iandol on fri 24 aug 01
As I understand the process, pit firing is a post bisque low temperature =
technique which uses the effects of local reduction through the =
proximity of carbon bearing materials to alter the colours of the clay =
in such a way that the flow of the fire adds beauty to the pots. It also =
changes the colours of copper salts giving metallic reds and lustre =
flashes. Very impressive stuff.
If the information which has been made available to me is correct, Mr =
Paul Soldner was one of the first to use Common Salt, aka Sodium =
Chloride on the clay in his Raku firings. In correspondence I had with =
him about twenty years ago he said he used it because it gave the =
effects he was seeking.
I have pots which have been fired up to cone 8 plus that have =
effloresced salt months later, from supposedly impervious stoneware. I =
also had discussions with a gallery proprietor who would not entertain =
salt glazed mugs because they were prone to tainting the beverages put =
in them, even when liner galzes were used. Since most salt glaze pots =
have crazed glazes I could understand her feelings
Your salt solution will dry in the pores of the bisque clay, or in raw =
clay. It melts at about 805 deg Celsius and acts as a solvent for clay =
and silica. If the temperature of your pit does not achieve that =
temperature there will be no chemical reactions of note. As the =
temperature rises Sodium chloride (NaCl) begins to react with mica and =
felspar in a complex reaction which releases Potassium Chloride (KCl). =
This reaction is slow at the temperatures of a pit firing when the =
temperature gets above the melting point of the salt but is very rapid =
above cone 8, a temperature often used for Saltglazing. It is my opinion =
that this leaves your pit fired clay in a questionable condition. Any =
unreacted NaCl will leach out of the clay. Any KCl which has formed will =
also leach out of your clay. If there was free silica and Sodium =
Silicate has formed, this is also water soluble so the surface may at =
times feel "sticky" if touched with a damp palm.
Provided the technique is recognised for what it is and the situation is =
well understood I see no problems. Just let the salt whiskers grow and =
enjoy their beauty.
Ivor Lewis. Redhill, South Australia.