eutectics, phase diagrams and the melt space--long reply to dan

updated fri 16 feb 07

Dave Finkelnburg on fri 9 feb 07

Dan,
Yikes, you are getting into this deeply! :-) I
don't know much about this but I've thought about it a
lot. I do know this is of interest for some of us but
not necessarily highly useful to artists. Helpful,
yes, all understanding is helpful, but it's certainly
not a requirement to make great-looking glazes.
You ask, "how does melting occur?" I have found
W.D. Kingery's short article, "Sintering in the
Presence of a Liquid Phase," in the book, "Ceramic
Fabrication Processes," to be a concise introduction.
Keep in mind there are solid-state reactions which can
also occur, but generally you want to read about
liquid phase sintering. Randall German has literally
written the book on this subject.
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.
On the point of intimate contact of materials, I
will argue with you.
Also, eutectics are not analogies. They are
material properties of crystalline solids. They
follow very precise natural laws. Glasses are sloppy
and exist within limits--we love 'em but they're
different! :-)
First, though, intimate contact of materials.
Consider two scenarios. In the first we will place
two minerals of different compositions in a dish in a
glaze firing. How do they melt? I don't know, but it
might be like this. We picked minerals that will not
form a eutectic composition with a lower melting point
than that of either mineral. The first mineral to
melt will obviously be the one with the lowest melting
point, provided the melting point is below the peak
firing temperature. When the melting point of the
first mineral is reached it melts, a pool of liquid
forms in place of every piece of that mineral and
liquid phase sintering can proceed.
Phase diagrams, by the way, were what told us the
melting points of the minerals we picked.
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.
One your point 2) the first part is NOT necessarily
so. While a phase diagram is a map of what can exist
at a particular temperature, one must also consider
kinetics. That is the rate at which a mineral crystal
will form from any melt. As the melt cools "a solid
mixture of the phases" will only form IF kinetics
permits. We cool our glazes too rapidly for some
phases to separate, thus we wind up with glass or a
mix of crystals in glass. The literature of geology
has examples of aluminosilicates which crystallize
incredibly slowly despite high temperature because the
atoms aren't very mobile in an alumina-rich melt
(which is very much like a glaze).
You are absolutely right when you say, "Most glazes
as far as I can tell very likely are not eutectic
compositions..." Even if you mix some eutectic
composition, when you fire it on a clay body, the body
takes cations from the melt and contributes cations to
the glaze, so the melt no longer has the composition
you mixed.
All this last bit is of greatest interest to me. I
think it has a big influence on glaze flaws. Today I
received new, unpublished melting point data for a
whole series in the flux:alumina:silica system. If
the data is as good as it appears, I hope to present
it at NCECA. I won't have time to dwell on it, but it
will be part of the glaze panel presentation.
Good glazing!
Dave Finkelnburg

From: Dan Semler
a eutectic has two
basic features - 1) it is the lowest melting point
composition of a
mixture of N components, and 2) at that temperature it
will transition
from a liquid to a solid mixture of the phases, not
two a solid liquid
combination. It appears that in its original use
(Thanx Tom, Ivor) its
expected that the solid is crystalline. Given glazes
are commonly
substantially, non-crystalline, I'm not sure how far
the use can be
pursued.

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

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

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

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

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

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




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Ivor and Olive Lewis on mon 12 feb 07

Dear Dave Finkelnburg,

Though I do not dispute the veracity of your statement, I would be =
pleased to look at evidence from your firings that leads to this =
conclusion <presented evidence in my Masters thesis at Alfred that this is exactly =
what occurs in a glaze on porcelain.>>

What was your Hypothesis? What were the conditions of your observations? =
Was this an isothermal reaction or was temperature still rising? What =
were the melting points and compositions of the materials you were using =
? If the fluid phase is increasing in volume <'...melting proceeds...'> =
what is its source and what was the source of the original fluid ?=20

It seems to me as though you have only confirmed what is common =
knowledge.

Best regards,

Ivor Lewis.
Redhill,
South Australia.

Dear Ivor Lewis,
You ask a great many more questions than I have
time to answer in the detail they deserve. Perhaps it
would be best if you read the thesis.
How would you explain the onset of liquid
formation in a non-homogenous (on an atomic scale)
powder? It is obvious liquid forms in this process or
the glaze would never melt. Do you feel the entire
powder begins to liquify simultaneously? What
evidence do you have to support that? What would the
driving force be for simultaneous melting?
My investigation was not about glaze melting as
such, but rather about the development of bubbles in
glazes. By observing the exceptionally non-random
nature of bubbles in some glaze systems I hypothesized
that the glaze could not be "bulk" sintering, that is,
it could not be melting all at once.
To observe this in action would only take an
Environmental Scanning Electron Microscope (ESEM) with
a "hot" stage capable of achieving glaze melting
temperature. If you have a spare $1,000,000US plus
change, you too can have one of these in your studio!
What you ask for may be within our reach, if not our
budgets, but I have not had the opportunity to see it.
Good glazing,
Dave Finkelnburg

--- Ivor and Olive Lewis
wrote:
> Though I do not dispute the veracity of your
> statement, I would be pleased to look at evidence
> from your firings that leads to this conclusion <> 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.>>




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