22 inch slabs successfully fired after a change. What?

Giant thin meter-square tiles are fired flat and crack-free by tile companies. How? Kilns that heat evenly from above and below (the tiles are on rollers). But these round tiles are being fired in an electric kiln, a device incapable of heating a large slab evenly. They are so large they reach almost to the outer walls. That means the outer edges receive direct radiant heat from the elements, this sets up a temperature gradient running from the edges toward the center. Passing such a piece up and down through quartz inversion thus creates a wave of sudden expansion and contraction moving through the piece. The artist was losing every one of these to dunting. It is not really advisable to even try this - but he was determined to do it anyway. One change in the process brought this one through: Slowing down to 50F/hr up and down through the quartz inversion (950-1150F).

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Drying and firing a 31-inch porcelain plate without cracks. How?

31-inch porcelain plate successfully fired

It takes extreme care. Three months of it! Porcelains are fine-grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a piece like this down through quartz inversion (~1200-900F) without dunting is only possible if done very slowly. The fact that ancient Chinese potters made very large porcelain pieces means they knew about slow cooling also (and it was a natural consequence of the heavy kilns they used). But our modern kilns cool quickly so the drop must be slowed. Peter adds an extra level of "super humanness" by actually lustre-firing these pieces, which means 2 more trips through the hazardous quartz inversion territory! If you do this be prepared to fire super slow (e.g. 25 degrees per hour) through this range.

Here is what it can take to fire heavy pieces in an electric periodic kiln

This eight-step firing to cone 6 took 33 hours, that is our conservative effort to assure this test bowl with inch-thick walls and base would not crack. It is true that the speed of firing is not the determining factor, the key is the avoidance of uneven distribution of temperature across the piece at all stages of firing, especially near quartz inversion. But achieving that in an electric kiln having no airflow and radiant heat is tricky. Two things helped assure success: The custom setter on which this was fired and the °/hr being controlled both upward and downward. To be even safer I slowed to 75°F/hr between 950 and 1150°F. A 30-minute hold at the beginning and end of each of these slow-downs added further insurance against any temperature gradients developing in the piece.

Links

Glossary	Ceramic Tile Tile manufacture is the largest sector of ceramic industry. Engineers overcome the very difficult technical challenges of drying and firing defect-free, flat and durable tile. Potters can do it too.
Articles	Electric Hobby Kilns: What You Need to Know Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, being aware of the limitations and keeping them in good repair is very important.
Troubles	Dunting and Cracking of Clay Bodies During Firing Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.

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