Q's & A's about glass

Second guessing the temperature indicator

There have been incidents where kiln operators have altered, or disregarded, the readings of instruments to conform to their expected behaviour of a particular glass.

This can be called the “I expect my glass to do something at a particular temperature and if the temperature indicator reads different; then the instrument is wrong” syndrome.

This syndrome is not all that rare. For example, the American Kiln builder Skutt, who claims to be advised by leading glass artists, includes instructions in their literature for offsetting of instrument readings if (the operator believes) the readings are predictably and consistently incorrect.

A digital temperature indicator senses a voltage from a thermocouple and translates that to a temperature value. 

The most modern indicators do this with precision; with accuracy generally within ±2° C. right across the scale. They are designed to eliminate the earlier problem of an instrument reading correctly at room temperature and significantly in error at higher readings.

For more, see “How hot is that” and other info in Advanced kilnforming.

Pyrometric cones are clay shapes of precise dimensions and composition, designed to slump and “kiss the batt” at a specified temperature when heated at a specified rate of temperature rise.

Varying that rate of rise will change the temperature at which they respond.

• Too slow a rate will allow more time for the heat to penetrate and soften the inside; so they drop at a lower temperature.
• Too fast a rate will allow less time; so they drop at a higher temperature.

To keep this short, here’s some points of guidance;

• Varying the rate of temperature change can vary the temperature at which a particular change takes place in ceramic (pottery and glass) bodies.

• Varying the thickness of the body can have a similar effect.
• Varying the position of the body in a kiln, or changing kilns, can also have similar effect.
• Temperature indicators show the temperature only at the end of the probe, nowhere else.

If there are valid reasons for suspecting the temperature reading of an indicator, have the devise checked against a known standard, not against the behaviour of a piece of glass.

Can I get a kiln suitable for both pottery and glass?

Most kilns will fire both, except that they will generally be designed to do one job better than the other.

The two operations have different temperature and power requirements. Lets have a look at them;

• Pottery. To 1300 degrees C. High temperature demands low element load and this dictates slow rate of heating. Load is dense so can be tightly packed. Matters little if stacked high in  shelves. Kiln can be tall and narrow.
• Glass, to about 900 degrees C. Lower temperature allows higher element loading. Fast heating preferred to avoid devit.  Load is light, shallow, sensitive to minor temperature differences, so best done at the one level; on a hearth or a single shelf. 

Most kiln components, including the element wire, are suitable for use at pottery temperatures. The major differences are the shape of the kiln and the top temperature for which the element is designed. 

The higher the peak operating temperature the lower the amount of power which can be dissipated from a given length of element coil. More power can be packed into a given length of element in a glass kiln than in a pottery kiln.

Glass can be fired in kilns designed for pottery temperatures, but the heating rate will be slow and the narrow width will limit the size of pieces which can be fired. In tall kilns, firing on one shelf means that space and energy will be wasted.

The elements in glass kilns will be overlaoded if fired to pottery temperatures and their life will be shortened. They won't fail immediately, but they won't last as long as if they had no been subject to temperatures for whivh they weren't designed.

As ever, the choice is yours.