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Firing stages
Initial Heating from Room Temperature (room temperature to 1200
°
F range):
During the initial heating, the glass is very brittle and susceptible to breaking (thermal
shock) i
f it‟s heated up too quickly. During this stage, it‟s best to take a conservative
approach and slowly heat up the glass. At the end of this range, it‟s a good idea to add
a heat soaking period to allow the project to equalize to the same temperature
throughout. Never peek in the kiln during this stage, or you risk thermal shock breakage.
Process Heating (1200
°
F to 1500
°
F range):
The glass becomes softer and more fluid.
During the process heating stage, the glass can be fired more quickly to the target
temperature and soaked only long enough to achieve the desired look. It‟s important not
to hold the project at these temperatures for a long period of time, or you run the risk of
devitrification (a visible clouding of the glass surface due to crystallization).
Fast Cooling (1500
°
F to 1100
°
F range):
After the finished look has been achieved,
it‟s important to cool the inside of the kiln and the glass as quickly as possible to stop the
firing action so that it “freezes”. During the fast cooling stage, the surface of the glass is
cooler and it has contracted more than the heated center which remains expanded, thus
introducing stress into the glass piece.
Annealing (1100
°
F to 700
°
F range):
At the beginning of the annealing stage, it‟s
necessary to heat soak the glass for an extended period of time to allow the glass to
equalize in temperature throughout and release the stress that‟s in the glass. This
makes the glass more stable. Then the glass is slowly cooled through the annealing
temperature range to better ensure the glass piece is free of internal stresses and is
physically stronger. NEVER OPEN THE KILN DURING THIS STAGE!
Cooling to Room Temperature (700
°
F to room temperature range):
After the glass
temperature has cooled below the annealing temperature zone, the kiln can be turned
off to allow it to cool
at it‟s own pace to around 150°F – 200°F. Again, to avoid thermal
shock, do not open the kiln until it has cooled down to at least 200°F. Even at this
temperature, the glass is very hot, so do not attempt to pick it up in your bare hands.
Simply open up the lid of your kiln and allow the glass and the kiln shelf to cool down to
room temperature.
Factors to consider before selecting a firing schedule
First envision what you want your finished piece to look like and ask yourself the
following questions:
What is the size (diameter) of the piece? How many layers thick will it be?
What is the desired finished look?
Will it be a full fuse, a more textural tack fuse, a combination of both?
Will it be slumped or draped to become a more 3 dimensional and possibly
functional piece?
Will it be embellished with any surface decorations like enamels or metallic
paints?
