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eutectics, phase diagrams and the melt space

updated sat 10 feb 07


Dan Semler on fri 9 feb 07

Hi ya,

I've been re-reading Parmelee on conversion of batch to glaze. I =20
have wondered about the use of eutectics, both as an explanatory =20
device and a practical tool. As I understand it a eutectic has two =20
basic features - 1) it is the lowest melting point composition of a =20
mixture of N components, and 2) at that temperature it wil transition =20
from a liquid to a solid mixture of the phases, not two a solid liquid =20
combination. It appears that in its original use (Thanx Tom, Ivor) its =20
expected that the solid is crystalline. Given glazes are commonly =20
substantially, non-crystalline, I'm not sure how far the use can be =20

Most glazes as far as I can tell very likely are not eutectic =20
compositions in the sense above. Further, even the simplest glazes =20
(well Ivor just sent some examples which contradict this, but in =20
general) have far more components than most of the readily available =20
phase diagrams.

It seems to me that eutectics are a nice shorthand analogy in =20
describing melt, but like all analogies, inaccurate at some point, and =20
misleading thereafter. Another thing about eutectics that bothers me =20
is the general comment that the materials must be in initmate contact. =20
This implies, finely ground materials, and well mixed batches. Of =20
course, I couldn't quantify either. Additionally, there seems to be a =20
geometric limitation to the intimate contact requirement as the number =20
of components grows.

If a eutectic is merely a key (low melting point) composition then =20
the more interesting question is this : how does melting occur ? =20
Hence my reading material.

That said then what are eutectics and phase diagrams good for ? I =20
liked Tom's point of know approximate melt temperatures and thus being =20
able to avoid certain compositions that may be hard to melt. Of =20
course, we know that simple CaO-Al2O3-SiO2 combinations that won't =20
melt at our firing temps, can be made to do so by the addition of =20
other materials.

So I think that the phase diagram can serve as a thermal map of the =20
melt space, and a map of the phases that one might expect to see in =20
crystallised melts of compositions that broadly match those in the =20
diagram. There is may be useful as a guide.

Otherwise, though, not really sure. They do not have enough =20
components in them to match a glaze for complexity, they speak of =20
equalibrium conditions, and they do not address issues of other =20
desirable characteristics in glazes. The question can be asked, is a =20
eutectic composition a good base for a glaze, in the general case, or =20
in the specific case ? I'm not sure at all. Does such an approach =20
necessarily lead to more durable, lower leaching glazes ?

Please leap in and correct anything here as I am trying to get this =20
all straight at present. But this roughly describes my current view.