search  current discussion  categories  glazes - chemistry 


updated sun 11 feb 07


Dan Semler on sat 10 feb 07

phase diagrams and the melt space--long reply to Dan

Hi Dave,

Thanx for the detailed response. I'll see if I can get Kingery's =20
article. I have got Kingery, Bowen and Uhlmann on loan at present and =20
will look there.

I did not mean to imply that eutectics themselves were analogies, =20
but rather that their application to glazes, was analogous rather than =20
strictly accurate. Now, whether that is true remains open to debate. =20
Your point about kinetics is well taken and relates (I believe) to my =20
brief mention of equilibrium in my final par. Most glazes do not =20
achieve that kind of equilibrium, either at melt or finally. Our =20
firings and coolings are just too fast.

Your description of the initial part of melting :

> In the second scenario we pick two powdered
> minerals and mix them together in some intentionally
> non-eutectic composition. However, we have picked
> minerals which, in some proportion, do have a lower
> melting point eutectic than either alone. When that
> eutectic temperature is reached, only in those
> locations where two different crystals are touching
> will there be an opportunity for melting to begin. A
> tiny bit of liquid forms and then melting proceeds. I
> presented evidence in my Masters thesis at Alfred that
> this is exactly what occurs in a glaze on porcelain.

is rather what I was getting at. In a complex multi-component =20
system melt seems most likey to occur in stages, from parts of the =20
complex batch. This is another reason why I'm not sure how to apply =20
eutectics, more than broadly, to questions of melt. It seems that melt =20
in a glaze surface will begin all over the glaze at points where low =20
melting point substances, or combinations of them, can begin to melt =20
as the temp rises. Other material will then be dragged into the melt.

One question I would like to clear up, and its why I presented the =20
defintion of eutectic that I did. As I understand it, a eutectic is =20
the lowest melting point composition of, say 2 (or N) materials. The =20
fact that two components in any proportions will melt at a lower =20
temperature than either alone, does not define a eutectic. Rather it =20
is the specific composition of the, say 2, materials that give the =20
lowest melting point, that is the eutectic. Is my understanding on =20
this point correct ?

The reason I ask the above is this. It appears that the fundamental =20
thing is not that a eutectic exists but that the combination of =20
materials melts at a lower temp than either alone. Eutectic, it =20
appears, is used as a shorthand for this behaviour. Forgive the =20
pedantry here please, but these things are complex and a clear =20
understanding helps.

> It appears you have an excellent understanding of
> eutectics and phase diagrams. I especially like how
> you succinctly define a eutectic and describe the
> phase diagram as a thermal map.

Well thanx, but I'm sure I could disprove this :) The idea of the =20
thermal map came to me from orbifolds. There is a guy I read about in =20
Time or Newsweek, who has mapped musical space. He contends that =20
musicians spend most of there time exploring only the locale they most =20
like or are familiar with. Orbifolds (don't know where the term =20
originates) are maps of the musical space. I have no musical bones in =20
my body, that I've ever discovered, but the concept seemed applicable. =20
Ceramists also stick to a somewhat restricted portion of what might be =20
term glaze space, not least governed by there chosen temp, atmosphere =20
and clay bodies.

One final thing for this salvo. It is worth noting, as you do =20
below, the variability of composition. So-called interesting glazes =20
are often varigated, often wildly so. Naturally this implies =20
non-uniformity in composition. Some parts of the glaze will contain =20
crystalline structures, others glassy ones. Some will contain =20
colouring impurities. And of course, this is further complicated by =20
the glaze body interface, as you point out.