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 ¾” from the elements to
avoid uneven heating and distortion of the piece.
Use high fire kiln wash. Some prefer to load stoneware onto surfaces sprinkled with silicaflour,
sometimes called “flint.” Keep “flint” away from elements.
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.
Methods of Temperature Indication
Pyrometric Cones
There are two basic devices for monitoring the temperature inside the kiln. The first, most
essential and most widely used method is the pyrometric cone. A cone is a small elongated
pyramid shape 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.
There is a whole series of cones available (see cone number – temperatures chart, page 15-4),
made to melt at different temperatures. Cones are available in two sizes. The large (witness)
cones may be used at any location in the kiln to check temperature uniformity and firing progress
while the small cones or Orton bar cones are designed for use in the Kiln Sitter
® (shutoff).
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 Kiln Sitter
®. Place the cone 2” to 3” behind the peephole to avoid cooling
drafts and obtain and accurate indication. Do not place it to close to the elements. Be sure to
position it so that you will be able to see tip when it bends. It is important to always stand
pyrometric cones at the pre-cut angle provided by the base of each cone; this should be
approximately 8 degrees away from vertical in the direction the cone is expected to bend.
Setting the cones consistently at this angle assures that each cone (of the same number) will
bend at uniformly the same temperature. Cone plaques or wire cone holders are
commercially available. Holders may also be made from brick or clay. Holders made from
fire brick are generally reusable and trouble free.
To Next Page
Loading...