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cone 05 glaze not fully transparent

updated wed 19 dec 07


Kerri Pero on mon 17 dec 07

I just fired a batch of 04 earthenware to cone 05 with a transparent glaze.
Everything came out sort of milky looking and not totally clear. The glaze I
used was gerstly borate, flint, EPK and a frit. (I don't have the recipe in
front of me here) It was a regular recipe I got from my ceramics professor.
I brush applied 2 coats - not super thick. It was a fairly fast firing.

I don't know if it is due to the fast firing or what. Should I try to refire
more slowly or at a higher cone? Please help!

Ivor and Olive Lewis on tue 18 dec 07

Dear Kerri Pero,=20

It is not unusual for low fired glazes containing ample Boric oxide to =
cloud. This effect seems to be caused by the molten mixture separating =
into two phases which are immiscible. Like oil and water, which when =
finely dispersed, becomes an opaque emulsion.

You have two rich sources of Boric oxide, The Gerstly mineral and a =
manufactured Frit.

Without the recipe I cannot comment further.

Best regards,

Ivor Lewis.
South Australia.