Vince Pitelka on thu 17 may 07
Ivor Lewis wrote:
"I have not read or seen Ron Meyers in action nor do I recall seeing
illustrations of his pots. So any comment I might make would be sheer
speculation. Perhaps you can explain and send me some pictures. I have a
stock of terra cotta and could rig up a temporary kiln for a couple of
firings if you can fully describe the process. After all, good observation
is at the foundation of science."
Ivor -
As promised, I did some research at school. I had forgotten how many books
I had on the salt firing process. I did not find anything on low-fire salt
in Jack Troy's book, but in Rosemary Cochrane's book she talks about
low-fire salt quite extensively. She claims that the best results are
achieved when small packets of salt are placed among the wares, and the kiln
is rapidly heated to cone 06, around 1000C, over about four hours. One
cannot expect much gloss at that temperature, but the clay is obviously
affected by salt vapors, which tend to bring out variable color and flashing
in different clays and slips. She also says that terra sigillata finishes
are especially appropriate for low-fire salt effects.
In Phil Rogers's book on salt glazing, he says "Potter such as Paul Soldner
in the USA and David Miller in France have incorporated salt into the raku
process by coating a fairly standard raku-type claybody
with slips that contain a low-temperature alkaline frit, often in
conjunction with copper oxide (present in the slip or the salt). This can
result in spectacular flashed pinks and reds."
Rogers also says "Salting in the temperature range of cone 1 to cone 5 is
also possible by adding nepheline syenite to the body in place of feldspar.
. . . Nepheline syenite fluxes the body to a state able to accept salt vapor
at a lower temperature than we are normally used to."
In Janet Mansfield's wonderful book on salt firing, she makes several
references to low-fire salt. She talks about Aruni Constantanidi's low-fire
salted sculptural work, where the salting brings out a range of
subtle shades in porcelaneous claybodies and slips. To me, the surfaces
look a lot like the best obtained from pit-fired or sagger fired work where
people often use salt and fire to low temperatures.
Mansfield quotes David Miller: "The function of salt, at a temperature below
that at which a glaze is formed, is to draw out subtle nuances of color in
both the claybody and the slip. In order to arrest the
formation of a glaze, the temperature must not exceed 1000C. The way in
which the kiln is set has an important influence on the distribution of heat
and the circulation of the salt vapor. The pieces can be stacked without
kiln shelves, but they should be protected from direct exposure to the flame
by a bagwall. The flame picks up salt which has been placed in bowls and
packed among the pieces along with other combustible materials, and carries
the vapor through the maze of pots."
I was looking at the Ron Meyers terracotta pieces I have, and they are all
glazed with a low-fire liner glaze on the interior food-contact surfaces.
On the outside, the salt seems to make the surface quite a bit harder and
denser, and while there is no real gloss, the surface color is intensified,
and very beautiful.
It seems apparent from all of these citations that low-fire salt is not a
process for functional food-contact surfaces. In several books they talk
about early German and American functional stoneware being salt-fired in the
cone 4 to cone 6 range, but never lower than that. But what is also
apparent from all of these citations is that the salt volatilizes at
low-fire temperatures and deposits upon and within the claybody and slips.
Some citations refer to tossing salt into the firebox, where the
temperatures could conceivably be considerably higher than low-fire, but
others refer to placing the salt in small dishes or packets among the wares,
sometimes within a sagger. In those cases, it is clear that the salt
volatilizes quite fully at low-fire temperatures.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on sat 19 may 07
Dear Vince Pitelka,=20
Thank you for your summaries. I got out Jack Troy's book and that of =
Peter Starkey.
All give good accounts of the technologies used but there is little that =
explains why things happen. When I was trying to solve the Science of =
Salt Glaze I found that all modern accounts lead back to Daniel Rhodes. =
He is quite clear about the whole thing. He said the way it was =
explained was a Theory. The Physical and Chemical properties of the =
reactants support that proposition, that it remains a theory
Your view that common salt volatilises completely at low temperatures is =
simple to demonstrate. Placing a sample in an open Nickel (Platinum is =
better but expensive) crucible and heating it rapidly with a gas torch =
to bright red heat would prove how quickly this happens.
In 1886 records of experiments by M. Alex Gorgeu were published in Paris =
by M Friedel. They described the behaviour of various solutions which =
were dropped into a red hot porcelain basins containing Kaolin. The =
results of using Sodium Chloride were presented as a new way of making =
Hydrochloric acid.
In 1670 Rudolph Glauber mixed Common salt with potter's clay and baked =
it in a kiln. It was done this way because when he threw his salt onto =
the hot coals crystals exploded and flew out of the fire. He collected =
the effluent in water then redistilled it to get Spirits of Salt.
These are the roots of the Theory given by Daniel Rhodes. But that does =
not help you understand the process as used by Ron Meyers.=20
Best regards, and thanks for the discussion.
Ivor Lewis.
Redhill,
South Australia.
.=20
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