Alisa og Claus Clausen on mon 11 jun 01
Dear Stephanie,
Hamer and Hamer say in abbreviated form,
Colemanite:
Boracalcite. Hydrated calcium borate. Pricerite, Pandermite. =
2CaO.3B2O3.5HxO. A variable crystalline mineral that provides calcium =
and boron oxides in a relatively insoluble form. Occurs in the Western =
US associated with =FAlexite. Colemanite is a useful means of =
introducing boron oxide into a glaze without using frit. About 10% of =
the glaze recipe is the usual limit for colemanite, improves brilliance =
of color and gloss. Popular glaze material but has it's advantages and =
disadvantages.
Colemanite is a powerful flux. Introduces clacium oxide which begins its =
action at 1100c (2012f) and boron oxide which begins to fuse at red =
hear 600c (1112f). In glazes for the 1000cto 1100c (1832f to 2012f) =
range it gives some opacity. Over 1100c the colemantie becomes a very =
active flux, giving very glossy glazes.
Boron oxide has a low coefficient of expansion. If boron oxide is used =
in a glaze, the glaze will contract less when cooled after firing. =
Therefore less likely to craze.
Disadvantage of colemanite is it's tendecny to encourage crawling. Other =
minor disadvantages are cration of a flushing opacity if not wnated, its =
cration of a dull opacity when used in large amounts. Therefore =
Colemanite is often limited to 2 or 3% of a glaze.
Gerstely Borate
A variety of colemantie which includes sodium ox. Name is derived from =
the man who operated a colemantie mine in California.
Practical use of Colemanite as a substitute for Gerstely Borate?
I had made many tests with the above sub. I found that most of my =
recipes with G.B. called for over 10% up to 50%. The Colemanite in some =
cases produced a similar glaze, but in most trials, the Colemanite test =
compared to the G.B. tests were muddy and opaque. I also experienced a =
lot of spitting in the glazes during firing.
Tom Buck advised me to screen the Colemanite because many times it is =
from Turkey (mineral called Pandermite), and is contaminated with Gypsum =
(alabaster, material that plaster of Paris is made of). When I did =
this, the results were better. Tthe spitting was eliminated, however, =
the glazes with under 10% G.B. came the closest to the original look =
when subbed with Colemanite. Glaze recipes with over 10% Colemanite =
were muddy.
In the archives there are many posts about Colemante Vs. Gerstely =
Borate. It was about a year ago that this was discussed so you will =
find a lot of info. there.
Hope this helps,
Best regards,
Alisa in Denmark
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