iandol on fri 13 oct 00
Dear Hank Murrow,
The following may be of interest to you
Tests on pure Aluminium hydrate and pure Silica show that they do not =
react to sodium chloride in a salt glaze kiln when tested at Orton cone =
8. Even commercial Kaolin only forms an indifferent surface, hardly =
sufficient to be called a glaze. All the evidence from the last century =
leads to a belief that there is an exchange reaction with Potassium =
minerals in the clay, be they natural residuals or added as part of the =
clay body composition. Just look in the books at recipe formulations for =
salt glaze clays and count the number that incorporate potash felspar. =
Many ball clays react with salt to give a good orange peel effect, which =
can be explained in this way.
I suspect that orange peel texture is the result of out gassing from the =
vitreous solution just before the glass transition temperature is =
reached during cooling. Surface tension of the glaze may be sufficient =
to partially heal the craters after the bubbles burst but the glaze is =
to viscous to cause the surface to flow out to a smooth gloss finish.
My own opinion is that popular published chemistries, and there are =
several, for this way of making glazes are totally erroneous.
Ivor Lewis, Redhill, South Australia.=20