As Fast as Possible Firings

I have long advocated that it is best to avoid as fast as possible firings because the way controllers work.  They compare the temperatures several times a minute (the number depending on the manufacturer) to determine the rate of increase.  This allows big overshoots at the top temperature with fast rises.  This was reinforced this morning by observing a different factor.
 
I took a piece out at 68°C to put another in.  During the time the kiln was open, the air temperature dropped to 21°C.  I filled the kiln and closed the lid and idly watched the temperature climb before switching the kiln on for another firing.  It took a bit more than two minutes for the thermocouple to reach 54°C with the eventual stable temperature being 58°C.  I had not been aware how long it takes the thermocouple to react to the change in temperature.  Yes, it takes a little time for the air temperature in the kiln to equalise with the mass of the kiln, but not two minutes.
 
With a two-minute delay the recorded temperature can be significantly behind the actual air temperature.  For example, a rate of 500°C per hour is equal to 8.3°C (15°F) per minute or 16.6°C (30°F) overshoot of the programmed temperature. Even at 300°C it is a 10°C (18°F) overshoot.  This effect, added to the way the controller samples the temperatures, means the actual overshoot can be significant for the resulting glass appearance.
 

This is just another small element in why moderate ramp rates can be helpful in providing consistent results for the glass.