Ten Basics of Electric Firing

Ten Basics of Electric Firing
Ten Basics of Electric Firing
by Bill Jones
Firing is the most critical part of the ceramics process because it is the one thing that
makes clay durable, hence ceramic. Here
are some of the principles of firing and getting the best results with electric kilns.
From Mud to Ceramic
Firing converts ceramic work from weak
clay into a strong, durable, crystalline
glass-like form. Ceramic work is typically
fired twice: it is bisque fired and then glaze
fired. The goal of bisque firing is to convert
greenware to a durable, semi-vitrified porous stage where it can be safely handled
during the glazing and decorating process.
It also burns out carbonaceous materials
(organic materials in the clay, paper, etc.).
As the temperature in a kiln rises, many
changes take place in the clay. The Firing
Chart (available at www.ceramicartsdaily.
org/education/resources/) explains what
happens to clay as it heats up.
How Hot
All clays and glazes are formulated to
mature at certain temperatures. Firing clay
too high can cause it to deform or even
melt, too low and it will not be durable.
Firing glazes too high can cause run-off
on the pot, too low and they will be dry
and rough. To fire to the right temperature,
pyometric cones are used. Cones are made
from various oxide mixtures and bend at
known temperatures (figure 1).
In general, the following cones are used in
the pottery studio:
• bisque fire (cone 08–05)
• low fire (cone 06–04)
• mid-range (cone 4–7) and high fire
(cone 8–10).
Heat Transfer in a firing
Heat in an electric kiln is transferred to the ware
being fired in three ways:
• Conduction–heat transferred through physical
contact (1)
• Convection–heat rising through the air (2)
• Radiation–heat emanating from all of the kiln
elements (3)
How it works:
Electricity passing through coiled heating elements (made especially for high temperatures)
generates radiant heat, which rises and is absorbed by everything in the kiln.
©2010 Ceramic Publications Company
Using Cones
1
Cones are used in every firing. Typically, a threecone system (either large or self-supporting), consisting of a guide cone that is one cone below the
target temperature, the firing cone and a guard
cone (figure 2) provides the best information
about the firing. Bar cones and small cones are
used in a properly adjusted Kiln-Sitter®, an automatic shut-off device (figure 3). While the three
large cones are not required for kilns equipped
with a KilnSitter or an automatic controller, they
do provide a second point of reference for how a
kiln is operating.
Get Ready
2
Before firing any kiln, vacuum it out if necessary—bottom, sides, element channels and lid.
Check the elements for breaks, and chisel off
any glaze drips on the shelves. Visually check
the electrical cords and connections. Make any
repairs required (see owners manual or call your
local supplier for service).
Kiln Furniture
An assortment of kiln furniture (figure 5) is
needed to hold and support ware during a firing. Furniture consists of shelves, posts, stilts and
tile setters, all of which are made from refractory
materials. Kiln furniture is designed to withstand
the repeated heating and cooling to high temperatures without deforming.
The Bisque Load
3
Loading a bisque kiln is a fairly simple task, but
there are some basic rules. Fire full loads to take
advantage of conduction heating and also save
electricity. All work should be bone dry. If the
work is cool or cold to the touch, it is not bone
dry. Handle all work very carefully because it
is extremely fragile at this stage. Place the bottom shelf on 1-inch stilts to aid circulation, and
Kiln Controllers
Many electric kilns are now equipped with kiln controllers. Kiln controllers use a signal from a thermocouple
(a sensing device that detects temperature) that’s located in the kiln. When the controller senses the temperature, it compares this information with a computer program that tells the relays to turn on or off. The relays
control current going to the elements. Controllers take the guesswork out of when and how high to turn up
the heat on the kiln. Because they are accurate at sensing temperature, they are more efficient than manually-fired kilns. They come with preset programs, or you can even easily input programs to adjust to special
firing requirements.
©2010 Ceramic Publications Company
keep ware 1 inch away from elements,
walls, thermocouple and KilnSitter (figure
6). Unglazed pieces may touch each other.
Place a small cone in the KilnSitter and/or
a cone pad on the middle shelf. Fire to cone
08–05, depending on the type of clay and
amount of porosity you want for glazing.
5
The Bisque Fire
During the bisque firing a lot of damage
can take place. Thicker pieces with moisture or air bubbles create the biggest problem. Clay needs to dry evenly through its
entire thickness. If the outside dries faster,
it seals off the escape route for the interior moisture. The interior moisture turns to
steam and forces its way out (explodes) during the bisque. To avoid this, start off slowly
when firing a bisque kiln. Turn on one element to low. If you do not have a downdraft
exhaust system, prop the lid open, take the
peephole plugs out and keep the temperature below 212°F until all the moisture is
gone. Close the lid and check for moisture
(hold a mirror or piece of glass up to the
top peephole to see if it fogs up). Turn on
all elements to low for at least an hour then
to medium for an hour before turning all elements on to high. The firing is done when
the firing cone falls.
6
7
The Glaze Fire
Vacuum the kiln, especially if any pieces
exploded during the bisque. When firing
glazed pieces, make sure there is a thin
coating of kiln wash (available from suppliers) on the shelves (figure 7). You do
not need a fresh coat for each firing, but
any bare spots should be coated. Built-up
kiln wash becomes bumpy and should be
cleaned off with a chisel. All glazed pieces
must be checked to make sure there is no
glaze touching the shelf. Coat with wax at
least ½ inch from the bottom of the piece.
Sort work by height and place on shelves
with a minimum of ½ inch between pieces
and 1 inch from the walls, elements and
KilnSitter. Turn the kiln on low for about an
hour and then medium for about an hour
before turning on to high. The higher the
cone you are going to, the longer it will
take to fire.
What’s That Smell?
Clay and ceramic materials change their
chemistry when fired. Carbonaceous materials burn out between 500°F–1450°F.
Firing clay materials in electric and gas
kilns produces carbon monoxide, formaldehyde, sulfur dioxide gases and more.
Some of the byproducts are harmful so
vent kilns to the outside. A downdraft
vent system works best, but an updraft or
crossdraft system is better than nothing.
All kilns must be vented to the outdoors.
©2010 Ceramic Publications Company
Kiln Firing Safety
Firing is a potentially hazardous activity and all students must obey safety
rules to avoid injury. Instructors must read and understand all the safety
information that came with the kiln, and assure that the kiln is properly installed and maintained. If a manual is not available, many companies post
them online or you can request a replacement copy from the manufacturer.
For operating the kiln, students must:
• Turn off kiln prior to loading or unloading. Disconnect the kiln for any
servicing or when kiln is not in use.
• Do not touch heating elements with anything since they carry high voltage.
• Do not place any combustibles within 12 inches of any surface of the
kiln.
• Do not leave kiln unattended while firing.
• Never look into a hot kiln without properly tinted safety glasses
(e.g., welder’s glasses). Sunglasses only block ultraviolet light.
• Make sure the ventilation system is working properly.
• Never add extra insulation around a kiln to conserve energy.
Extra insulation can cause the wiring and the steel case to overheat.
• Remove all tripping hazards. Keep the power cord out of the way.
• Do not fire with cracked shelves. They can break during firing, which
could damage the ware inside the kiln. Store kiln shelves in a dry area.
• If you smell burning plastic, turn the kiln off. Examine the wall outlet and
power cord for signs of burning.
• Never wear loose-fitting clothing around a hot kiln.
• Do not open a kiln until it has cooled to room temperature.
Pots may break from thermal shock.
• Keep the kiln closed when not in use, and never place anything on the
kiln lid, even when the kiln is idle—you may forget.
• Always keep unsupervised children away from the kiln.
• Do not place any objects under or around the kiln stand.
Blocking airflow changes the kiln’s heating characteristics.
• Remove all flammable materials from the kiln room.
©2010 Ceramic Publications Company
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