June Perry on sun 27 jul 97
Found my little blurb from an old Ceramics monthly article by Richard
Behrens, I believe. I only cut out the meat of the article so I can't give an
issue date. But he says in relations to gerstley borate and colemanite:
Both colemanite and gerstley borate contain the active flocculating
(coagulating) metal, calcium and this may cause a glaze to flocculate on
standing. Flocculation can take place when slip of glaze contains substantial
amounts of dissolved electrically charged acids, cacium, barium or alumina
particles(ions). These behave somewhat like a magnet with positive and
negative poles that attract or reject each other. When such
particles(including clay) are present in slip or glaze, the attract each
other and ten to group together in a floc. In this case the glaze may become
less fluid. The thickening may often improve the ease of applying glaze to
ware, but additions of water for thinning may result in an over saturated
glaze which could crack or peel away from the ware due to excessive
shrinkage.
The articles goes on to say that coagulation or flocculation can be reduced
by the addition of as little as one tenth of a percent soda ash, or (with
somewhat less efficienty) three-tenths of a per cent gum. So you may be able
to salvage your batch with the soda ash. You may want to mix up the batch and
take a small quanity out and test the soda ash in that.
Regards,
June
| |
|
