peachblow pottery on sun 11 mar 01
Mel, Both times I've sent this I have received notice that it was sent =
to you. No big deal, thanks, Tom
----- Original Message -----=20
From: peachblow pottery=20
To: clayart=20
Sent: Saturday, March 10, 2001 10:19 AM
Subject: whiting and cornwall
We've experienced some chemical change problems and other potters have =
been asking about them, so I thought I'd post the info. Whiting and =
dolomite can both be labelled Limestone and we have seen drastic =
differences between what is being sold as whiting. Several years ago I =
ran out of an old stash of whiting, bought some new, and our bright , =
gloss copper red came out matt pink and about two cones underfired. I =
traced the problem to the whiting ran more tests, confirmed that it was =
the whiting, got into my files to see what I used to use, found a =
supplier, ran more tests, and confirmed that there was a vast difference =
with available "whitings". For years I had used Pfizer's Vicron 41-8 =
which is higher in calcium and lower in magnesium. The mattness was =
magnesium crystals floating on the surface. It shows up on our Temmoku =
also. I called around and found a warehouse in Cincinatti, Ohio that =
carried the Vicron and bought several hundred pounds. I'm out again and =
will buy 1000 pounds next week. The Vicron is a Massachusetts whiting =
with 1.5% magnesium carbonate, and 96% calcium carbonate. Pfizer has an =
even purer whiting called Pficarb H which has 98.3% calcium and almost =
no magnesium(.6%), but the Vicron 41-8 is fine for our glazes. The 41-8 =
refers to mesh size and is their finest grind. They offer a 15-15, =
25-11, and a 31-6, but again the 41-8 works fine. For information on =
the Vicron, availability, mesh size, or price, contact Mr. Mark Leverone =
at C.L. Zimmerman Co. 2501 Commodity Circle, Sharonville, Ohio =
45241. Or call 800-543-1121. You can check with your local supplier to =
see if they carry Pfizer Vicron before you call Zimmermans. Now, if you =
are doing magnesia matts, or even clay matts, then the higher magnesium =
whitings would help you out. If you don't like the little crystals =
floating in your Temmoku's, then the lower magnesium would help you out. =
The other problem and more recent for us, has been Cornwall. Again I ran =
out of an old stash and bought a new bag, which was wet and I had to dry =
it in the electric kiln before I could even use it. My wife mixed some =
Temmoku and it came out bleached and over fired by about two cones. =
Naturally the first assumption was that there was an error in mixing. We =
did some testing and they were over fluxed. I called around and Tim =
Mather said he saw a more recent chemical analysis of Cornwall, and =
compared to 20 year old Cornwall, the new has more calcium and less =
silica. A friend gave me some of the blueish colored Cornwall and it was =
a little better, but still not what we had used for years. I'm sure some =
of you have experienced glaze results that didn't seem to make sense, =
but didn't remember that you purchased some new chemicals. The other =
thing that adds to the problem is that we have received chemicals from =
studios that had shut down or the potter had died, and these were older =
chemicals. So I guess the bottom line is be aware that each bag of =
material can and could be different from what you had been using; and =
when you open a new bag of something, test it before you mix 20,000 =
grams of a glaze.
Happy testing,
Tom Turner
A sophmore in stoneware (-;)
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