Tom Buck on tue 13 mar 01
Yes, Alisa:
Colemanite mineral from Turkey reaches most continents; it is a
very bad actor for potters if it is not processed to remove the Gypsum
usually contained in the mineral from the mine.
The Turkish colemanite that comes to North America seldom goes to
potters; it is chemically upgraded for many uses in industry.
There is scuttlebutt that colemanite from Argentina is being used
by North American potters nowadays. and it too may contain varying amounts
of Gypsum, since Calcium Borate and Calcium Sulfate likely were "salted
out" from waters of an inland sea that went dry. and this deposit now is
being mined.
If the "spitting" glaze in question contained a substanital amount
(15%+) of colemanite, and the gypsim is not removed, the glaze will
undergo "lime" spits during the firing.
If a potter plans to use colemanite as currently available, one
should take the precaution to screen the colemanite (60 mesh or finer) and
discard the larger granules...these will be mostly gypsum because it is
a harder mineral than colemanite.
til later. Peace. Tom B.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Alisa og Claus Clausen on tue 13 mar 01
Dear Dawn,
I also found this receipe on in the archives and tested it. I did not =
have the problems you experienced. I am looking at my test bowl now. I =
tested for the same reasons I like to test Albany Slip glazes (2 =
components). It is not a "glaze" in terms of anything that is stable; =
very fluid and I do not think much glass. But for other reasons, I try =
them. I tried it again on a very rough clay and it did not move nearly =
as much as on the smoother body. Again, these types of receipes for me =
are more for interest to see how few components can react together make =
a " primitive glaze" I always leave a high bare margin at the bottom of =
the cup, but I miss sometimes anyway. I am sorry that you had such a =
miserable experience with it. =20
I am wondering about your colemanite. I noticed that many, many of the =
tests I made where I subbed colemanite for Gerstely Borate, also left =
halos around the test bowls, and some of them really spit.=20
Tom Buck advised me (not directly in relation to this problem) that =
colemanite can often be contaminated with gypsum. It is best to screen =
the colemanite to get the contaminants out. I have not used too many =
receipes with colemanite lately since my local frit has similar =
properties to Gerstely Borate. However, since I have screened the =
colemanite (mine comes from Turkey), I have not experienced any spitting =
or halos.
Perhaps Tom will confirm or not, if contaminated colemanite could be the =
culprit.
Best regards,
Do not give up, there will 10 bad to 1 good test, and then 1 good pail =
of glaze to 10 previously good glaze tests! But when they work for you, =
it is of course worth the troubles.
Alisa in Denmark
| |
|
