iandol on wed 13 nov 02
Dear Carol,
I think the chemistry of your glaze is quite complex
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5.50 Lithium Carb
28.0 Flint
30.0 Custer Feld
11.0 EPK
4.50 Magnesium Carb
3.50 Fusion Frit F-12 (like 3134)
15.50 Wollastonite
2.00 Bentonite
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I believe there is not enough melting flux at 39% (Lith. =
carb+Custer+Frit) to adsorb all of the high melting point materials at =
Cone 6. This includes the Wollastonite which has a given melting point =
of 1544=B0C. Without a high solvent action, sufficient undissolved =
materials will remain to act as nuclei during the cooling phase. Even =
much of the silica may remain undissolved and you are relying on this =
dissolving to give sufficient vitreosity to overcome the crazing. =
Holding at top temperature for 40 mins may not give sufficient time for =
everything to go into solution. Also, the slower the cooling will =
influence the degree of growth on a point by point basis.
Naturally Calcium Silicate (Wollastonite)will grow well, then such =
magnesium oxide as has been taken into solution will come out of =
solution during cooling, forming Enstatite (Magnesium mono-silicate, =
considered by some as a gemstone). Providing more silica will promote =
this if it dissolves. If the glaze remains deficient in silica then it =
may be Periclase (crystalline mineral Magnesium oxide) which forms if =
the oxide which forms during the decomposition of Magnesium carbonate is =
taken into solution and then precipitates during cooling.
This is speculative thinking and there could be side reactions of which =
we know nothing. Well designed and potentially costly investigation are =
needed to sort these things out.
Best regards,
Ivor Lewis=20
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