Stuart Ridgway on fri 19 jun 98
On drying pots before fireing.
The pottery books tell the student to give his pots a
substantial amount of time drying before fireing. One test was
to hold the pot to the cheek, and if it was cool there was still
water to be evaporated. But how long? And how accomplished is
my temperature sensing cheek?
After I had lost many pots by explosion in the kiln
that had seemed to be quite dry before firing I decided to try
the chemists scheme of drying to constant weight. A pot was
thrown of Laguna B-mix cone 5. Once the pot was dry enough to
move from the wheel it was placed in an unheated slightly closed
garage, and weighed periodically as natural drying took place.
Relative humidity varied between 30% and 50%, and temperatures
varied from about 75 F in the day to 60 F at night. Careful
measurements were made before and after each trimming operation
so that weight of clay removed was not counted in the water loss.
The final dry weight of the pot turned out to be about 1600
grams. The pot was 7 inches tall, 4.5 inches diameter, with a 8
mm thick wall. The bottom was a little thicker.
The water loss rate was approximately constant for the first
three days at about 3 grams/hour. It then began to decline, and
at the end of a week it was about a twentieth of that, about 0.15
grams/hour. Was all the water out? No, it was not!
The pot was placed in a 200 deg F electric oven and the
weight followed. The loss rate became 10 grams/hour.
Drying was not fully complete in 6 further hours but finally
seemed to be slowing down. About 50 grams more water were driven
out by the 200-250 deg F oven treatment, which was 3% of the
final dry weight. It was then successfully fired.
Drying is slow. If the pot is not fully dry when kiln
fireing starts, it better have many hours in the warmup stage
before the kiln temperature rises above the boiling point of
water, or 210 deg F or so.
Since then I have baked dried pots at 200 degrees F for at least 6
hours before kiln fireing, and no longer have had kiln explosions.
Stuart Ridgway
Ron Roy on mon 22 jun 98
Hi Staurt,
Just read this - thought it is an excellent post - I was surprised at how
long it took but then realized - some clays take longer to dry of course.
The BMix must be very fine and not much material that provides channels
out. This explains some of the drying problems some have with fine textured
clays.
One of the sessions at Cermatech (Clays and Clay Bodies by Robert Hawkins)
addressed this in a way I had never heard before. He explained that some
clays can get "case hardened" - the surface pores can close up from fast
drying and make it more difficult for the inside water to get out. The cure
is to slow down the drying so that the ware dries more evenly - from the
inside.
Clays with more granular and less plastic content suffer less from this problem.
>----------------------------Original message----------------------------
>On drying pots before fireing.
>
> The pottery books tell the student to give his pots a
>substantial amount of time drying before fireing. One test was
>to hold the pot to the cheek, and if it was cool there was still
>water to be evaporated. But how long? And how accomplished is
>my temperature sensing cheek?
>
> After I had lost many pots by explosion in the kiln
>that had seemed to be quite dry before firing I decided to try
>the chemists scheme of drying to constant weight. A pot was
>thrown of Laguna B-mix cone 5. Once the pot was dry enough to
>move from the wheel it was placed in an unheated slightly closed
>garage, and weighed periodically as natural drying took place.
>Relative humidity varied between 30% and 50%, and temperatures
>varied from about 75 F in the day to 60 F at night. Careful
>measurements were made before and after each trimming operation
>so that weight of clay removed was not counted in the water loss.
>The final dry weight of the pot turned out to be about 1600
>grams. The pot was 7 inches tall, 4.5 inches diameter, with a 8
>mm thick wall. The bottom was a little thicker.
>
> The water loss rate was approximately constant for the first
>three days at about 3 grams/hour. It then began to decline, and
>at the end of a week it was about a twentieth of that, about 0.15
>grams/hour. Was all the water out? No, it was not!
>
> The pot was placed in a 200 deg F electric oven and the
>weight followed. The loss rate became 10 grams/hour.
>
>
> Drying was not fully complete in 6 further hours but finally
>seemed to be slowing down. About 50 grams more water were driven
>out by the 200-250 deg F oven treatment, which was 3% of the
>final dry weight. It was then successfully fired.
>
> Drying is slow. If the pot is not fully dry when kiln
>fireing starts, it better have many hours in the warmup stage
>before the kiln temperature rises above the boiling point of
>water, or 210 deg F or so.
>
> Since then I have baked dried pots at 200 degrees F for at least 6
>hours before kiln fireing, and no longer have had kiln explosions.
>
>
>Stuart Ridgway
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
Christopher J. Anton on mon 22 jun 98
I, too, have found that (at least for some objects) checking water loss by
weighing can be quite effective. I am currently handbuilding solid spheres
ranging from two to six inches in diameter. I have had minor cracks from
raku firing, but have had no explosions either in bisque or in final firing.
- Chris
Ray Carlton on mon 22 jun 98
mmm the second post tonite on this subject. the moisture content of the pot
is irrelevant. The speed at which the temperature advances in the kiln is
not..To avoid blowing up your pots take it very easy with the region form 0
deg to 200 deg centigrade. leave the kiln on very low all night....in the
morning it will be close to 200 degrees...... you can check for moisture
content by holding a piece of glass up to the spy hole. A moisture fog will
appear on the glass.... If it clears within 10 - 15 seconds you can begin
to raise the temp.......if it does not clear, leave the kiln for a bit
longer ...check it at 1 hour intervals
good luck
At 09:20 19/06/98 EDT, you wrote:
>----------------------------Original message----------------------------
>
>On drying pots before fireing.
>
> The pottery books tell the student to give his pots a
>substantial amount of time drying before fireing. One test was
>to hold the pot to the cheek, and if it was cool there was still
>water to be evaporated. But how long? And how accomplished is
>my temperature sensing cheek?
>
> After I had lost many pots by explosion in the kiln
>that had seemed to be quite dry before firing I decided to try
>the chemists scheme of drying to constant weight. A pot was
>thrown of Laguna B-mix cone 5. Once the pot was dry enough to
>move from the wheel it was placed in an unheated slightly closed
>garage, and weighed periodically as natural drying took place.
>Relative humidity varied between 30% and 50%, and temperatures
>varied from about 75 F in the day to 60 F at night. Careful
>measurements were made before and after each trimming operation
>so that weight of clay removed was not counted in the water loss.
>The final dry weight of the pot turned out to be about 1600
>grams. The pot was 7 inches tall, 4.5 inches diameter, with a 8
>mm thick wall. The bottom was a little thicker.
>
> The water loss rate was approximately constant for the first
>three days at about 3 grams/hour. It then began to decline, and
>at the end of a week it was about a twentieth of that, about 0.15
>grams/hour. Was all the water out? No, it was not!
>
> The pot was placed in a 200 deg F electric oven and the
>weight followed. The loss rate became 10 grams/hour.
>
>
> Drying was not fully complete in 6 further hours but finally
>seemed to be slowing down. About 50 grams more water were driven
>out by the 200-250 deg F oven treatment, which was 3% of the
>final dry weight. It was then successfully fired.
>
> Drying is slow. If the pot is not fully dry when kiln
>fireing starts, it better have many hours in the warmup stage
>before the kiln temperature rises above the boiling point of
>water, or 210 deg F or so.
>
> Since then I have baked dried pots at 200 degrees F for at least 6
>hours before kiln fireing, and no longer have had kiln explosions.
>
>
>Stuart Ridgway
>
cheers :) Ray Carlton
McMahons Creek Victoria Australia
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