iandol on fri 10 may 02
Dear Heather,
I take it as a given that you have read the standard texts on salt =
glazing and the newer information about using soda and other sodium =
salts. If not I suggest Salt Glaze Ceramics by Jack Troy which covers =
the necessities of preparing, loading and firing your kiln. Add to that =
Salt Glaze by Peter Starkey. They provide a firm technical foundation =
for anyone wishing to be successful with these processes.
There may be several answers to each of your questions but I will take =
No 5 for starters. In spite of the rhetoric that both salt and soda =
create the glaze by reacting with free Silica, this is only true for =
Sodium Carbonate. Sodium Chloride reacts strongly with Potash felspar =
which is a major ingredient in Porcelain clays but Stoneware clays are =
may be deficient in this ingredient. Hence a minimal build up on your =
clay. Give the stoneware surface a few brushstrokes of potash felspar =
and make a comparison.
Many reasons why lids stick. Salt vapour may exaggerate the usual cause =
which is sintering. This is a solid state reaction. Small wad may work, =
so does adding alumina sand to your wax resist if you are incorporating =
a liner glaze but be sure to make certain no glaze adheres to the =
gallery or the seat on the lid.
Warping is due to the fired clay getting soft at high temperatures. =
Flanges can drop as well as the plate rocking. Some salt glazers =
incorporate a good proportion of refractory clay in their mixtures. This =
may mean adding more pure Kaolin or a good high alumina fireclay.
To get a good build up of salt or soda on your pots you need to cut the =
draft by pushing the damper home. Hold the vapours in the kiln for as =
long as possible, but reduce fuel imput while you do this.
Black crystals can be Magnetite if you are using an Iron rich clay. They =
are known as "Spangles" and can form on the surface of your pots.
Hope that helps.
Best regards,
Ivor Lewis. Redhill, South Australia.
vince pitelka on sat 11 may 02
Ivor wrote:
"There may be several answers to each of your questions but I will take No 5
for starters. In spite of the rhetoric that both salt and soda create the
glaze by reacting with free Silica, this is only true for Sodium Carbonate.
Sodium Chloride reacts strongly with Potash felspar which is a major
ingredient in Porcelain clays but Stoneware clays are may be deficient in
this ingredient. Hence a minimal build up on your clay. Give the stoneware
surface a few brushstrokes of potash felspar and make a comparison."
Ivor -
With all due respect for your expertise on salt firing, the above is
misleading. With or without feldspar present, the sodium in both salt and
soda firing attacks the silica in the clay, whether it is free silica or
combined silica to form the salt glaze. In addition to the higher feldspar
content, the even particle distribution in porcelain clays produces the
smooth even coverage of glaze with minimal "orange peel" surface. Stoneware
clays, with their broader distribution of particle size, give a more
variegated surface, often with considerable orange peel. Silica sand
drastically increases the tendency to orange peel, as the sodium attacks the
large silica particles.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
iandol on mon 13 may 02
Dear Vince,
I knew I would get a rise on that one!
So I'm misleading everyone! Well Vince, twenty years ago I would have =
agreed with you, but not now. When I first discovered this anomaly in =
1982 I was told I was wrong and that Daniel Rhodes was right. I checked =
the reference. It is interesting reading and I agree with what Prof =
Rhodes wrote. Now where does your information come from?
I suggest you are looking for a spontaneous chemical reaction at the =
firing temperature. This can be tested in a discriminatory experiment. =
The reaction between sodium chloride vapour and free silica does not =
become spontaneous below about 1700 deg Celsius. This can be confirmed =
by doing the calcs. However, the reaction between Sodium Carbonate and =
free silica is spontaneous at nominal salt glaze firing heats. P.J. =
Durrant reported this in respect of the behaviour of sodium carbonate in =
the presence of porcelain in contrast to the effect of heating it in a =
Nickel crucible.
So, what do you teach your students?
The processes which form that reticulated surface called "Orange Peel" =
are possibly a little more complex than we realise. They may be due to =
differences in surface tension, or they might just be caused by =
evolution of gas from the glaze as it cools, even a combination of both. =
But at the moment I would not know how to measure one or observe the =
other.
Best regards,
Ivor Lewis. Redhill, South Australia
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