David Hendley on thu 5 aug 99
Contrary to what has been said here on Clayart last week,
the Colemanite of old and today's Gerstley Borate are far
from the same material.
The theoretical formula for Colemanite (yes, it was named for
a guy named Coleman, in California, in the 19th century) is
1.00 CaO
1.50 Ba2O3
Here's the latest analysis I have for Gerstley Borate (named
for a Mr. Gerstley, the president of U. S. Borax)
.73 CaO
.11 MgO
.14 NaO
.84 B2O3
.03 Al2O3
.42 SiO2
This is a HUGE difference, and, in fact, to supply a glaze with a
given amount of boron, almost twice as much G. B. is needed
as theoretical Colemanite.
My glaze tests have shown this to be true; to get results similar
to those from 25 years ago, much more G. B. is needed to replace
the Colemanite from those old recipes. Of course, with the other
added oxides in the G. B., this require recalculating the whole glaze.
According to the Hamer Dictionary, Colemanite is associated with
ulexite, which is similar but contians soda. We all know that G. B.
is a variable material, and I'd speculate that what we are offered
today is a Colemaite, ulexite, who-knows-what-else mixture that
does not contain nearly as much boron as what was offered 20 or
30 years ago.
The history of the discovery of boron in California and all its uses
is a great story. If you happen to be passing through the desert,
make a stop in the town of Boron, California and visit the museum.
You can't miss it: at the corner of Boron Street and Twenty Mule Team
Boulevard.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
June Perry on sat 7 aug 99
Dear David:
Colemanite and Gerstley have always had a difference in the B2o3. An old
Ceramics Monthly article from 25 years ago lists Colemanite with 48% boron
and Gerstley with approximately 28%; but they've always been able to be
directly substituted for each other, in the past, because of the powerful
activity of sodium in the Gerstley. In other words, their fluxing capacity
was pretty equal in the finished glaze with Gerstley having a bit of an edge
for resulting in a brighter glaze and it being, at that time, a more
consistent material. I believe this was a Richard Behrens article.
I still have old stash of Gerstley and has reformulated some of my glazes for
frits to eliminate all or most of the Gerstley. I know from Tom Buck that
there is a lot of stuff in the newer batches of Gerstley that may be
impacting negatively on glazes. So I would suggest that any one who still
wants to use this material, to get a current breakdown of the material and if
you decide you want to use it, reformulate your glazes accordingly.
Additionally, in the article he recommended 1/10 of 1 % soda ash added to the
glaze to reduce flocculation.
Warm regards,
June
Martin Rice on tue 12 nov 02
Hi,
For a change (NOT) I'm confused. In Charles McKee's "Ceramic Handbook=
: A
Guide to Glaze Calculation, Materials, and Processes," in the three p=
laces
where there's a page number in the index for Gerstley borate, he has
"Colemanite (Gerstley borate)" as though they were synonyms.
On the other hand, in Richard Behrens' "Ceramic Glazemaking," some re=
cipes
refer to Colemanite and some to Gerstley borate with not implication =
at all
that they are the same thing.
Could someone please explain this apparent contradiction for me?
Thanks so much,
Martin
Lagunas de Bar=FA, Costa Rica
http://www.rice-family.org
Snail Scott on tue 12 nov 02
At 01:48 PM 11/12/02 -0600, you wrote:
>...in the index for Gerstley borate, he has
>"Colemanite (Gerstley borate)" as though they were synonyms.
Well, way back when dinosaurs roamed the earth, and
dirt was shiny and new (sorry...) ;)
Anyway, once upon a time, colemanite got unreliable,
so gerstley borate was widely adopted as a one-for-
one substitute. It's not identical, but it's pretty
close for most purposes. Besides, both gerstley and
colemanite now vary so much from batch to batch as
well as from their earlier analyses, that any
blanket statement of the differences would be
fairly meaningless. I'd say, don't get too attached
to either of them, and try some of the better-known
borate substitutes. At least you'll have a reliable
chemical composition to work with.
-Snail
Martin Rice on wed 13 nov 02
Thanks, that helps a lot. Is there anywhere I can go on the web to find a
list of some of the better-know borate substitutes?
Thanks,
Martin
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Snail Scott
Sent: Martes, 12 de Noviembre de 2002 08:28 p.m.
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: [CLAYART] Colemanite and Gerstley Borate
At 01:48 PM 11/12/02 -0600, you wrote:
>...in the index for Gerstley borate, he has
>"Colemanite (Gerstley borate)" as though they were synonyms.
Well, way back when dinosaurs roamed the earth, and
dirt was shiny and new (sorry...) ;)
Anyway, once upon a time, colemanite got unreliable,
so gerstley borate was widely adopted as a one-for-
one substitute. It's not identical, but it's pretty
close for most purposes. Besides, both gerstley and
colemanite now vary so much from batch to batch as
well as from their earlier analyses, that any
blanket statement of the differences would be
fairly meaningless. I'd say, don't get too attached
to either of them, and try some of the better-known
borate substitutes. At least you'll have a reliable
chemical composition to work with.
-Snail
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June Perry on wed 13 nov 02
There was a nice article about these materials in Ceramics Monthly, many
years ago. They have been used interchangeable because although they get
their fluxing from different materials in their formula, the melt is very
similar. It has been said that the gerstley gave a bit brighter glazes; but
as it has mentioned many time before here on Clayart, both of these materials
have been unreliable.
I should get into my studio and see if I have my little elbow pots melts of
these materials as well as a bunch of frits,etc. and scan them and put them
up on my web site.
If possible, you'd be better off converting your gerstley and colemanite
formulas to frit compositions using one of the good software programs
available, i.e. Matrix, Insight, etc.
If these are old glaze recipes from the 60's or 70's, make sure you use the
gerstley and colemanite formulas that are closest to what was available at
that time, when you do your conversion.
I keep two versions of Tony Hansens insight program on my computer. One has
the old gerstley formula and the newer one has his generalized formula, which
according to feedback he's had from some of the lab people at the boron mine,
is a bit off. Keeping that in mind, you may have to play with the formula a
bit to get it right.
Since many of the glazes people are trying to convert to frits are old
formulas, I use the older gerstley formula when doing the conversions since
the new one has too much magnesium and much less boron than what was
available in gerstley twenty or thirty years ago.
Regards,
June Perry
http://www.angelfire.com/art2/shambhalapottery/index.html
Alisa Liskin Clausen on wed 13 nov 02
One of the earliest testing I did when I started up my studio here three
years ago, was to try to use G.B. and Colemanite interchangeably in my
recipes. Rather, sub. Colemanite for G.B. Somewhere in the archives are
all of those results. However, I basically concluded that recipes with 20%
or higher of G.B. did not do well with the sub. of Colemanite. They were
dryer and muddier. From these tests, I found, by testing and accidents,
that a local frit, does the job very well as a G.B. sub.
regards from Alisa in Denmark
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