judd cullen on mon 12 nov 07
Hello All,
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I am having a bit of a problem with my transparent earthenware glaze. It ha=
s started to shiver when used in conjunction with my decorating slips.
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The slips are 60% black ball clay, 40% china clay with added oxides.
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The glaze is one that we have used for years and has only recently started =
to have problems:
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Lead Bisilicate 65
China Clay 10
Potash Feldspar 10
Flint 10
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The vessels are terracotta bisqued upto 960 C with no problem, and then fir=
ed up to 1120 C with a 30 minute soak.
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I'd say that it may be the black ball clay, but the problem occured before =
we started mixing your own slips. I am using black ball clay as we have inh=
erited a lot of it (75kg at least), an the only information I could find is=
on it is that it performed the same, but had more carbon and had greater p=
lasticity.
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During tests, the slips perfromed wonderfully on both E/W and S/W with no s=
hivering at all.
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In the meantime I will try and test at different temperatures.
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If anyone could shine some light on the problem I'd be very grateful.
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Georgina :)
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Jon Pacini on wed 14 nov 07
Greetings All ---Hi Georgina, you wrote--I am having a bit of a problem with
my transparent earthenware glaze.
I'm going to paint with a broad brush here, but ---earthen ware clays,
slips and glazes don't fuse together like stoneware clays slips and glazes
that physically melt together. Especially the lead-free variety of glazes.
And I see a lot of shivering when the bisques are not hot enough. The softer
the slips and clays are, the less they want to bond together with the glaze.
You may want to try bisquing higher, maybe up to ^04.
Another tact if firing hotter is not in the cards for you, is to add some
flux to the slip to help it all bond together better. Try 5% increments of
borax in a line blend to the slip.
Best regards,
Jon Pacini
Clay Manager
Laguna Clay Co
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