Michael Banks on tue 30 mar 99
Michele,
There is some confusion here whether you're referring to scumming on fired
clay, or crystalline deposits on top of glaze. The two phenomena are
different in cause and effect.
If you're talking about the latter, it can be more correctly termed
efflorescence. Efflorescent occurs typically on high alkali and high boron
glazes with compositions outside of the normally accepted limits. It is in
fact a sure sign that one or more of the ingredients has exceeded the glazes
ability to keep the component in solid solution.
The surface deposits (often borates) are usually fairly soluble and could be
expected to be removed by the methods quoted. This cure will only be
temporary and the efflorescence will continue over a prolonged time. It is
aided by hydration reactions with humid air. Such "glazes" are totally
unstable and unusable, hence the quotation marks.
Michael Banks,
Nelson,
New Zealand
>----------------------------Original message----------------------------
>Hi everyone:
>
>I recently read that the scumming that occurs on pots with some alkaline
>glazes is the result of overfiring and can be removed with a mixture of
>vinegar and salt. My ceramics teacher disputes this to be true, saying the
>scumming has appeared on pots that she fired for only 6 hours.
>
>Does anyone know why the scumming occurs? Is it the glaze or the firing or
>both? Is there a solution? Thank you in advance to anyone who replies.
>
>Michele
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