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oading stoneware and porcelain
It is necessary to fire stoneware and porcelain at a much higher temperature than that used
for low fire ware or glaze. At this heat the ware becomes much softer than ceramic bisque. For this
reason it may stick to other pieces and so should not be stacked or nested. It will also tend to distort in
shape unless the maximum temperature is accurately controlled and the piece is properly supported.
Hollow greenware pillars made of the same material as the ware are often used for support so that the
expansion and shrinkage of the support is the same as that of the ware. Do not support high-fire ware
on ordinary stilts. Often simpler shapes are supported by a reusable “setter” shaped for a particular
piece. Do not place ware closer than 3/4" from the elements to avoid uneven heating and distortion of
the piece.
Glazed porcelain and stoneware pieces, because of the high temperatures used, are always
dry-footed to prevent them adhering to the shelves.
Use high fire kiln wash. Some prefer to load stoneware unto surfaces sprinkled with silicaflour,
sometimes called “flint”. Keep “flint” away from the elements - it will cause them to burn out.
Loading glass
Do not fire small thin pieces of glass and large thick pieces in the same load. Glass sagging
is very sensitive to variation in temperature. Load only one or two shelves; keep the bottom shelf 3"
or more off the kiln floor and keep glass pieces at least 3" or more from the kiln top. Do not crowd
pieces at any time. Terra cotta molds, dusted with whiting (calcium carbonate) to prevent the glass
from adhering to the mold, are used in sagging sheet glass to shape. Do not use ceramic bisque molds
for sagging glass.
PYROMETRIC CONES
The most widely used method for monitoring the temperature achieved inside a kiln is the
pyrometric cone. A cone is a small elongated pyramid shaped indicator made of ceramic material which
is formulated to melt when subjected to a sufficient amount of heat. It does not indicate temperature
per se but the effects of temperature over a period of time. Cones react very similarly to the ceramic
ware, since they are similar in composition. They deform as a result of the influence of heat, time, and
kiln atmosphere, revealing what is happening in the kiln and when the proper firing temperature has
been reached.
A whole series of cones is available (see cone number - temperatures chart), made to melt at
different temperatures. Cones are available in two sizes. The large (senior or witness) cones may be
used at any location in the kiln to check temperature uniformity and firing progress. Small (junior)
cones are used specifically in kilns equipped with Kiln Sitter
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mechanical shutoff devices.
It is a good idea to make use of witness cones during firing, especially if exact temperatures are
critical. They are helpful both during the firing, allowing you to check on how the firing is progressing
and enabling you to turn the kiln off should maturity be reached before the kiln shuts off, and after the
firing, when you can make notes of the results to use in adjusting the programming and increasing the
accuracy of future firings. Witness cones also monitor the accuracy of the thermocouple, which may
degrade over time and eventually need replacing.
Always use a large cone of the number corresponding to the maximum desired temperature
behind each peephole during every firing in order to monitor firing progress and check the operation
of the control. Place the cone 2" to 3" behind the peephole to avoid cooling drafts and obtain an ac-
curate indication. Do not unplug the lower peepholes for more than a few seconds to avoid creating a
convection draft which can cool the cones and shock the ware. Do not place the cone too close to the
elements. Be sure to position it so that you will be able to see the tip when it bends and so that it will
not contact any ware when it bends. At high temperatures it is difficult to see the cones; using dark