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salt glaze. salt and water reaction

updated thu 15 jun 00


iandol on wed 14 jun 00

Some time ago I requested information about the process of salt glazing =
with sodium chloride and followed advice which was given to me. I have =
now read the article from Ceramics Review by Peter Meanly and Dr. W. =
I have no doubts that their experimantal results are true and accept =
that effluent from the kiln will test positive for Hydrochloric Acid. =
This is not in dispute. Results from the late 1950s and early 60s which =
prove this are in the records of the British Ceramics Research =
Association. This earlier work was cited at the 5th NCC in Sydney in =
1988 at a meeting attended by Mick Casson. Barringer said, in an article =
published before 1910, that Hydrochloric Acid was due to a secondary =
reaction after the salt had reacted with the clay.
The problem with current established Salt Glaze Theory is the assumption =
that there is a chemical reaction when salt is heated with water, or =
rather, superheated steam. The search I have carried out leads me to =
believe this is unproven. Science, when used to explain chemical =
reactions, must have a control, or a blind test. The control in this =
case would be to react sodium chloride vapour with steam in the absence =
of any ceramic or silicate substance and then test for chlorine and =
hydrogen chloride separately. Discrimination between HCl as a gas and =
other chlorides which are water soluble is an exacting analytical =
There is also on record an account of a comparison of the rate at which =
Sodium Carbonate will decompose when heated to red heat. In a platinum =
crucible there is no noticeable loss in weight over a reasonable period =
of time, but in a porcelain crucible, loss of weight was rapid and =
I remain convinced that both sodium chloride and sodium carbonate react =
directly with the substances of clay. When NaCl is used, it is known =
that the effulent is almost fifty percent Potassium Chloride.
The sources of Potassium in clay are the added flux such as Potash =
Feldspar and Potash Mica. Other effluent chemicals include Aluminium =
Chloride and Silicon Tetrachloride. These compounds react with water to =
form acidic solutions which will give a chloride reaction. These three =
chemicals are not accounted for in the popular theory.
I trust this information will be of use to those who continue to uphold =
the ancient treaditions or follow the modern trend.
Ivor Lewis. In South Australia