|Monthly Tech-Tip |
This porcelain plate has inch-thick walls, it weighs more than 30 lbs. The weight is part of the aesthetic appeal of the pieces. However the artist was losing almost all pieces when refiring was needed. In a typical loss the crack was like this, with the force of the fracture blowing the two halves apart. This is the earmark of a quartz inversion failure, it could be happening on the heatup or cooldown. The solution is a controlled firing curve that greatly slows both upward and downward ramps so that heat gradients are not carried into the 1000-1200F danger zone.
This bowl, although only 33cm (13in) in diameter weighs 6kg (13lb). The walls are an inch thick. It would very likely not be possible to fire this up and down through quartz inversion sitting on a kiln shelf, almost certainly a significant temperature gradient would develop between the outer walls and the base. And when a gradient enters into the zone of quartz inversion, especially on the way down, it is trouble. I have already made the situation better by making the wall and base thicknesses as even as possible. This refractory setter was the next step. I made it using the L4543C recipe, it is wonderfully plastic and it was easy to throw on the potters wheel. This setter enables radiant heat and airflow (what little there is in an electric kiln) access to the base, reducing thermal differences between the walls and the center. An additional benefit: It is so refractory that cone 6 does not even touch it, the platform stays absolutely flat, thus the bowl stays round.
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.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down a temperature window centering on 573C.