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NotesOnProgrammingElectricKilns

Notes on Programming Electric Kilns

The Bisque

• Needed dryout times vary based on the following factors
  • Thickness of the greenware
  • Wetness of the greenware
  • porousity of the clay
  • how well the water moves through the clay
  • the strenght of the greenware
  • how much water there is to evaporate and the ventilation of the kiln or the relative humidity in the kiln.
  • The temperature the dryout happens at.

Because of hydroscopic chemicals and structures, clay is never dried completely at room temperature. Water retained in the clay if quickly turned to steam will blow up the work. Each spoonful of water makes 1600 spoonfulls of steam. The dried mud cannot hold the pressure this can create so it can blow up. Created slowly it can escape through the pores before pressure builds. The more pores, the sooner it escapes and the faster the temperature can rise without trouble. The biggest troubles are from water deep in the clay that becomes steam. Water vapor on the surface needs to travel little distance to escape.

People often dry work out at temperatures near the boiling point of clay. This is good practice. But it has some added benefits. Water at higher temperatures looses much of its surface tension and viscosity allowing it, even within the clay to spread out. The more surface it has exposed to air, the faster the drying. If you are drying out work wetter than leather hard, this lower surface tension, speeds the movement of water throughout the clay helping to keep it an even wetness. This helps prevents cracks from differential drying. The problem is that wet ware appears to need the surface tension for the wet strength of the clay. Wet handles and spouts and more fragile forms can sag from drying near the boiling point of water.