Tom Buck on fri 28 mar 97
Cheryl Shoemaker:
Congrats on your good maths; unfortunately your basic
assumption is at fault: gerstley borate is NOT the same as
colemanite although in most glaze mixes an equal weight of GB
or Colemanite (if available) would have much the same fluxing
effects. However, in many glaze recipes the chemical
differences sometimes do change results often sadly (but this
is not vital in Raku where it is mostly used).
US Borax has a giant deposit in California that it mines
to process into Boron compounds (borax, others). The potters'
market is a tiny fraction of its mine output so we get whatever
comes ... the mine sells 3-inch pebble to Hamill & Gillespie and
Laguna Clay (perhaps others?) who pulverize, screen, and bag
the powder sold to potters. The chemical makeup of the pebble
probably varies from time to time but there is no way H&G/Laguna
can control the boria (B2O3) content and other components.
In 1996, Laguna cited this composition (ex-US Borax):
Boric Oxide (B2O3) 28% (+/- 3%)
Sodium oxide (Na2O) 5.3%
Calcium oxide (CaO) 20.6%
Bound water (crystallization) 25.0%
Moisture (average) 0.3%
Insoluble minerals (shale, limestone, ?) 18.7%
US Borax does not determine a value of importance to potters, namely,
the Loss on Ignition (LOI).
In 1992, I received this analysis from H&G and the company
has yet to supply a more current one (some data is better than none):
B2O3 28.3%; Na2O 4.7%; SiO2 9.8%; Fe2O3 0.35%; Al2O3 1.3%;
MgO 3.5%; CaO 17.9%; K2O 0.14%; H2O (moisture) 0.52%. And the LOI
value obtained by calcining to 750 deg C was cited as 29.1%.
This analysis sums to 95.6% so one can guess that GB contains
some 4.4% of inerts (non-participants in the routine analysis procedure).
H&G said the chief components of the ore were colemanite and ulexite (a
sodium/calcium borate compound).
I am using the H&G values in my glaze calc program (Insight)
and the error introduced is tolerable, since it is less than the
likely variation in the Boria content from month to month. Also, if one
puts 10% GB in a glaze mix, the difference between 25% B2O3 and 31% B2O3
becomes 2.5 vs 3.1% in the recipe, quite possibly not a big enough swing
to change the glaze's behaviour. This likely would not be true for
recipes containing in excess of 20% GB.
Having made clear what Gerstley Borate really is, now let's
address your question: if GB were calcined before use, what amount of
calcine would equal 100 weight units of raw GB?
H&G's LOI says 29.1% would be lost on ignition. But the Laguna
numbers suggest it probably is higher. Bound water + moisture = 25.3%
plus perhaps a third of the "insoluble minerals", or another 6%, which
mean approx 30-33% loss. So as a ballpark figure, use until revised,
one could say 70 weight units of calcine would be equivalent to 100
weight units of raw GB. And the Formula weight of GB would be 204
and this would supply 0.84 moles of B2O3.
Finally, since it is known that BG contains some Gypsum (calcium
sulfate), a material that can cause "pop-outs" if left in the glaze mix,
one should always pass the GB powder through an 80/100 mesh screen and
discard the on-screen stuff. And since GB is slightly soluble in water
(especially hot water), one should routinely add some Epsom salts to
the water to be used for dispersing the solids; this inhibits formation
of sodium ions which can gel the glaze mix. Most mixers add 1 teaspoon
or 5g of MgSO4 per kilogram of solids.
Cheers Tom Buck (new email address: )
old:
Hamilton, Ontario, Canada
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