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3 soda questions

updated tue 17 aug 99


Peter Atwood on mon 16 aug 99

Hi Folks,

My friend Jackie does not have internet access so she asked me to post a
couple of questions concerning her soda firing last weekend. In the firing
were several tests of different porcelain bodies as well as numerous glaze
tests. This was the first soda firing that she has done in her new Olympic
Torchbearer updraft kiln. She chose to apply a mixture of 70% Tile 6 and 30%
Alumina Hydrate to the shelves instead of using wadding and she sprayed a
total of 2 lbs of soda ash mixed with one gallon of water into the kiln. The
pieces all came off the shelves easily-no sticking. The kiln was fired to
^9 in 10 hours with a cooling period of 24 hours.

Overall I would say that the results were great with nice orange peel
texture and vivid glaze colors. Her underglazes were gorgeous and there were
a lot of promising combinations. A few problems plagued her however...

1) Stress cracks in the bottoms of some of the porcelain cylinders. The
Miller porcelain and the Sheffield Grolleg both developed tiny stress cracks
in the bottoms. The cracks are not s cracks and she definitely compressed
when throwing etc, and they were not there when she bisqued. The other two
bodies that she used, Sheffield T23A and Sheffield 92700 both tile 6
porcelains were fine and exhibited no cracks. Any ideas why this could

2) The lid of this top loader stuck a little bit and she lost some of the
edges of the brick where the soda had bound the soft brick together. The
kiln was coated with ITC100. Can she still apply a wash of some sort over
the ITC for the next firing? SHould she make a gasket out of wadding to put
around the edges of the lid to prevent sticking?

3) Although she got a nice orange peel effect, I noticed that there was very
little of the orange flashing that I always associate with soda. I have
applied soda ash by itself to white stoneware in a regular reductuion firing
and got the orange but in this firing the porcelain was pure white under the
clear. Why is that? There was a little bit of flashing evident on a couple
of the stoneware pieces that were in the kiln but none of that deep color.
Jackie says that to get the flashing you would have to apply a flashing slip
but it seems to me that some orange should have occurred. I guess I'm asking
what makes the orange appear. Is it resistance to the soda or a combination
with trace amounts of iron in the body or what?

Any thoughts on this stuff would be much appreciated.

--Peter Atwood

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