David Hendley on sun 11 mar 01
Every time you buy materials from your supplier ask for the
current analysis. Every boxcar load of material has an analysis
specifically for that batch.
Save the analysis from your previous purchase, and you will have
a side-by-side comparison every time you buy a new batch.
You will know if you need to adjust before you ever weigh up a
gram. If necessary, just plug the new analysis into the materials
data base of your glaze calculation program, re-calculate the
glaze, and you should be right on the money.
'Natural', as opposed to processed materials, such as Cornwall
Stone and other feldspars, are the most likely materials to vary
from batch to batch.
This is another reason to buy only whole bags of materials,
and several years worth of materials used in greater amounts.
I'm still using the 50 lb. bag of copper carbonate I bought in
the 70's, and only recently had to buy a new bag of rutile.
I draw the line at tin oxide and cobalt, since I don't have several
thousand dollars to tie up in raw materials.
--
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com/
----- Original Message -----
From: peachblow pottery
To:
Sent: Sunday, March 11, 2001 8:35 AM
Subject: Fw: whiting and cornwall
Mel, Both times I've sent this I have received notice that it was sent to
you. No big deal, thanks, Tom
----- Original Message -----
From: peachblow pottery
To: clayart
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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