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real salt glazing and re: non-salt salt glazing

updated sat 14 oct 00


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.

Best regards,

Ivor Lewis, Redhill, South Australia.=20