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eutectics - martell

updated tue 7 jul 98


gambaru on mon 6 jul 98

Don't Stop!! It is always so informative to read of approaches to problem
solving or the methods of the search. All this information is very much
appreciated. MB
-----Original Message-----
From: Craig Martell
To: Multiple recipients of list CLAYART
Date: Saturday, July 04, 1998 9:15 AM
Subject: Re: Eutectics -how practical are they, anyway??

----------------------------Original message----------------------------
At 01:30 PM 7/3/98 EDT, you wrote:
>----------------------------Original message----------------------------
>I have been reading a bit about eutectics and that's left me somewhat
>confused. If some of the glaze theory people have time to comment, I'd
>really appreciate their thoughts.

>Are most working glazes near the eutectic or the eutectic plus more silica?
>Someone said (I think) that the best clear glazes were at the eutectic
>Is the main importance of knowing about eutectics the understanding that
>melting points aren't linear? And that there is the possibility of mixing
>or overlapping two seemingly stable glazes and having them run madly


I hope my wrist holds out so I can voice an opinion about this!

At present, I'm working on some new cone 6 glazes for my wife and also a
supplier who wants to buy some new bases if I can come up with anything
interesting. That's always debatable isn't it???

I'm looking for all kinds of eutectic points at cone 6 so I can add more
silica and alumina to make real durable, functional glazes for this temp.
The first approach is to line blend stuff that is known to form eutectics.
Feldspar and calcium will form a eutectic as will boron and barium for
example. I'm doing flux triaxials and line blends of everything I can think
of, including frits and slip clays. My first considerations are visual
examination of the melted blends to see what proportion of materials will
give the most active melt. Then, I can start blending in the alumina and
silica with the idea that the more aggressive melts can take more Al and Si,
making harder glazes. The other big consideration is color response and
texture which will be influenced by the fluxes present in the glaze. Once I
have glazes that look promising, the next step is to put them into the Seger
formula to see where they are in terms of the suggested limits for that
temp. Anyway, this is sort of a round about answer to your question Autumn.

It may well be true that a lot of good clear glazes are based on a eutectic
point. One would certainly stand a better chance of clarity and good fit
when the proper amounts of Al and Si were blended in. I don't think all
glazes are based on eutectics. Perhaps ash glazes and calcium matts are an
example of this. The calcium in the glaze is much more than needed for a
eutectic melt with feldspar and the main consideration in the matts is
devitrification and crystal development to make the glazes matt. Ash glazes
have high amounts of calcium to promote stringing and separation. The
calcium level in ash glaze Seger formulas is usually about .8 to .9, which
is beyond the magic amount for a spar-calcium eutectic. There are lots of
other examples of eutectic and non eutectic glazes. Shall I go on?? OK, I
can hear you guys snoring so I'll quit.

Gee, my wrist feels pretty good. Maybe typing is good therapy?

later, Craig Martell-Oregon