Tom Buck on fri 31 jan 97
Marshall: You were close; however, the term "crystallization" as you used
it is confusing. What is involved is a bit of solution chemistry combined
with colloidal particles in a water suspension (ie, particles smaller than
1 micron, 10-6 metres, in diametre).
Powdered Gerstley Borate (a mineral containing Na2O.CaO.2B2O3
hydrate or "ulexite" and CaO.B2O3 hydrate or "colemanite") is slightly
soluble in water, producing Na+ ions (and other ions) which make the glaze
liquid slightly alkaline via induced OH- ions. The OH- ions will cause
colloidal particles to link up with water molecules and this in turn will
often produce a "gel", that is the glaze slurry viscosity (consistency)
rises noticeably, making it difficult to dip-glaze bisqued ware
efficiently (altho ok for leatherhard ware). If the glaze slurry gels it
is very hard to break this gel and restore a colloidal suspension
(dispersion). If a gel forms, the mix will not settle.
However, if one puts a small amount of epsom salts (magnesium
sulfate) in the water used to disperse the glaze ingredients, then the
Mg++ ions interfere with the solubilizing of Na+ ions from Gerstley
borate, and thus it is most unlikely that a gel will form. Also, the
presence of Mg++ ions inhibits the clumping together (agglomeration,
rather than "crystallization") of the colloidal particles and so this
avoids a hardpan from forming when the ingredients settle out as they will
on standing. Instead of a hardpan, the glaze components settle into
segmented layers that are easily stirred to reconstitute the glaze slurry
for dipping.
Sorry for the physchem lesson. But you did ask.
TomBuck Hamilton ON Canada URL http://digitalfire.com/magic/tombuck.htm
Bugash@aol.com on sat 8 feb 97
How much is "a small amount" of epsom salts per recipe of glazes using G.
Borate? In layman terms does the epsom salts flocculate and if I used Calgon
it would deflocculate?
A very confused "Newby"
Ginger in Germantown, MD
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