Finding Your Kiln’s Natural Cooling Rate

You need to observe how your kiln behaves while cooling without any power to be sure when you can safely &turn it off and let it cool without power.

Assuming you have programmed your kiln for a shut off at 370C, you need to observe every quarter hour or so to record both time and temperature.  From those observations you can calculate the cooling rate at the various temperatures.

Say at 6:00 your kiln was at 370C;

At 6:15 it was at 310C;

At 6:30 it was at 265C;

At 6:45 it was at 230C;

At 7:00 it was at 200C;

At 7:30 (you missed the quarter hour) it was at 160C;

At 8:00 it was at 140C;

At 9:00 it was at 125C;

At 10:30 it was at 110C.

To calculate the rate, you divide the temperature difference by the proportion of an hour between observations, as demonstrated in the following table.

Kiln Name/Description
Size
Shelf composition
Amount of glass
Observations
	Time	Temperature	minutes	Proportion	temperature	Rate of
1st	06:00:00	370	difference	of an hr	difference	cooling
2nd	06:15:00	310	15	0.25	60	240
3rd	06:30:00	265	15	0.25	45	180
4th	06:45:00	230	15	0.25	35	140
5th	07:00:00	200	15	0.25	30	120
6th	07:30:00	160	30	0.50	40	80
7th	08:00:00	140	30	0.50	20	40
8th	09:00:00	125	60	1.00	15	15
9th	10:30:00	110	90	1.50	15	10

Although this is an example, it shows how the cooling rate slows down as the kiln cools. 

If you were cooling a flat piece 12mm thick, you might get away with turning the kiln off at 370C, as a flat piece can cool as quickly as 300C/hr.

If you were cooling a piece 19mm thick, the natural cooling rate of the above kiln is too fast. 19 mm thick pieces need a cooling rate of 150C/hr, so according to the figures above you need to programme this kiln down to 230C to get the appropriate final cooling rate.

If it is a tack fused piece with a 6mm base and areas of two layers of tack fusing, you should fire as though it is 24mm thick.  In this case, the final cooling rate needs to be 90C/hr.  For the kiln in the example above, that rate is not achieved until below 160C, so that is the minimum temperature for switch off.

This method can be used for any temperature range.  For example, you may want to know the rate of cooling from the top temperature to the annealing temperature.  This method will work there too. You may want to record the temperatures more frequently than every quarter of an hour though.

Further information is available in the ebook Low Temperature Kiln Forming.

You really need to know your kiln’s natural cooling rate before you can be confident of switching the kiln off at 370C.  This blog shows a method of determining the natural rate of cooling.