Gavin Stairs on thu 28 mar 02
The problem of solubility is two fold. The first is that the soluble
materials are much more mobile on the bisque than the insolubles, and they
don't stay put. Solubles can migrate into the body and flux the clay,
rather than the glaze. This can lead to obvious problems like bloating and
underfluxed glazes. Of course it can also be used to advantage, as in
Egyptian paste ware, and Dannon Rhudy's famous soda ash sprays.
The second problem is the one the original post was about. The soluble
salts in the glaze bucket exchange with the relatively insoluble ones, like
calcium and magnesium, The relatively insoluble salts can now drift off to
a new site, and exchange again. This reaction can go a lot quicker than it
does without the soluble fraction, since the return path is free, so to
speak. So the reaction rate is higher by a factor of the relative
solubilities of the two salts, which can be enormous. The net result is
that insoluble salts grow from dust into granules, and from granules into
lumps. Since the reaction is biased in favour of larger particles, the
original granularity of the glaze gets bigger and bigger. At first this is
not very noticeable, but at some point, the bigger granules begin to fall
out of suspension, and a hard pan develops at the bottom of the
bucket. This is essentially the way that limestone rock forms in the
ocean, so the hardpan can be very hard indeed. The longer you leave it,
the harder it gets.
The worst characters for this are any form of chloride, especially common
salt. This can exchange with an almost insoluble calcium carbonate, for
example, and the result is two soluble salts: sodium carbonate (soda ash)
and calcium chloride (road salt). This is practically a free ride for the
calcium to move about wherever it likes. The only common chloride salt
which is insoluble is silver chloride, a fact which is used in wet chemical
analysis to isolate silver. All the other common chlorides are soluble to
some degree. This little fact is a clue to how nodules form on the sea
floor. Sea water is a weak solution of chloride salts.
The solution is not to have solubles in the glaze in the first place, which
is the basic reason for washing ash, to get rid of the soluble
alkalis. The remainder is relatively insoluble alkali earths, mostly, so
the treatment tends to make the flux more refractory. The remaining
problem is how to include alkalis that aren't soluble. The general
solution is to include them in silicate form, but even there they can be
troublesome, as in nepheline syenite, for example. The frit solubility,
and the same problem with the Colemanite/Gerstley Borate group of fluxes,
arises out of an excess of alkali, and not enough silicate to stabilize it,
or possibly insufficient melting of the frit.
The old potters often accepted the problem, and simply used the glaze
rapidly, before the reaction had a chance to go too far. I recall in
Peterson's book on Hamada a description of the glaze he used, which
included a small boy whose sole task was to keep the glaze stirred. That
was partly because the glaze had very little clay, but also, I suspect,
because it had soluble salts, which would turn any sludge into a hard pan.
At 05:19 PM 28/03/2002, Craig wrote:
>...I don't understand how the frit's solubility prevents the reconstition
>of the glaze. Could you explain? ...
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