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eutectics - long again!

updated tue 14 jul 98


Stairs interlog on mon 13 jul 98

Hi Everybody,

I guess I can't resist this one any longer.

A eutectic is, as the name sort of says, a mixture in which every thing
melts at the same time. There are several tricks to it though. Every
multiphase material (ok, ok, there are some wierdos like borates) has at
least one eutectic composition. Take Ron Roy's mixture, for example:

>CaO - 23.3
>Al2O3 - 14.7
>SiO2 - 62.2

He says this is supposed to melt at 1170C. And it probably does... the
second time around. But the first time through, it will not melt until at
least the first raw material has begun to melt. Actually, to split hairs,
it may begin to melt at a somewhat lower temperature at which the atoms
have time to diffuse across the distance of the particle size. This is our
old friend heat work again. But lets stick with the temperature for a while.

If you made this mixture up out of pure materials, you'd have to go up to a
rather high temperature (I don't have the references in front of me, so I
can't say exactly what) to get the CaO to melt first. Then the silica and
reluctantly the alumina would begin to dissolve into the flux. When that
process was completed, you would have an equilibrium state (thermodynamics
term meaning that no further changes will occur no matter how long you heat
it) in which the liquid is homogenous and stays that way even when you cool
it past the solidification temperature. That's a eutectic mixture. Now,
if you heat it again, it will melt at the eutectic temperature, all at the
same temperature, like a cube of ice. This is what frits are about, by the
way. Well, partly, anyway. That's another can of worms I'd better not touch.

Now, for a 3-phase mixture like Ron's, there are actually more than one
eutectic. That is, more than one different composition of these materials
that behave as I have described, having different melting points. The more
materials you mix together, the more complex the picture, and the more
eutectics you get, in general. Also, the lower the melting point of the
lowest melting point eutectic. So, adding many materials is one of the
means we have of making low melting point glazes. Just how this works is
the next twist.

If you have a mixture which is just off the eutectic point (I'm ignoring a
few subtleties here: wait for the advanced lessons for those), the mixture
will begin to REmelt at the eutectic temperature, but it will not form a
liquid: it will form a slurry of a solid (of the excess material) in a
liquid WHICH HAS THE EUTECTIC COMPOSITION. Sorry for the caps. Got
carried away. So, the local governing eutectic sets the REmelting
temperature of the glaze, a point we call the solidus. As the temperature
rises, more of the excess material dissolves until finally it all dissolves
and we have a liquid with no chunks. At this point there is no eutectic
phase, and we have reached the temperature called the liquidus. The glaze
has just now flowed off the pot.

The reason eutectics are not good glazes are several. First, they are
touchy: one moment they're solid, and the next they're liquid. And,
because of the little stuff that I omitted above, if you get the materials
wrong on one side of the equation the temperature can be quite different
than if you get them wrong on the other side. So they're touchy. Next,
when they go liquid, they are completely fluid, and may flow off the pots.
Normal glazes have some degree of thixitropy, which is a big word to say
that they tend to gel and resist flow to some degree. This is good.

There's much more to say, of course, but this is long enough already, don't
you think?

Bye from Geneva, Gavin