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: safe glazes

updated wed 10 dec 03


iandol on tue 9 dec 03

Dear Marianne Lombardo,

You ask << Does firing at ^10 versus ^6 make glazes high in manganese =
safer? >>

Interesting question, since it allows discussion of the factors which =
allow us to make standard mature glazes that are stable when fired to =
particular cone values. The concepts involved in this process are =
thoroughly discussed in "Mastering Cone Six Glazes" published by Ron Roy =
and John Hesselberth.

The next step is to determine why additives change the chemistry of a =
glaze. Ask this question "Why should compounds containing certain =
metallic elements change an amorphous silicate glass from being almost =
insoluble to being soluble in weak acids ?" It is certainly true that =
Copper compounds cause this change but no one has yet given a =
satisfactory answer as to why this is a fact. When we have that =
knowledge we can make the predictions which would answer your question.

Manganese Carbonate and Dioxide do not behave the same was as Copper =
compounds in a clear bright glaze and changes in firing atmosphere will =
affect both elements. Manganese compounds seem to be highly soluble in =
silicate melts. Their solubility seems to increase as the maturity =
temperature of a glaze increases. With higher concentrations colour =
changes from pale yellow to dark honey brown. But there is a limiting =
value beyond which a dark metallic lustrous stain begins to form at the =
surface of the glaze. This is encouraged if there are coarser particles =
in the colourant.

Since manganese oxides and carbonate are given as being soluble in =
mineral acids and there appears to be a saturation point at which =
compounds are no longer taken into and retained in a silicate solution, =
people who use these colorants should be cautious. The "Lemon Wedge" =
test is easy to apply and samples can always be sent to a laboratory for =

In the end, research needs to be done to establish the Physical =
Chemistry of Silicate Glazes in terms which are consistent with modern =
scientific concepts and principles.

In the end Marianne, I believe the best answer to your question is "We =
just don't know. We may guess, but we just don't know"

Best regards,

Ivor Lewis. Redhill, South Australia