Ron Roy on mon 8 jun 98
I find soaking improves most glazes but there are exceptions. I also think
many glazes can be adjusted so that they can take a soak. There are glazes
which are best not soaked - but don't use any of those.
All kilns fire differently so the first thing to find out is how your kiln
fires. Ist step is to do a normal firing and have LARGE cones throughout
your kiln, on the floor, on the next shelf, in the middle, at the top spy
and near the top - keep them away from the elements so you will get a
better picture.
After you find out where the cooler zones are you can then decide the way
to soak in order to even out your firings.
There are three different kinds of soaking - soaking should be designed to
eliminate uneven temperature gradients in a kiln and solve different glaze
problems.
At CerMatech one of the sessions was on this subject - what happens when
clay is fired. One area stressed was giving the ware a chance to even out -
especially at the end temperature.
The first type of soaking is a slowing down before the top temperature is
reached - good for more even firings and giving glazes a chance to settle
down.
Another type is trying to hold at the top temp - difficult without a
pyrometer and a controller.
The third type is a controlled slower cooling - in some cases the best -
depending on the problem.
I suppose all three could be used - a cinch if you have a controller that
is versatile enough.
I think slow firings are best - up to a point - because they eliminate more
problems. I find that many of the problems I can't solve by changing clays
or glazes are with those potters who insist on firing fast.
I also noticed some comments about some ramps built into controllers not
matching cone bends - naturally - controller use a pyrometer to sample
temperature in the kiln - they do not measure the time - remember we are
always dealing with heat work which is not only temperature but the time it
takes to get there. Speed of firing is affected by how tight or loose a
kiln is stacked - I always use my controller to get me there but I always
shut it off manually when I see the cone bending the right amount.
I always make my own ramps - that way I can change the final hold temp if
it does not match the cone bend. Easy to do - even during a firing - and I
have arrived at a number that will work every time - it goes into soak mode
just as the cone starts to bend - I watch it till it bends halfway down and
then shut off manually - piece of cake - especially if you fire slow - I
try to keep a 50 to 60C per hour throughout the whole firing cycle. There
are times when I think it is OK to go faster but too much bother for me
most of the time.
>----------------------------Original message----------------------------
>I have never tried to soak using my electric kiln. However, the recent
>discussions on this subject makes me wonder if I should't try. My question
>is how to adapt or adjust my current schedule to accommodate the soaking
>period.
>
>I have a 7 cubic foot paragon, top loading, three tier, electric kiln with
>an installed kiln sitter, and overhead exhaust fan. I don't have a
>programmable controller or pyrometer. I do use kiln sitter cones. My ^5-6
>firing schedule is very routine:
>
>1. First hour: lid propped, all spys open, all switches on low
>2. Next 3 1/2 hrs.: lid down, bottom spy in
>3. Next 3 1/2 to 4 hrs.: all switches on medium, middle spy in
>4. Finally 1 1/2 hrs+ or -: all switches on high
>5. Kiln sitter shuts off: top spy in
>
>Once the kiln sitter shuts off, what's next? Which switch(s) goes back on
>and for how long? Do I leave all the spys in during the soaking process?
>
>Does soaking generally improve glaze characteristics?
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Cameron Harman on tue 9 jun 98
Referring to Ron Roy's comments:
Although I agree with what Ron has said I would like to take it a
little further. Firstly, where I am coming from: I am a ceramic
engineer and have specialized in drying and firing for about 40
years so my opinions are based on a lot of practical experience
world wide.
To me soaking is ONLY holding the temperature level at some point
in the firing. It is not uncommon for people to soak at lower
temperatures in order to finish a process. For example burning out
of organic materials can take some time, so some operators soak
the kiln to complete the binder burnout at a temperature that
allows a slower, more controlled rate of oxidation. It is done at
other temperatures for other reasons as well.
When the rate of firing is adjusted so as not to reach the peak
temperature at too fast a rate, that is called common sense. It is
considered bad practice to fire a kiln at a high rate of
temperature increase then soak it. It is, however, acceptable to
fire a kiln a high rate of speed to a peak firing and then stop.
However, this should only be done in kilns with very good
temperature uniformity or with products having a wide range for
the end point of firing
When firing a fluid (low viscosity) glaze it is not unusual to
lower the temperature slightly after soak and let the kiln cool
very slowly until the glaze begins to get stiff. This action
preserves a good gloss. It is not practiced everywhere, but is
fairly common with German tunnel kilns.
I hope this helps everyone with a little better understanding.
Cameron Harman
--
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215-245-4040 fax 215-638-1812 visit us http://www.@kilnman.com
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