20
Firing Speed
The main consideration for firing speed is that you should not fire faster than the ware will absorb heat, and
you should not cool faster than the ware will release heat. A specified amount of heat over a certain amount of time
is necessary to produce the chemical changes that result in a finished piece. Firing and cooling rapidly will result in
stressing and even cracking or crazing the ware due to uneven expansion. Very thick pieces, such as hand molded
sculptures, require very slow heating and cooling. Slow heating also allows moisture to escape without damaging the
ware, and slow cooling allows glazes, which may bubble when gases escape during the vitrification process, to return
to a smooth finish.
The maximum degree of heat and the length of time necessary vary widely according to many factors such as
the type of ware (porcelain, earthenware, stoneware, etc.) or glaze, paint or decal, the thickness of the piece, the size
of the load being fired, the voltage available and the condition of the elements.
Remember that during firing you will not endanger your ware by turning the kiln off before maturity. If you
ever hear pieces cracking or falling, or if your kiln has been jarred, turn the kiln off. Wait until it is cool, then open and
check for problems that may have developed. Replace all cones with new ones before firing again.
Element life varies depending upon the frequency and temperature of firings. They will last for many firings
if treated properly. Keep the element grooves free of debris of any sort which will eat through the element and cause
it to burn out. Also keep in mind that after repeated firings elements become brittle and will break if struck or scraped.
Do not fire your kiln hotter than the temperature for which it is rated.
Venting
Conventional venting consists of propping the lid during the first part of the firing cycle and firing with the
top peephole unplugged to allow fumes to escape. The optional Cress Fanfare kiln exhaust system is an excellent way
to provide ventilation and has many advantages. Besides removing fumes coming off the ware from the kiln room and
eliminating the need to prop and then lower the lid during firing, the Fanfare increases temperature uniformity, provides
for less cross contamination of colors during glaze firings, and achieves the clearest, brightest and purest colors pos-
sible.
Firing Glass
Glass is very sensitive to variations in temperature of only a few degrees. Glass must be allowed to pass through
the lower temperatures slowly to prevent shattering, then it may be fired rapidly up to temperature (firing rapidly at this
point helps preserve colors). Glass softens quickly once the critical temperature is reached and sagging begins (usually
approximately 1500
O
F for glass slumping); it is wise to check it often at this point (at 10 minute intervals) and when it
has slumped properly, turn off all switches, crack the lid at the lower lid prop position for up to ten minutes to prevent
over softening of the glass, then close the lid and allow to cool completely.
Glass is slumped onto terra cotta molds dusted with whiting (calcium carbonate) to prevent sticking. Be sure
to the glass loading section.
Firing charts
It is a valuable practice to keep a written record of each firing, noting the settings (even the degree of deforma-
tion of witness cones and their locations.) This firing chart allows you to repeat good results and successful firings and
avoid repeating less than satisfactory firing results.
AFTER FIRING
When the firing cycle has been completed, the display will read CPLt. If you need to monitor the tem-
perature as the kiln is cooling, press "stop", then turn the power switch off at the end of the cooling. Otherwise turn
power switch off when the display reads CPLt.. Allow the kiln to cool naturally, and do not open the peepholes or
prop the lid (unless working with glass, metallics, or lusters in which case some special venting techniques may
be necessary) until the kiln has cooled to at least 130
O
F and you can unload it using your bare hands. If the kiln is
opened prematurely, the ware and even the kiln could be damaged. You should expect the cooling period to take at
least twice as long as the firing cycle took to complete.