Craig Martell on thu 5 jan 06
Eleanora was asking various things about kaolin:
Hello Eleanora:
The only answer I can come up with about which is the "best" kaolin is: It
depends. I really don't think there is a best kaolin for a throwing
body. There are a lot of good ones but the differences in the many forms
of the mineral lead potter's to form their own opinions about what is
best. The total composition of the claybody is also a deciding
factor. You can put a stellar kaolin in a less than stellar claybody and
it won't perform well. The reverse can be true as well.
If whiteness and purity are an issue, the English China clays are the best
kaolins that are easily available to us. If you want a US kaolin with good
throwing properties that is fairly white, you are already using the best
one, EPK. If you don't mind a little iron and titanium in the kaolin, the
Sandersville, Ga kaolins, 6 Tile and Sapphire are very good ones to
use. They make much better throwing bodies than the English kaolins. Of
couse, this is just my opinion. Sometimes, as Jonathan mentions, Kaopaque
20 is added with 6 tile. This makes a whiter, more workable blend and the
ratio is 2 to 1, or 66% 6 tile and 33% Kaopaque. Kinda moot now because
Imerys has discontinued Kaopaque 20. It's a delaminated kaolin that
requires more processing and costs.
The 2/1 ratio of silica and alumina is one of the basic defining points of
the kaolin mineral but, as you noted, it doesn't tell you a lot about the
clays. Even when you look at the entire analysis you don't get any hints
about throwing properties. Particle size will give more indicators about
that. You can get a lot of info about kaolins online and a good thing to
note is whether the clay is a primary or secondary kaolin. The secondaries
throw better and have more plasticity. Just about all the US kaolins are
secondaries. The English kaolins are primary kaolins. Primaries are a
rare thing.
My advice would be to try some different kaolins and see which one you
think is best. It's a subjective thing. I've been working on porcelain
bodies regularly for more than two years. I have two that I'm using now
and they both have blends of different kaolins. The throwable, very white
body is a blend of ECC Super Standard, ECC Standard, and the dreaded EPK
plus a very small amount of ball clay. It's a very strong body and has
very acceptable shrinkage rates. 6% raw shrinkage and 13.5% fired
shrinkage. The other body is a blend of Sapphire and Kingsley kaolins with
some J-2 ball clay. A bit more grey but one of the nicest throwing bodies
I've made. When I run out of the ECC clays I'll use this one
exclusively. Super Standard has tripled in price and I just can't afford
it any more. I still have over a ton left though. 8>)
probably way more than you wanted, Craig Martell Hopewell, Oregon
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