crackle and tiny bubbles

[Carol H.]

I have recently fired a cone 6 faux celadon glaze and ended up with crackle on some pieces and tiny bubbles which have produced a cloudy appearance on others.  Can anyone say why I may have gotten two different results in the same firing.  Here is the recipe if that will help...

Birk's Turquoise

Custer feldspar 58

Cal. carb. 17

Silica 14

OM4 (ball clay) 6

Zinc oxide 5

Copper carb. 1

Thank you!

:)

[Min]

There is a heck of a lot of custer in that recipe, custer contains high expansion fluxes, the calcium carbonate is another high expansion flux, so without even looking at the calculated coefficient of expansion, COE, you can tell that the glaze will have a high COE which will cause crazing on a lot of claybodies. Chances are any uncrazed pots with that glaze on the same claybody will craze over time.  If only some of the pieces developed the tiny bubbles my hunch would be those pots were either in a cooler part of the kiln or the glaze was applied thicker.

BTW that glaze is also known as Burke's Strong Celadon.

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