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how fast does your kiln cool - rr

updated mon 19 nov 01


Bob Hamm on sun 18 nov 01


If I understand the experiment you are proposing, you would like to know =
how different switch settings affect the temperature decline of a firing =
and ultimately the effect it would have on the firing. I once collected =
data that seems to be similar to what you propose. Unfortunately I think =
I threw out the records because I decided they did not have any value =
for me. I think do have all the records for the last 3 years or so. The =
following is what I did and my conclusions.

I have been firing electric kilns for 26 years, most of that as a =
production potter. I have been manipulating the switches during the =
cooling cycle for 25 years and kept records for each firing. I have 4 =
kilns. They range from 3/4 cu. foot to 15 cu. foot. All are electric and =
home made. The 2 larger kilns have 10 elements each with an infinite =
heat switch for each element. Both of these kilns have 4 1/2 in. ifb =
walls and lids with 5 in. ifb bottoms. These are the 2 kilns I used for =
the following experiment. I was firing to cone 8 at the time, and now =
fire to cone 6.

I recorded temperatures going up and down to about 850C and all switch =
changes going up and down. For a short time in the mid 80s I did keep =
track of the number of posts, the number and types of shelves and the =
finished weight of pots in each load. I wanted to see if the density of =
the load had any effect on the length the firing and the glaze results.=20

My conclusions were that only one factor was important in reproducing a =
reliable glaze result and that was the rate of temperature rise and more =
importantly its fall. In the end I decided the best thing I could do was =
watch my pyrometer to track the rate of temperature rise and fall and =
use cones for the final determination of reaching temperature. Using a =
standard switch settings schedule just was not reliable. Switch settings =
from one load to another are generally similar but can change. A dense =
load will take longer to fire and will cool more slowly on its own so =
the switch settings will be lower during the cooling cycle. A low =
density load is the opposite with higher switch settings during the =
cooling cycle. Other factors that effect the temperature and switch =
settings are the air temperature in the room, the age of the elements, =
and whether some elements or switches have burnt out during the firing.=20

If I got lazy and tried to fire using experience ( and I had a lot of =
experience with these kilns and glazes ) instead of babysitting the =
kiln, quality suffered. I like to fire slowly. 50C max. temperature rise =
for the last 150C, and often only a 25C rise that can stall or even drop =
before all the cones are down. On the cooling side, 6 to 7 hrs. back =
down to 850C with at least 4 hours between 1050C and 850C.

These slow firings and cooling cycles can have a dramatic effect on the =
glaze. Some of the glazes I use would be gloss if cooled fast but are a =
nice soft matte when cooled slowly. The tactile qualities of the glaze =
surface can improve and the glaze can look warmer and have more of a =
reduction look to it. More then one potter has picked up one of my pots =
thinking it was fired in reduction. The results vary with the materials =
and how they are used and fired.=20

I hope this helps.

Bob Hamm

Ph (250) 765-8876 Fax (250) 765-0497
email web site

6750 Highway 33 East=20
Kelowna, B.C. Canada V1P 1H9

For information about the Kelowna Clay Festival 2001, go to =

----- Original Message -----=20
From: Ron Roy=20
Sent: Sunday, November 18, 2001 10:26 AM
Subject: Re: How fast does your kiln cool

This subject is an excellent opportunity for some experimentation - =
wish I
had the time but no way these days.

What is needed.

A working pyrometer - calibrated with pyrometric cones for top =
- or a controller - calibrated the same way.

A kiln loaded with shelves and posts to simulate a standard load - =
will depend on the size if the kiln. This can be determined by =
weighing the
pots, shelves and posts from a normal firing and duplicating that =
with just shelves and posts.

A kiln with 3 switches with high, medium and low settings.

After temperature is reached - but keeping the kiln turned on - start =
turning individual switches down for each firing as follows.

Control firing - the actual rates of cooling temperature (all switches =
at top temp) of a typical firing the one you base the weight of ware =
shelves for the experimental firings.

#1 All switches down to medium.
#2 All switches turned down to low
#3 If kiln fires cooler on bottom - bottom switch on medium - top two =
on low
#4 If Kiln normally is cooler at top - bottom two switches on low - =
top on

There are other combinations one on high two on medium, one on high =
two on
low but perhaps not necessary - WARNING - the kiln must be monitored
carefully at the start of the cool down to make sure it is actually
cooling. Best to have two witness cone packs visible with a higher =
cone to make sure heat is not increasing in different parts of the =

As each test firing cools - the indicated temperature should be noted =
lets say every half hour down to 700C (1300F) - best to use a timer =
stay with the kiln - or take the timer with you. - if you leave the =

Anyone willing to do this - feel free to contact me directly if I have =
included information you need. This kind of information would be an
excellent basis for an article for potters.

Information on type of kiln, size, amount of insulation and age of =
will make the information more useful.

I would also think that having a series of glazes - on test tiles - =
glazes - different firings - would make these experiments sensational.