Tom Buck on thu 13 aug 98
This post is a follow-up to Kristin Busch seeking answers to
problems with a claybody that broke apart either in the kiln or
afterwards, and the crazing being encountered with a traditional
recipe, Chappell's "Floating Blue", a midfire glaze that exhibits
a variegated colour and surface.
And Bonnie Hellman has commented on her experiences with
Floating Blue in light of my suggestion to Kristin that the
NephSy/Gerstley Borate version be substituted with Tony Hansen's
5x20 glaze that uses FF 3134 and Custer (or G200) feldspar, and
that colour testing then proceed.
K.Busch cited this recipe:
Floating Blue C5/6 ox
47.3 Nepheline syenite
27.0 Gerstley Borate
5.4 EPKaolin
20.4 Flint/silica
add 4.0 Rutile; 2.0 Iron Oxide Red; 2.0 Cobalt Carbonate. In
molecular terms, the Seger is:
0.39 CaO/0.11 MgO 0.06 Fe2O3 0.2 TiO2
0.5 KNaO 0.47 B2O3 3.80 SiO2
(K2O=0.1; Na2O=0.4) 0.56 Al2O3
SiO2/Al2O3=6.8 COE=8.0x10-6
A year ago, Debra Corbedi asked for a revision to this glaze to
increase the B2O3 content. So I said raising the Gerstley Borate
from 27.0 to 33.0, leaving the other igredients as is, would
probably make the glaze somewhat more fluid and more suitable to
her claybody (unknown to me). She said thanks and since then no
word so I presume all is well with Debra.
Now Bonnie Hellman says she uses both the above recipe and
also Tony Hansen's 5x20 clear base plus colourants. She gets
different results with both recipes.
Ok. This sets the scene. Why do three potters in three different
locales in N.America get different results from the same Chappell recipe?
In simplest terms, because not one of them gets the "same" Raw Materials
although the names remain unchanged.
Glazes do NOT travel well
Years ago, when he designed Floating Blue, Chappell named
Colemanite (40-44% B2O3) as his source of Boric Oxide, but this
mineral is no longer mined in N. America. Instead, the crude
calcium borate ("colemanite") now is replaced by Gerstley Borate,
a mineral from California that has a significant amount of sodium
in its makeup. GB as mined contains "ulexite", a double salt (like
dolomite) that combines equal amounts of sodium borate and calcium
borate into its crystalline structure, and GB also contains
additional amounts of colemanite that has fossilized along with the
ulexite. A decade or so ago, GB had a B2O3 content of 34% (more or
less); today, it runs 28% or less. (Ron Roy cited the current
presumed composition here last month).
Potters in Europe and Asia have the dubious choice of
expensive US Gerstley Borate or Turkish Colemanite containing
varying amounts of gypsum. Neither material makes for happy
potters.
To understand why potters in different locales in USA often
have different problems with the same recipe, one must appreciate
that mined materials often exhibit a variable composition from time
to time, and even from bag to bag.
An earlier post on Clayart stated that the Gerstley Borate
deposit is worked primarily for sale of the mineral (as 3-inch
pebble) in truckload lots to Laguna and H&G who then pulverize it,
bag it, a ship it to their dealers. Apparently, once in a few
years, US Borax sends in mining equpiment to the deposit, mines up
to a thousand tons of the rock, puts it in pile so trucks can
subsequently be loaded. And we potters get whatever is in that
rock, without any chemical processing or change.
Put simply, Gerstley Borate cannot be regarded as a material
with a reliable chemical composition. Since actual chemical testing
is done on a very irregular basis, one uses Gerstley Borate and
hopes for the best. Mix the recipe, test it, if it works fine; if
not, well perhaps some juggling of the amounts might do the trick.
Even glaze analysis data may prove of little help because we lack
an accurate chemical composition of the current GB being shipped.
Fortunately, Nepheline Syenite, from a Canadian mine is
produced in large quantities for various industries, and the mine
is huge, so a uniform product is certainly guaranteed by the
producer in Canada. Much the same is true for a Nepheline Syenite
operation in South Africa.
Similarly, both EPK and Flint come from huge holdings, and
their composition stays fairly consistent. Ditto for Cobalt
Carbonate.
That leaves RIO and Rutile. Both these materials come from
many plants around the world. And if your RIO is "synthetic" it
surely will vary markedly from my "Spanish Red" from a mine in
Spain. In N.America there are at least four different sources of
Rutile, and each can have a different composition. Some rutiles
have twice as much FeO as others (10% vs 5%).
Let me wind-up this brief dissertation by issuing a caution to
potters generally. We are using some fairly inconsistent materials
in our some of our glaze mixes, so be happy when everything goes
well. But when it goes sour, please be prepared to seek possible
solutions and do the tests that will verify the solutions. In
chemical terms, your studio, etc., in no way matches mine; what
works for me may bomb for you. If fixes can be achieved, by
controlled testing, then all should rejoice.
Good tests. Tom.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
| |
|
