Controlled cooling

It is sometimes stated that you can simply turn the kiln off below 370C and let the kiln’s natural rate of cooling take over the cool down.

This works for most flat 6mm pieces in most kilns, but as you work thicker or with greater contrasts in thickness, lots of tack fused elements or in a small rapidly cooling kiln, you do need to control the cooling toward room temperature.

The first thing you need to know is the natural cooling rate of your kiln.  

The rate of cool is not just about the annealing soak. The soak at annealing temperature is to equalise the temperature throughout the blass to have a differential of not more than 5C. 

The rate of cool is about avoiding thermal shock, too. The glass needs to maintain the temperature variation to less than 5 degrees Celsius difference throughout the glass as it cools.  This requires a slow controlled cool.  

You may program a cool of 100C to 370C thinking that the kiln will maintain that rate or less.  If the natural cooling rate of your kiln at 370C is 200C/hour, you risk thermal shock due to the rapid increase in the cooling rate.

You really do need to know the natural cooling rate of the kiln from the point you turn the programmer off to room temperature to be safe from thermal shock.

The alternative to turning off at 370C is to program the schedule all the way to room temperature.  The kiln will use no energy unless the kiln cools too quickly on its own.  At which point the program will kick in to slow the cooling of the kiln.