Chris Hogg on sun 21 mar 99
> ---------------------------Original message---------------------------
> Sheldon Bieber wrote:
>
> Hello,
> Does anyone have any idea how I could use ground glass as a glaze or part of a
> What might be added to it to raise its melting point? Where could I buy suita
> ground glass?
Ground glass, sometimes called cullet, can be a component in glazes, but
doesn't make a very good glaze on its own. It is a source of silica,
lime and soda, but it contains hardly any alumina, so will run badly if
used alone, and won't be very durable. It used to be sold by Wengers in
the UK many years ago when they existed. It had the following molar
formula: SiO2 3.25, Al2O3 0.03, CaO 0.35, MgO 0.18, K2O 0.02, Na2O 0.46.
You need to get some more alumina in to stiffen it up. The problem is
that most alumina comes with silica (e.g. kaolin or feldspar), and
there's already enough of that in there unless you fire very high. You
could try something simple like 50:50 cullet:nepheline syenite, which
might be OK at ^8+ or so, but it contains too much soda and not enough
lime for my liking and would probably craze badly. I also fould cullet
made the glaze bubble at high temperatures, and sometimes it blackened
in reduction. Maybe mine contained a little lead or boron not shown in
the analysis. To get a balanced glaze composition, you end up putting in
so much of everything else and so little cullet that it hardly seems
worth the effort. You could make your own cullet by crushing up some
broken bottles or window glass and grinding in a ball mill. At least you
would have a reasonable idea what went into it. Wear goggles and heavy
gloves for the first bit!
Chris.
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