doorways. You can also limit increased
temperatures caused by the sun by using existing blinds, curtains,
drapes, or exterior shutters.
•
In temperate zones, reduce temperature levels during the winter.
Lowering the set point of the heating equipment by several degrees
raises the interior relative humidity to stabilize conditions overall.
Note: Gradually reduce the temperature over a period of weeks. In the
spring, gradually raise the temperature back to the appropriate set
point. See the next section for a more in-depth discussion of
humidistatic control.
•
Store objects in cases, boxes, and folders. Containers are a very
effective method of buffering temperature and RH fluctuations. They
also limit light damage and protect collections from pests.
•
To control relative humidity levels for sensitive objects (for example,
some metals, textiles, paper, pyritic mineral, and fossil specimens) you
may need to create a microenvironment to stabilize and maintain
conditions that are different from the general museum environment.
The use of a properly sealed storage cabinet or exhibit case with
buffering material (for example silica gel) can provide a proper
microclimate for sensitive objects. Refer to Conserve O Gram 1/8,
“Using Silica Gel in Microenvironments” for guidance on using silica
gel. There are a variety of other materials that can be used to produce
microenvironments for storage of particularly sensitive objects.
Consult with a conservator to discuss various options for your
particular problem.
•
Many materials in a museum environment absorb water and give off
water. This slows changes in RH and buffers the environment around
the object. Damage can be limited by slowing down RH changes.
Natural organic materials (wood, textiles, cotton, and paper) are
especially good at buffering. You can use this property to help limit
changes in an environment. For example, when packing objects for
shipping, wrap them in layers of paper and use pads of paper to fill the
box and limit RH changes.
Active methods of control. A properly designed heating, ventilation and
air conditioning (HVAC) system can maintain appropriate levels of relative
humidity and temperature and filter particulate gases from the air.2
Installing an HVAC system that achieves and maintains the environment to
the levels described in this chapter is not easy. In some cases, especially
with historic buildings, this approach can be detrimental to the historic
building. Before embarking on a program to install, upgrade, or design a
new HVAC system, assemble a team of experts and plan a system that
protects both the collections and the museum building. Choose team
NPS Museum Handbook, Part I (1999)
4:31
members with expertise in historic collections care, preservation,
mechanical, electrical, and structural engineering.
You must have good information from your ongoing monitoring program to
help you identify the needs and problems of your current system. Working
from this information, your team can design a practical system that will
preserve both the collection and the building.
In some cases, you may choose to use portable humidifiers, dehumidifiers,
heaters, and air conditioners. In the short-term this equipment can do a lot
to improve the environment in a museum collection space. It is also less
expensive than installing a new HVAC system. Refer to the NPS Tools of
the Trade for sources of equipment.
•
Humidifiers quickly add moisture to the air. Use a humidifier in the
winter to counteract the drying effect of a central heating system. Use
only an unheated evaporative humidifier. This type of humidifier does
not disperse minerals in the air, and if the humidistat (a switch that
turns off the equipment when a certain RH is reached) malfunctions,
this type of humidifier will not raise the RH level above 65-70%. Be
sure air is well circulated. You may have to use fans for circulation.
You must select the size and number of humidifiers based on the size
of the space, the air exchange rate, differences between the inside and
outside of the building, and the number of people using the room. The
Humidification Handbook 3 listed in the bibliography provides useful
information about humidification.
•
Dehumidifiers remove moisture from the air and lower the RH. Don’t
use this equipment as a permanent corrective measure – instead, find
out why the air is so damp and work to remove the source of the water.
There are two types of dehumidifiers:
−
Refrigerant dehumidifiers work on the same principle as a
refrigerator. Cool air cannot hold as much moisture as warm air
and it condenses within the machine. Use this type of
dehumidifier in warm climates. You must drain dehumidifiers at
least daily.
−
Desiccant dehumidifiers force air through a moisture-absorbing
material (for example, lithium chloride) to reduce moisture. Hot
air is blown over the desiccant to regenerate it. Desiccant
dehumidifiers are useful in colder areas where refrigerant
dehumidifiers may ice up and stop working.
The Cargocaire Dehumidification Handbook is a basic sourcebook for
understanding dehumidification.4
9.
What are humidistatically
controlled heating and
ventilation systems?
Humidistatic control is a way to control relative humidity in a building
without using a HVAC system. The basic idea behind humidistatic control
takes advantage of the inverse relationship between temperature and
relative humidity.
Humidistatically controlled heating is based on the idea that if the absolute
humidity of a given volume of air changes, it is possible to maintain a
stable RH by manipulating and varying the temperature. A humidistat
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NPS Museum Handbook, Part I (1999)
sensor adjusts the temperature up and down to maintain a stable RH. If the
RH rises above a set point, the heat is turned on until the RH drops back
down. However, using this system, temperatures can drop very low, so this
type of environmental control system is best used in areas that are
infrequently accessed.
Humidistatically controlled ventilation is used in areas with high relative
humidity. If interior RH is lower than exterior RH, dampers are opened by
sensors and the air is circulated through the building. If exterior RH is too
high, the dampers remain closed.
Both of these techniques may be cost effective ways of improving the
environment in historic buildings that were not built to house museum
collections. They are generally less intrusive to the building fabric, and
maintenance and energy costs are lower than typical HVAC systems. If
you are considering using humidistatic controls work with an engineer or
architect who has experience with the technique.
10. What is the time-weighted
preservation index (TWPI)?
The time-weighted preservation index is a mathematical model developed
to estimate how long some organic materials will last at certain temperature
and RH levels. Using the TWPI you can make educated decisions choosing
a setpoint for RH and temperature in your collection.
The TWPI was developed specifically for paper-based collections and is
more commonly used for archives and libraries. It may not be appropriate
for mixed collections found in many NPS parks, though its use in various
types of collections is being actively researched. However, you can use it
to develop microenvironments, or separate storage rooms for your paper
and photo collections. See the reference by James Reilly et al. (1995) for a
complete discussion of the TWPI.
F. Light
Light is another agent of deterioration that can cause damage to museum
objects. Light causes fading, darkening, yellowing, embrittlement,
stiffening, and a host of other chemical and physical changes. This section
gives an overview of the nature of light. It will help you understand and
interpret monitoring data and the standards given for light levels in museum
storage and exhibits.
Be aware of the types of objects that are particularly sensitive to light
damage including: book covers, inks, feathers, furs, leather and skins,
paper, photographs, textiles, watercolors, and wooden furniture.
1.
What is light?
NPS Museum Handbook, Part I (1999)
Light is a form of energy that stimulates our sense of vision. This energy
has both electrical and magnetic properties, so it known as electromagnetic
radiation. To help visualize this energy, imagine a stone dropped in a pond.
The energy from that stone causes the water to flow out in waves. Light
acts the same way. We can measure the “wavelength” (the length from the
top of each wave to the next) to measure the energy of the light. The unit of
measurement is the nanometer (1 nanometer (nm) equals 1 thousand
millionth of a meter). We can divide the spectrum of electromagnetic
radiation into parts based on the wavelength. The ultraviolet (UV) has very
short wavelengths (300-400 nm) and high energy. We cannot perceive UV
4:33
light. The visible portion of the spectrum has longer wavelengths (400-760
nm) and our eyes can see this light. Infrared (IR) wavelengths start at about
760 nm. We perceive IR as heat.
The energy in light reacts with the molecules in objects causing physical
and chemical changes. Because humans only need the visible portion of the
spectrum to see, you can limit the amount of energy that contacts objects by
excluding UV and IR radiation that reaches objects from light sources.
All types of lighting in museums (daylight, fluorescent lamps, incandescent
(tungsten), and tungsten-halogen lamps) emit varying degrees of UV
radiation. This radiation (which has the most energy) is the most damaging
to museum objects. Equipment, materials, and techniques now exist to
block all UV. No UV should be allowed in museum exhibit and storage
spaces.
The strength of visible light is referred to as the illumination level or
illuminance. You measure illuminance in lux, the amount of light flowing
out from a source that reaches and falls on one square meter. We measure
illuminance in museums because we are concerned with the light energy
that falls on our objects, not how much light energy comes from the source.
When you measure light levels (see Section H), hold your meter at the
surface of the object to catch the light that is reaching that surface.
Illuminance was previously measured in footcandles. You may find older
equipment or references that list footcandle levels. Ten footcandles equal
about 1 lux.
When considering light levels in your museum you should keep in mind the
“reciprocity law.”
The reciprocity law states, “Low light levels for extended periods cause
as much damage as high light levels for brief periods.”
The rate of damage is directly proportional to the illumination level
multiplied by the time of exposure. A 200-watt light bulb causes twice as
much damage as a 100-watt bulb in the same amount of time. A dyed
textile on exhibit for six months will fade about half as much as it would if
left on exhibit for one year. So if you want to limit damage from light you
have two options:
•
reduce the amo unt of light
•
reduce the exposure time
Note: Even small amounts of light will cause damage. Damage as a result
of exposure to light is cumulative. It cannot be reversed. However, you
can stop the continuation of damage by placing an object in dark storage.
Cases, boxes, and folders are the first defense against light damage.
If lighting is too close to or focussed on an object, IR can raise the
temperature. It may also lower the water content of porous materials. You
can get heat buildup from:
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NPS Museum Handbook, Part I (1999)
•
sunlight
•
incandescent spotlights
•
fluorescent ballasts
•
lights in closed cases
Design exhibits so there is no heat buildup from IR generated by lights.
2.
What are the standards for
visible light levels?
You can protect your exhibits from damage caused by lighting by keeping
the artificial light levels low. The human eye can adapt to a wide variety of
lighting levels, so a low light level should pose no visibility problems.
However, the eye requires time to adjust when moving from a bright area to
a more dimly lighted space. This is particularly apparent when moving
from daylight into a darker exhibit area. When developing exhibit spaces,
gradually decrease lighting from the entrance so visitors’ eyes have time to
adjust. Do not display objects that are sensitive to light near windows or
outside doors.
See the next section for ideas on how to control visible, UV, and IR light.
Basic standards 5 for exhibit light levels are:
•
•
50 lux maximum for especially light-sensitive materials including:
−
dyed organic materials
−
textiles
−
watercolors
−
photographs and blueprints
−
tapestries
−
prints and drawings
−
manuscripts
−
leather
−
wallpapers
−
biological specimens
−
fur
−
feathers
200 lux maximum for less light-sensitive objects including:
−
NPS Museum Handbook, Part I (1999)
undyed organic materials
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•
−
oil and tempera paintings
−
finished wooden surfaces
300 lux for other materials that are not light-sensitive including:
−
metals
−
stone
−
ceramics
−
some glass
In general don't use levels above 300 lux in your exhibit space so that light
level variation between exhibit spaces is not too great. With this method,
people's eyes will not have to keep adapting to changing light levels, and
they will be able to see objects exhibited at lower levels much more easily.
These standards should serve as a starting point for developing lighting
standards for your collections. In order for collections to be seen and used
in various ways (for example, long-term exhibit, short-term exhibit,
research, teaching) you should take into account a variety of factors:
•
light sensitivity of the object
•
time of exposure
•
light level
•
•
type of use
color and contrast of object
For example, if a researcher needs to examine fine detail in the weave of a
textile, but will only be working on the object for one day you should allow
more light (up to 1350 lux) than if the same textile were going on exhibit
for two years.
Note: These light levels are a compromise between the need to see exhibits
and the need to preserve the objects. All light exposure will cause damage
to sensitive objects. There is no minimum level where damage will not
occur.
See references in the bibliography for more information on light and
designing appropriate lighting for your exhibit and storage areas.
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NPS Museum Handbook, Part I (1999)
G. Monitoring and Controlling
Light
To be sure that light levels are at required levels and to be sure that any UV
filtering material is still effective, you should measure light levels at least
once a year. If you change lighting fixtures, take new measurements to be
sure the changes are within recommended levels. If the source of light is
daylight (for example, in a historic house museum) you should measure
light in the morning and afternoon throughout the seasons.
1.
How do I monitor light levels?
You monitor light levels using specialized equipment. This equipment is
necessary because your eye is not a reliable guide as it easily adapts to
changes in visible light and can’t see UV or IR light. Use a visible light
meter to measure visible light and a UV meter to measure ultraviolet light.
Use a thermometer to measure heat buildup from IR. Several different
meters are available for measuring visible and UV light. See NPS Tools of
the Trade for sources for light monitoring equipment.
•
Visible Light Meter: Use a visible light meter to measure the visible
portion of the electro magnetic spectrum. If you purchase a new meter,
you should be sure to purchase one that measures in the standard unit,
lux. The meter you choose should be sensitive enough to measure
light levels as low as 25 to 50 lux with a reasonable degree of accuracy
(10% or better).
•
Ultraviolet Meter: The Crawford UV Monitor is the standard piece
of equipment used in museums for measuring UV levels. This monitor
gives UV readings in microwatts per lumen. Older models depended
on adjusting a knob until one red indicator light jumped to another
light, giving a fairly inaccurate measure. Newer models are more
accurate, providing the reading on a direct analog scale. There are also
models of UV meters from different manufacturers that will provide a
digital readout.
Use a standard set of procedures when monitoring light levels with either
piece of equipment. Aim the sensor toward the light source so you are
catching the light that is hitting the object you are monitoring. Be sure no
shadows from your hand or body are in the way. Make sure the sensor is
parallel to the surface of the object and aimed toward the light source. If the
object is larger than about one foot square, take several readings. Before
using any equipment, carefully read the manufacturer’s instructions for
operation and maintenance.
2.
How do I improve the lighting
to minimize damage to
objects on exhibit or in
storage?
You should develop a plan of action first, to determine if a lighting problem
exists, then to determine the cause, and solutions to correct the problem.
To develop your plan you first need to collect information, evaluate the
data, and then develop solutions to problems that you find.
Obtain and record the following information:
NPS Museum Handbook, Part I (1999)
•
types of existing lighting fixtures, ballasts, and filters
•
movement of sunlight in the room throughout the day
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•
seasonal variations in light
•
unusual events that occur (for example, filming in a historic structure,
drapes removed for cleaning)
Once you have identified the types of light and variations in lighting, you
need to evaluate how light may affect the objects. Identify museum objects
that are most susceptible to light damage and establish monitoring sites
nearby. You will use these same sites for each monitoring session.
Abandon old sites and es tablish new ones as conditions change. Document
your monitoring and any corrective actions that you take:
3.
How do I limit light damage
from research use?
•
Prepare a floor plan for each exhibit or storage space that indicates the
location of each monitoring site.
•
Record data on the Light and Heat Measurement Record illustrated in
Figure 4.8.
•
Note any corrective actions taken in the comments section, for
example:
−
curtains drawn
−
historic awnings replaced
−
UV filtering film installed over windows or fluorescent tubes
−
electric voltage stepped down
−
light fixtures replaced
−
new procedures to turn off lights when room is not in use
When historic objects, archival materials, and natural history specimens are
used by researchers they are exposed to light. Set up separate work spaces
and research rooms so that your entire collection is not exposed to light
when people are working with individual objects. Incorporate the
following practices into research room use to limit the damage that occurs
from this use.
•
•
Develop procedures so that collection items are exposed to light only
while the researcher is using them.
−
Keep documents in boxes or folders.
−
Remove objects from cabinets only when the researcher is ready
to work.
Limit the number of times an individual document can be photocopied.
See Museum Handbook , Part III, Appendix D: Planning a Research Space,
and Conserve O Gram 19/7, “Archives: Reference Photocopying,” and
4/14, “Planning a Research Space,” for more information on limiting light
damage from collection use.
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NPS Museum Handbook, Part I (1999)
Evaluate the data you collect using the Light and Heat Measurement
Record. Look carefully at the information and use it to decide how to
minimize the damage from light. You may want to consult the regional/SO
curator and/or a conservator for help with your evaluation. Think about the
following:
•
Which areas have acceptable levels of light for the objects? How long
have objects been on exhibit in these areas? Do they show signs of
damage? Remember, not all damage can be detected by visual
inspection.
•
Which areas have light levels that are too high? You will need to make
changes in these areas and evaluate whether your changes have helped.
For example, if UV filtering film is installed on glass window panes,
monitor for UV before and after application. Has the UV been
eliminated? Has it affected visible light? Does the visible light still
exceed the standard?
•
Compare existing light levels with historic lighting conditions. Do
objects on exhibit receive more or less light than they did historically?
Will reducing light levels in a historic structure improve the
interpretation of the building and the collection, and at the same time
improve preservation?
•
How often are collections used? Where and how are they used? What
levels and duration of light exposure do they normally receive? Can
you reduce light levels in research rooms to improve preservation and
still provide adequate access?
•
How often are archival collections copied? Can you produce a
‘duplication master’ so that originals will not have to be continuously
copied?
You should keep the data you collect. It can help you make a case for
needed changes in lighting or removal of threatened objects. Keep a
permanent file of all light monitoring data.
4.
How do I fill out the Light and
Heat Me asurement Record?
NPS Museum Handbook, Part I (1999)
Follow these instructions for filling out the Light and Heat Measurement
Record shown in Figure 4.8. You should use building floor plans in
conjunction with this record.
•
Identify the park and structure in the appropriate blocks.
•
Enter the day, month, and year, and the time of day in the appropriate
blocks.
•
On the building floor plan enter a number at a light measurement site.
Enter this station number on the Record in the Location block.
•
Record the ultraviolet light reading from the UV meter. If the reading
is above 75 microwatts per lumen, you should take corrective action.
4:39
4:40
•
Record the visible light reading from the visible light meter in the Lux
block. If you are using a meter that records in footcandles, convert to
lux by multiplying by 10 (1 footcandle = approx. 10 lux).
•
Record the temperature in the Temperature block.
•
Record any information about the type of light source, the weather, and
other comments in the comments box. Record any unusual
circumstances in this block.
NPS Museum Handbook, Part I (1999)
NATIONAL PARK SERVICE
NPS Museum Handbook, Part I (1999)
Figure 4.8. Example Light and Heat Measurement Record
Comments:
Comments:
Comments:
Comments:
Comments:
4:41
LIGHT AND HEAT MEASUREMENT RECORD
Structure: Historic Exhibit Building____________________
Date
Time
Location
UV Reading LUX
(µw/lumen)
Reading
4/11/99
11:00
Dining Room table in front of bay window
10
100
Comments: Cloudy day, drapes open
5/10/99
11:30
Same as above
10
75
Comments: Bright hot day, drapes closed
6/11/99
1:30
Same as above
100
100
Comments: UV filtering film is peeling in one corner of window, cloudy day, drapes open
68
70
68
Room Temp.
5.
Is there any way to directly
monitor light damage?
You can directly monitor light damage by using Blue Wool light standards.
Blue Wool light standards are specially dyed textiles made so that the most
sensitive sample fades in half the time needed to fade the next most
sensitive sample. There are eight samples to a set. You can use the Blue
Wool standards in two ways:
•
Place one set of standards at the place you want to measure. Place
another set in total darkness.
•
Place aluminum foil over one half of a set of standards.
By comparing the two sets of standards, or two halves of one set, you can
determine the light fastness of a material. The standards will not help you
estimate how much exposure to light a material will stand in a particular
situation. You can use Blue Wool standards to help you make an argument
that light damage is occurring and that changes are needed to protect
museum objects.
6.
How do I control light levels?
All light causes damage and the damage is cumulative. Therefore, you
must control all light in museum spaces that contain museum objects.
There are several control methods that you can use. Be creative and use a
variety of strategies to minimize light. Always monitor before and after to
be sure that your changes have really helped. Remember, your eye is not a
good tool for measuring light levels —use monitors.
Visible light must be maintained at or below the recommended levels. You
can obtain these levels using any of the control methods below:
4:42
•
Use window coverings such as blinds, shades, curtains, shutters, and
exterior awnings. Close window coverings as much as possible to
prevent light from reaching museum spaces. If windows must be
uncovered for visitors, install UV filters and work out schedules so that
windows are uncovered for only part of each day.
•
Use opaque dust covers (for example, cotton muslin or Gortex®) to
cover light-sensitive objects, including floor coverings. Dust covers
should be used whenever visitors are not present for extended periods.
They are useful in storage areas and exhibit areas that are not open to
the public for part of the year.
•
You can use tinted light filters (for example, films or glazing) on
windows or over artificial lighting. Don’t use reflective films or tints
that call attention to the windows or are historically inappropriate.
Consult the park or regional historic architect and your regional/SO
curator to be sure filters are appropriate.
•
You can reduce the amount of light from fixtures by using colored
filters, lowering the wattage of incandescent bulbs, using fewer
fixtures, using flood light bulbs instead of spots, and turning off lights
when people are not present. You can install motion detectors in
exhibit areas that activate lighting only when a person is present. You
can attach timers so that lights are on only for a specific period of time.
•
Use incandescent lights (which produce very little UV) instead of
fluorescent lights.
NPS Museum Handbook, Part I (1999)
Ultraviolet light should be completely eliminated. All of the techniques
used to limit visible light will also cut down on UV light. To block the
remaining UV light:
•
Install filtering material. Refer to NPS Tools of the Trade for sources
of UV filtering material. Types of filters include:
−
UV filtering film for windows or glass on framed objects
−
UV filtering plexiglass instead of glass
−
filter sleeves for fluorescent tubes
−
UV filtered fluorescent tubes
The plastic material that carries the UV filtering coating often breaks down
faster than the filtering chemical. You should replace filters whenever they
begin to turn yellow or crack. Monitor UV radiation at least every five
years to be sure the filtering material is still effective.
Infrared radiation (heat) generated by natural or artificial lighting should
also be controlled to prevent rapid changes in relative humidity. Window
coverings and filters and good air circulation systems (for example, fans
and air conditioners) help control heat buildup. You can control the heat
produced by artificial lighting fixtures by using filters and good air
circulation systems, as well as keeping lights outside exhibit cases.
Floodlights used for professional and motion picture photography and
photocopy machines can cause excessive heat buildup. Discourage
photography in museum storage areas. When photography is allowed in
museum areas request heat absorbing light filters and be sure the area is
well-ventilated with fans or air conditioners. Lights should be turned off
whenever filming is not taking place. If lighted rehearsals are necessary,
use dummy objects until the final filming will take place.
H. Dust and Gaseous Air
Pollution
Air pollution comes from contaminants produced outside and inside
museums. Common pollutants include: dirt, which includes sharp silica
crystals; grease, ash, and soot from industrial smoke; sulfur dioxide,
hydrogen sulfide, and nitrogen dioxide from industrial pollution;
formaldehyde, and formic and acetic acid from a wide variety of
construction materials; ozone from photocopy machines and printers; and a
wide variety of other materials that can damage museum collections. Air
pollutants are divided into two types:
NPS Museum Handbook, Part I (1999)
•
particulate pollutants (for example, dirt, dust, soot, ash, molds, and
fibers)
•
gaseous pollutants (for example, sulphur dioxide, hydrogen sulphide,
nitrogen dioxide, formaldehyde, ozone, formic and acetic acids)
4:43
1.
What are particulate air
pollutants?
Particulate pollutants are solid particles suspended in the air. Particulate
matter comes both from outdoor and indoor sources. These particles are
mainly dirt, dust, mold, pollen, and skin cells, though a variety of other
materials are mixed in smaller amounts. The diameter of these pollutants is
measured in microns (1/1,000,000 of a meter). Knowing the particulate
size is important when you are determining the size of air filters to use in a
building.
Some particles, such as silica, are abrasive. Pollen, mold and skin cells can
be attractive to pests. Particulates are particularly dangerous because they
can attract moisture and gaseous pollutants. Particulates can interact with
gaseous pollutants and cause deterioration in three different ways.
Particulates may be:
2.
What are gaseous air
pollutants?
•
a source for sulfates and nitrates (These particles readily become
acidic on contact with moisture.)
•
a catalyst for chemical formation of acids from gases
•
an attractant for moisture and gaseous pollutants
Gaseous pollutants are reactive chemicals that can attack museum objects.
These pollutants come from both indoor and outdoor sources.
Outdoor pollutants are brought indoors through a structure’s HVAC
system or open windows. There are three main types of outdoor pollution:
•
sulfur dioxide (SO2 ), and hydrogen sulphide (H2 SO) produced by
burning fossil fuels, sulfur bearing coal, and other organic materials
•
nitrogen oxide (NO) and nitrogen dioxide (NO2 ), produced by any kind
of combustion, such as car exhaust as well as deteriorating
nitrocellulose film, negatives, and objects
•
ozone (O3 ), produced by sunlight reacting with pollutants in the upper
atmosphere and indoors by electric or light equipment, such as
photocopy machines, printers, some air filtering equipment
When s ulfur and nitrogen compounds combine with moisture and other
contaminants in the air, sulfuric acid or nitric acid is produced. This acid
then causes deterioration in a wide variety of objects. Ozone reacts directly
with the objects causing deterioration.
The main sources of indoor air pollution come from building materials
and include:
4:44
•
wood, which can release acids
•
plywood and particle board, which give off acids from wood and
formaldehyde and acids from glues
•
unsealed concrete, which releases minute alkaline particles
NPS Museum Handbook, Part I (1999)
•
some paints and varnishes, which release organic acids, peroxides, and
organic solvents
•
fabrics and carpeting with finishes, such as urea-formaldehyde, and
wool fabrics that release sulfur compounds.
•
glues, used to attach carpets, that can release formaldehyde
•
plastics that release plasticizers and harmful degradation products such
as phthalates and acids
Museum objects themselves may also contribute to indoor air pollution.
For example, many plastics are inherently unstable and as they deteriorate
they give off acidic by-products. Examples of sources of pollutants from
museum objects include:
Object Materials
•
celluloid and other unstable plastics used to produce many 20th -century
objects
•
cellulose nitrate and diacetate plastic, used for film
•
pyroxylin impregnated cloth used for book bindings
•
residual fumigants, such as ethylene oxide
Deterioration
Primary Air Pollutants
Environmental Factors
Accelerating Damage
metals
corrosion/tarnishing
sulfur oxides and other
acidic gases
water, oxygen, salts
stone
surface erosion, discoloration
sulfur oxides and other
acidic gases, particulates
water, temperature
fluctuations, salt, vibration,
microorganisms, carbon
dioxide
paint
surface erosion, discoloration
sulfur oxides, hydrogen
sulfide, ozone, particulates
water, sunlight,
microorganisms
textile dyes and
pigments
fading, color change
nitrogen oxides, ozone
sunlight
textiles
weakened fibers, soiling
sulfur oxides, nitrogen
oxides, particulates
water, sunlight, mechanical
wear
paper
embrittlement
sulfur oxides
moisture, mechanical wear
leather
weakening, powdered surface
sulfur oxides
mechanical wear
ceramics
damaged surface
acid gases
moisture
Figure 4.9. Deterioration to Museum Objects Caused by Air Pollution
NPS Museum Handbook, Part I (1999)
4:45
I. Monitoring and Controlling
Particulate and Gaseous Air
Pollution
As with problems from other agents of deterioration, you need to monitor
your collections to identify whether or not air pollution is causing damage
to your collections.
1.
How do I monitor air
pollution?
There are a variety of monitoring devices that can be used to directly
measure pollutants in the museum. If you feel direct measurement is
needed, contact your regional/SO curator for assistance. There are other
steps you can take to identify and understand air pollution levels.
•
Inspect storage spaces (for example, floors, open shelving, tops of
cabinets and tables) for dust. Note how much dust has built-up since
the last cleaning. Watch for increased insect activity using your IPM
program. Increased insect activity is often related to an unacceptable
accumulation of dust.
•
In coastal areas, watch for pollution from chlorides by observing and
noting active corrosion on metal objects. Chlorides will react with
unpainted iron or steel objects, causing rust.
•
Observe and document a building’s air control system and the nature of
the structure. Concrete walls and adobe are sources of high levels of
dust. Some concrete dating from 1940-1975 contains asbestos, making
it a health risk as well as a source of particulates. Improperly filtered
air intakes can transfer high levels of pollutants into museum spaces.
•
Identify exhibit cases, storage cabinets, and shelving made out of
untreated wood or painted with the wrong paints that can outgas
formaldehyde and acetic acid.
•
Watch to see how much dust and dirt is tracked into spaces by visitors
and employees.
Some parks have on-going air monitoring research. You can also contact
the Environmental Protection Agency (EPA), Office of Air Quality
Planning and Standards to obtain information on levels of ozone, sulfur
dioxide, nitrogen dioxide, and particulates recorded in the park. These data
will assist park staff in identifying potential pollutant problems that may
exist. Areas with high concentrations of gaseous pollutants in the air will
definitely want to establish a program for monitoring signs of active
deterioration on objects in museum storage and exhibit areas.
2.
4:46
Are there ways to monitor for
air pollution?
There are several ways to monitor air pollutants that are simple to use in
museums. Each has good points and bad points so before you choose one
method, investigate each type of monitor and evaluate the type of
information you want to recover. You can get advice from a conservator or
your regional/SO curator. There is more information in the bibliography on
using and evaluating monitoring devices.
NPS Museum Handbook, Part I (1999)
Oddy tests: Oddy tests have been used for some time as a simple method
of evaluating materials that are used in contact with objects in storage or on
exhibit. In this test, metal coupons (small samples of metal) are placed in a
closed container with the material being tested and a small amount of
moisture. The container is slightly heated and after a set amount of time,
the metal is examined for corrosion. It gives you some idea of how ‘safe’ a
material is and whether or not it will cause deterioration? Problems with
this test include:
•
unusual reactions—because heat and moisture are raised in the
container, reactions may occur that would not happen in a normal
museum environment
•
little reproducibility—for a variety of reasons, results from this test are
widely variable
Passive sampling devices: These are devices that absorb particular
pollutants. They are placed in the area you want to test for some period of
time and then removed and sent to a lab to be tested for presence and levels
of pollutants. Each passive sampling device measures one type of
pollutant. For example, one device will measure for formaldehyde, another
for acetic acid. However, there are problems with these devices:
•
They may require off-site analysis.
•
The devices have varying sensitivities. Use devices that can detect
gaseous pollutant in parts per billion (1:1,000,000,000 ppb) or lower
levels.
A-D strips. These strips detect acetic acid. They were developed to detect
and measure acetate film deterioration or “vinegar syndrome” in film
collections. They change color as the level of acidity increases. They are
used to set priorities for film reformatting.
3.
How do I control air pollution?
NPS Museum Handbook, Part I (1999)
The NPS standard in the Checklist for Preservation and Protection of
Museum Collections on controlling air pollution states, “Eliminate gaseous
and particulate pollution to the lowest practical level.” There is no
minimum acceptable level of pollution. You can do the following to reduce
levels of air pollution:
•
In storage spaces, keep floors, tops of cabinets, and work surfaces
clean to minimize dust accumulation. Work with custodial staff to
keep areas clean. Use high efficiency particulate air (HEPA) vacuums
which catch more particulates. Regular vacuum cleaners simply throw
many smaller particles up into the air.
•
Separate office and curatorial work spaces from museum collections
storage spaces. Areas that are not accessed often will stay cleaner than
high traffic areas.
•
Upgrade and maintain seals and weatherstripping around doors and
windows to keep pollutants out.
4:47
4:48
•
Store sensitive objects in appropriate museum specimen cabinets.
Maintain sound gaskets on all storage cabinets. Replace old gaskets
with neoprene gaskets. Refer to NPS Tools of the Trade for the source
of retrofit gasket kits. NPS Conserve O Gram 4/8 explains how to
install the retrofit gasket kit.
•
Store archival materials in boxes, map cases, and folders.
•
Use dust covers to protect objects on open shelving. Dust cover
material should be chemically and physically non-damaging and
provide as complete a dust seal as possible, while allowing easy access.
Use clear polyethylene sheeting, unbleached cotton muslin, Tyvek®, or
Gore-Tex®. Refer to NPS Conserve O Gram 4/7, “Dust Covers for
Steel Shelving,” for additional information on constructing dust covers.
•
Segregate objects that outgas pollutants (for example cellulose nitrate
negatives or objects, diacetate negatives, or hardwoods such as oak,
birch or beechwood) from other objects.
•
Store, exhibit, and transport objects in appropriate cases. Avoid using
exhibit materials (for example, hardwoods) that outgas organic acids.
The adhesives used in plywood and veneers may be a source of
pollutants. See Figure 4.10 for a list of both harmful and safe
materials.
•
In areas with high air pollution levels you may want to install pollution
filtering in your HVAC system. These filters extract gaseous and
particulate pollutants before they get into a museum space. Work with
HVAC engineers to design a system appropriate to your facility. Do
not use filtering systems that generate damaging ozone.
•
You can use portable air filters with activated-carbon filters to remove
particulates from the air. These filters will also remove some gaseous
pollutants. Refer to NPS Tools of the Trade for sources for this
equipment.
NPS Museum Handbook, Part I (1999)
Storage and Exhibit Construction Materials Known to Release Harmful Substances
Materials
Harmful Vapors
wood (particularly oak, birch, beech)
organic acids
wood panel products
organic acids, formaldehyde
protein-based glues, wool
volatile sulfides
vulcanized rubber
volatile sulfides
some dyes
sulfur compounds
cellulose nitrate
nitrogen oxides
cellulose acetate
acetic acid
polyvinyl chloride
hydrogen chloride
polyurethanes
volatile additives
Storage and Exhibit Construction Materials That Appear to be Safe
metals
glass
ceramics
inorganic pigments
polyethylene and polypropylene
acrylic solutions (some acrylic emulsions are suspect)
polyester fibers
cotton and linen
Note: while these materials are considered safe, manufacturing processes may add coatings and additives that can
damage museum collections.
Figure 4.10. Types of Materials That Can Harm Objects and Types of Materials That are Considered Safe to
Use with Museum Objects for Storage and Exhibit6
NPS Museum Handbook, Part I (1999)
4:49
J. Selected Bibliography
Adcock, Edward P., Marie -Therese Varlamoff, and Virginie Kremp . IFLA Principles for the Care and Handling of
Library Material. Washington, D.C.: International Federation of Library Association and Institutions Core
Programme On Preservation and Council on Library and Information Resources, 1998.
Baer, Norbert S., and Paul N. Banks. “Indoor Air Pollution: Effects on Cultural and Historic Materials.” MM &C
4: 9-22, 1985.
______. “Particulate Standards for Museums Libraries and Archives.” In Air Pollution Control Association in
Detroit, 1986.
Brimblecombe, Peter. “The Composition of Museum Atmo spheres.” Atmospheric Environment 24B, no. 1 (1990):
1-8.
Canadian Conservation Institute. “Ultraviolet Filters for Fluorescent Lamps.” CCI Notes 2/1. Ottawa, Ontario:
Canadian Conservation Institute, 1988.
Carpenter, Jane, and Pamela Hatchfield. Formaldehyde: How Great is the Danger to Museum Collections?
Cambridge, Mass.: Center for Conservation and Technical Studies, Harvard University Art Museums, 1987.
Child, Robert E, ed. Electronic Environmental Monitoring in Museums. Denbigh, Clwyd, Wales: Archetype
Publications Ltd., 1993.
Cumberland, Donald C., Jr. “Dust Covers for Steel Shelving,” Conserve O Gram 4/7. Washington, D.C.: National
Park Service, 1985.
_______. “Calibration of Hygrometers and Hygrothermographs.” Conserve O Gram 3/ 16. Washington, D.C.:
National Park Service, 1988.
Cuttle, Christopher. “Damage to Museum Objects Due to Light Exposure.” CIBSE National Lighting Conference,
1996.
De Guichen, Gael. Climate in Museums: Measurement. 3d ed. Rome: International Center for the Study of the
Preservation and Restoration of Cultural Property, 1988.
Elovitz, Kenneth, ed. “Practical Guide to HVAC for Museums and Historic Renovation.” ASHRAE Journal 41, no.
4 (1999): 48-98.
Feller, Robert L. “Control of Deteriorating Effects of Light on Museum Objects: Heating Effects of Illumination by
Incandescent Light.” Museum News 46, no.9 (1968): 39-47.
Grzywacz, Cecily M., and Norman H. Tennent. “Pollution Monitoring in Storage and Display Cabinets: Carbonyl
Pollutant Levels in Relation to Artifact Deterioration.” In IIC Ottawa Congress: Preventive Conservation
Practice, Theory, and Research, pp. 164-170. Edited by Roy Ashok and Perry Smith. London: International
Institute for Conservation of Historic and Artistic Works, 1994.
Hackney, Stephen. “The Distribution of Gaseous Air Pollution within Museums.” Studies in Conservation 29
(1984): 105-116.
Hitchcock, Ann, and Gordon C. Jacoby. "Measurement of Relative Humidity in Museums at High Altitude." Studies
in Conservation 25 (1980): 78-86.
4:50
NPS Museum Handbook, Part I (1999)
Jessup, Wendy Claire. “Conservation: A Basic Overview for the Exhibit Specialist.” Exhibitionist (Summer 1990):
15019. Washington, D.C.: National Association for Museum Exhibition: Standing Professional Committee
on Museum Exhibition of the American Association of Museums.
La Fontaine, Raymond H., and Patricia A. Wood. “Fluorescent Lamps.” Technical Bulletin No. 7. rev. ed. Ottawa,
Canada: Canadian Conservation Institute, 1982.
Lull, William P., and M. A. Garrison. “Planning and Design of Museum Storage Environments.” Registrar 5, no. 2
(1988): 4-14.
Michalski, Stefan. “Towards Specific Lighting Guidelines.” ICOM Committee for Conservation Preprints, 1990.
______. “Temperature and Relative Humidity: The Definition of Correct/Incorrect Values.” Technical Appendix.
In Michalski, Stefan, A Systematic Approach to the Conservation (Care) of Museum Collections. Ottawa,
Ontario: Canadian Conservation Institute, 1992.
Padfield, Tim, David Erhardt, and Walter Hopwood. “Trouble in Store.” In Science and Technology in the Service
of Conservation, pp. 24-27, 1981.
Park, Sharon C. “Heating, Ventilating, and Cooling Historic Buildings: Problems and Recommended Approaches.”
Preservation Brief 24. Washington, D.C. National Park Service, 1991.
Raphael, Toby. Exhibit Conservation Guidelines. CD-ROM produced by Harpers Ferry Center. Harpers Ferry,
W.V.: National Park Service, 1999.
Reilly, James M. The Storage Guide for Color Photographic Materials. New York: The University of the State of
New York, New York State Education Department, New York State Library, the New York State Program
for the Conservation and Preservation of Library Research Materials, 1998.
Reilly, James M., Douglas W. Nishimura, and Edward Zinn. New Tools for Preservation: Assessing Long-term
Environmental Effects on Library and Archives Collections. Washington, D.C.: The Commission on
Preservation and Access, 1995.
Sheetz, Ron, and Charles E. Fisher. “Reducing Visible and Ultraviolet Light Da mage to Interior Wood Finishes.”
Preservation Tech Notes 2. Washington, D.C.: National Park Service, 1990.
Stolow, Nathan. “Chapter 2: Conservation Principles.” In Conservation and Exhibitions: Packing, Transport,
Storage, and Environment Considerations, pp. 4-24. London: Butterworths, 1987.
Thomson, Garry. The Museum Environment. 2d ed. London: Butterworths, 1986.
Thompson, John A., ed. Manual of Curatorship, 2d ed. London: Butterworths, 1992.
Tétreault, Jean, and R. Scott Williams. Materials for Exhibit, Storage, and Packing. Technical Appendix in
Michalski, Stefan. A Systematic Approach to the Conservation (Care) of Museum Collections. Ottawa,
Ontario: Canadian Conservation Institute, 1992.
Weintraub, Steven, and Gordon O. Anson. “Natural Light in Museum: An Asset or Threat?” Progressive
Architecture 5 (May 1990): 49-54.
______. “Use Ultraviolet Filters to Neutralize an Invisible Enemy.” Museum News 69 (1991): 85-87.
NPS Museum Handbook, Part I (1999)
4:51
K. Endnotes
1.
Relative humidity optimum ranges for materials included in Figure 4.2 are based on information from Climate
in Museums: Measurement (3d ed.) by Gael De Guichen, and on information included in curatorial care
appendices of this handbook.
2.
The ABCs of Air Conditioning: A Primer of Air Conditioning Types and Methods outlines the types of air
conditioning systems, introduces cooling load calculation, and compares the functioning of system types. A
copy of this publication may be obtained from:
Carrier Air Conditioning
P.O. Box 4800
Syracuse, NY 13221
(315) 432-6000
3.
The Humidification Handbook: What, Why and How, written by Bernard W. Morton, includes an introduction
to humidity theory and measurement, and provides specification information on the determination of
humidification load, methods of humidification, and system design considerations. It is available from:
Dri-Steem Humidifier Company
14949 Technology Drive
Eden Prairie, MN 55344
(800) 728-8336
4.
The Cargocaire Dehumidification Handbook discusses methods of dehumidification, system design, and
selection, and provides an introduction to calculations of moisture loads. A copy of this publication may be
obtained from:
Cargocaire Engineering Corporation
79 Monroe Street
P.O. 640
Amesbury, MA 01913
(800) 843-5360
5.
Illumination levels are based on information in The Museum Environment (2d ed.), “Light,” Part I by Garry
Thomson.
6.
Information included in Figure 4.10 is taken from “Trouble in Store” by T. Padfield, D. Erhardt, and
W. Hopwood, and "Materials for Exhibit, Storage, and Packing," by J. Tétreault and S. Williams.
4:52
NPS Museum Handbook, Part I (1999)
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