Craig Martell on tue 8 nov 05
Paul was saying:
>but Kingman and Custer are about as alike as any two feldspars have ever been.
Hello Paul and other glaze material enthusiasts:
In some respects Custer and Kingman don't have much of a difference and
with a lot of glazes a direct substitution is just fine. When I was working
on the blue celadons I found that there is quite a difference with respect
to that type of glaze. The feldspar that is closest to Kingman is
G-200. Both have more K20 than Custer and the ratio between fluxes and
silica is closer. The ideal for a potash feldspar with regard to the
flux/silica ratio is 1/6. Kingman and G-200 are both very close to this
but Custer is higher at about 1/7.14. When Moose Creek Feldspar becomes
avialable, that will be the spar of choice in my studio.
When I used Kingman and G-200 for the iron blue celadons the glazes were
bluer and fit was better. When I tried Custer I had to raise the
percentage of spar in the recipe to supply enough K20 for the blues. This
caused crazing due to an increase in Na20 which is also present in all of
the spars. With Custer, the blues were a bit greyer. With Kingman and
G-200 I was able to get sky blues. Also, when using G-200 in porcelain
bodies instead of Custer I found that I could decrease the feldspar and
raise the clay content of the body and get good vitrification.
If any of you folks are having questions about material content and
substitutions, I would suggest running the material through a search
engine. Digitalfire and Matrix both have websites with material databases
and analyses for many of the materials that we use. Then you won't have to
wait for a reply and someone else won't have to tell you stuff that's
already available. If you have questions about uses and other material
issues you can get advice on Clayart.
regards, Craig Martell Hopewell, Oregon
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