The Infinite Control Switch
Firing Flat Clay Slabs, by Vince Pitelka
THE INFINITE CONTROL SWITCH
The infinite control switch makes a clicking noise during firing. This is the sound of a bimetallic timer turning the elements on or off to control the firing rate.
When the switch is about to fail, it sometimes makes a faint popping instead of a clicking noise. This is a sign to order a new switch.
Other than normal wear, here are reasons that an infinite switch fails:
1) New elements are higher amperage than the kiln was designed for.
2) Loose push-on connectors over-heat the switch by creating a tiny electrical arc.
3) The kiln’s heat shield missing. (This is a sheet metal channel that helps guide heat away from the switch box. The heat shield is under the porcelain insulators of most kilns.)
4) A 120-volt switch is installed on a 240-volt kiln. Both switches look the same; the voltage is printed on the switch body.
5) There is too much heat in the kiln room. Switches burn up more frequently on hot summer days. In this case, use a fan to circulate air through the kiln switch box. But do not blow air against the outside of the firing chamber.
When you change a switch, remove and transfer only one wire at a time to the new switch. Be sure the push-on connectors are tight. Replace heat-damaged wires. Bend a wire to test it. If it sounds like it is cracking, replace it, because the insulation is flaking.
FIRING FLAT CLAY SLABS
By Vince Pitelka of Smithville, Tennessee (author of “Clay: A Studio Handbook”)
The shelf is much thicker than the ware, and insulates the ware from heating and from cooling. So, if you are firing a large slab piece flat on the shelf, it heats up around the outside faster than in the center, and the difference can be extreme. And it cools off faster around the outside while still remaining hot in the center. Depending on the type of clay body and the rate of heating/cooling, this can be a serious issue and can cause flat slab pieces to crack, either across the center, or from the edge towards the center.
If you elevate the clay piece off the shelf surface with wadding, the heat and atmosphere can circulate around the piece, causing it to heat and cool much more evenly. The same is true of a large bowl or platter--the rim will heat up faster than the center, and cool off faster. So, cracking can be a serious issue there as well, again depending on the clay body and the firing/cooling rate.
The safest thing to do with large slab pieces, bowls, and platters is to fire them on wadding. With flat slab pieces I use parallel rows of wadding rolled as coils. For large bowls and platters I use a sunburst pattern of coils radiating out from the center.
Thank you, Vince, for the excellent pointer.
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With best wishes,
Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 firstname.lastname@example.org / www.paragonweb.com