MELANIE SWANN on fri 13 dec 96
I've been lurking here on the list and sharing all your wonderful
knowledge with my high school ceramic students. Thank you. Now, I
have a question; I'm hoping someone can help.
The pots from our last sagger firing came out with large blisters.
Any ideas on the cause? Some of the blisters were as large as 3 cm in
diameter and extended from the surface of the form 1.5 cm. I broke a
few, and the clay forming the "skin" of the blister was 2-3 mm thick.
Half of the forms were made with a buff, cone 10 clay from Laguna
clay company, called Rodsbod; the other forms were constructed from a
white, cone 5 clay, WS-5. The pots were sprayed lightly with cobalt
and copper oxides after a cone 04 bisque. The sagger was packed
with saw dust from the school's woodshop. A small amount of soda ash
(about 1/4 a cup) was placed in amoung the saw dust in a 12 cubic
foot gas kiln. The interrior of the sagger was fired to 1600 degrees
F. and the interrior of the kiln fired to 2200 degrees over a six and
a half hour period.
A friend suggested that the problem was too heavy a reduction.
But the sagger packing and firing wasn't different from past firings,
and this is a new problem for us. My past professor, Dorthy Bearnson,
at the Univ. of Utah, suggested that plastics from the woodshop may
have been in the saw dust and caused the problem. However, the
woodshop teacher has no knowledge of plastics being used yet this
year in the shop.
The students were discouraged with the firing and are now
crunched for time for student shows and competitions and can not
afford another firing like this one. If you have any ideas, we would
be very grateful.
Melanie Swann
Woods Cross, Utah
Christopher L. Weeks on sat 14 dec 96
On Fri, 13 Dec 1996, MELANIE SWANN wrote:
> The pots from our last sagger firing came out with large blisters.
> Any ideas on the cause? Some of the blisters were as large as 3 cm in
> diameter and extended from the surface of the form 1.5 cm. I broke a
> few, and the clay forming the "skin" of the blister was 2-3 mm thick.
I once opened a sagger and found my two pieces covered with these
beautiful reddish blisters. My blisters maxxed out at about 1.5cm
diameter. I was much more pleased with my results than you/your students
seem to be. I've spent some time since then trying to replicate the
result, but sagger's just don't work that way for me.
The things that I did differently on the firing that resulted in my
blisters were: I added more salt than usuall, I sprinkled in some metals
with the salt and wood, I went hotter than usuall (maybe cone 7).
I am absolutely certain that there was no plastic in my kiln.
I'm not sure if this is of any help, but it's my experience...
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June Perry on sat 14 dec 96
One possible explanation could be that the sawdust had a high moisture
content which would create more of a reducing atmosphere.Were the pots
thoroughly dried after the application of the oxides?
Regards,
June
Carol Ratliff.clayart.CLAYART.MAILING LIST on sat 14 dec 96
I have used both of those clay bodies in saggar before and had no problems
like you described. I haven't ever used soda ash in the firing though. Do
you do this regularly? Are the students having any problems with these clays
in other firings..... occasionally it is a bad batch of clay.
carol in san diego
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