Bob Hamm on sun 18 nov 01
Ron,
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 bobhamm@look.ca web site www.bobhamm-art.com=20
6750 Highway 33 East=20
Kelowna, B.C. Canada V1P 1H9
For information about the Kelowna Clay Festival 2001, go to =
www.bobhamm-art.com/clayfest=20
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----- Original Message -----=20
From: Ron Roy=20
To: CLAYART@LSV.CERAMICS.ORG=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 =
temperature
- or a controller - calibrated the same way.
A kiln loaded with shelves and posts to simulate a standard load - =
which
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 =
weight
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 =
by
turning individual switches down for each firing as follows.
Control firing - the actual rates of cooling temperature (all switches =
off
at top temp) of a typical firing the one you base the weight of ware =
and
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
medium.
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 =
guard
cone to make sure heat is not increasing in different parts of the =
kiln.
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 =
and
stay with the kiln - or take the timer with you. - if you leave the =
room.
Anyone willing to do this - feel free to contact me directly if I have =
not
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 =
kiln
will make the information more useful.
I would also think that having a series of glazes - on test tiles - =
same
glazes - different firings - would make these experiments sensational.
RR
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