carrie jacobson on tue 7 jul 98
Hi all. For those of us who are novices, could someone provide a simple
explanation of eutetics?
Thanks, Carrie
Carrie Jacobson
Pawcatuck, CT
mailto:jacobson@brainiac.com
Craig Martell on wed 8 jul 98
At 08:40 AM 7/7/98 EDT, you wrote:
>----------------------------Original message----------------------------
>Hi all. For those of us who are novices, could someone provide a simple
>explanation of eutetics?
Hi:
Well, it's like Ron Roy has said many times: There are no simple answers to
this stuff. But let's try for one anyway! Also, if you have a copy of
Hamer's book, A Potter's Dictionary of Materials and Techniques, there is a
good section on this point under, "eutectic mixtures".
Lessee here??? A eutectic mixture is the proportion at which two materials
can be blended to give a very complete, fluid melt at a temperature lower
than one would expect. This is only in some cases though. The fluid melt
thing is mostly applicable to eutectic mixtures of fluxing oxides and or
fluxing oxides and silica. Alumina and silica will form a eutectic mixture
at 90% Si to 10% Al but this will not be very fluid, just a lower fusion
point than you would expect from two refractory materials.
So, if you have an oxide that melts at 1000 degrees C and you blend it with
another oxide that melts at 800 degrees C you would expect the melting point
of some proportion of the two to be at between 800-1000 degrees C. However,
a true eutectic mix of the two would be lower than 800 C and sometimes a lot
lower.
I think it's a good thing to be aware of eutectics and materials that will
form a eutectic mixture but eutectics are not something to get hyper about.
When I'm testing materials to make new glaze formulas I look for points
where flux mixtures fuse well but I'm not usually striving for Eutectic
Shangri-La.
Know whut I meen??? I think you do!!
later, Craig Martell-Oregon
Paul Lewing on wed 8 jul 98
Carrie,
In the simplest terms, the principle behind eutectics is this: the
combination of any two materials melts at a lower temperature than
does either material alone. A graph of most combinations (with
increasing temperature on the vertical axis, and going from 100%
Material A on the left to 100% Material B on the right) looks like an
inverted bell curve. However, it's not usually a smooth curve. There
is usually a particular ratio of A to B that melts at a dramatically
lower temperature than the ratios just to either side. So the real
graph becomes a curve with a sharp spike downward at one point. This
point is known as the eutectic point. Does that make sense?
Paul Lewing, Seattle (Glaze Guru Wanna-Be)
carrie jacobson on fri 10 jul 98
Thank you, all who have explained eutetics to this beginner. I have a
better idea of what it's about.
Carrie
Carrie Jacobson
Pawcatuck, CT
mailto:jacobson@brainiac.com
David Finkelnburg on sun 22 nov 09
Dear Ivor,
I missed most of your discussion, didn't even know about it until I got
an off-list note last yesterday thanks to Lee Love, but I'm trying to catch
up in this busy season. I beg your pardon if I've missed some important
part of this.
On November 21 you wrote, "Eutectics in the "Scientific" sense are
highly unusual events in the processing of the things potters and ceramic
artists make." On November 20 you posted, "The only natural example of
Eutectic melting that I have identified relates to a natural mixture of
minerals Soda Felspar and Potash Felspar."
I definitely agree the term eutectic is poorly understood and often
misused by artists but the phenomena exists and it is certainly common.
Eutectic mixtures can and do aid melting at high fire for all the common
alkali and alkaline earth fluxes. Liquid phase sintering may be more
important, especially in boron or lead containing systems, but that's an
entirely different, complex subject.
You once again cited a mix of whiting ( calcium carbonate, ground
limestone), aluminum hydrate (Al2O3 or alumina) and quartz (silica) as
melting at a particular temperature different from what Michael Cardew once
wrote it would melt at. I understand you believe Michael Cardew was wrong.
I don't know, but I accept your assertion.
I do know that whiting decomposes below 1,000C, and the resulting oxide
melts above 2,500C. Alumina melts above 2,000C. Silica melts above
1,600C. Put trays of powders of these three separate materials in a cone 1=
0
firing and they will emerge from the firing still powdery.
However, mix the three together in the ratio of 1 mole CaO: 0.35 moles
Al2O3: 3.5 moles SiO2 and put that mixture in the same firing and the
result will be a glass, a very simple cone 10 clear glaze. The quality of
the glass will depend on the speed of the firing, alumina being a poor
source of Al2O3 for glass or glaze making, but that's not the point. The
point is, the mixture melts at several hundred degrees C lower than the
melting temperature of the lowest melting point ingredient in the mixture!
I don't want to argue semantics. I don't know what you would call that
phenomena. I do know that if you mix alumina and silica and common fluxes,
the mixture melts well below the melting point of any of the ingredients
with the possible exception of potassia (K2O--I'm not sure at what
temperature that melts).
In my article in CM I made exactly the point Tony Ferguson posted, that
we deal with materials that are far more complex than simple phase diagrams
of ideal systems. However, the fundamental idea remains that mixtures of
glaze ingredients melt at lower temperatures than the individual oxides.
The concept of those mixtures melting due to different eutectic composition=
s
is a useful way to consider how glaze melting occurs. Do you have a more
accurate explanation of why the 1:.35:3.5 mix of whiting, alumina and silic=
a
melts at the low temperature it does? I'd really like to hear it and try t=
o
understand it.
All the best,
Dave Finkelnburg
Neon-Cat on mon 23 nov 09
Hi all,
I rarely look at phase diagrams or mess with eutectics. I have read
hundreds of articles where researchers work with and fire materials
under different conditions for different systems. I find these much
more enlightening and useful especially when coupled with hands-on
testing and the basic, relative guidelines we already have in place.
Phase diagrams are constantly being updated -- one fact no one has
mentioned. Lower limits have been found and been reported to be lower
and been accepted by the modern scientific world as lower, one thing
I've consistently noticed.
Science and scientific testing is constantly updating and adding to or
correcting what went before. In our world today we can't hope to
understand things correctly if we're using dated reference material
from the early or mid previous century. Even 5-10 years often makes a
huge difference in what is known and can be understood.
Preparation of materials has also grown more definitive as folks watch
and note particle sizes used and document their source materials.
Standard clays and spars and other natural and synthetic materials can
be procured for testing so everyone is on the same page. Repositories
are held and added to, many in the US, for the named and labeled
source materials. As potters our materials are variable, from particle
sizes to trace elements and oxides. Phase diagrams and an
understanding of eutectics, eutectic points, melting points, etc.,
etc., can get us close but are not the end-all.
We do not need to do high science from scratch to understand what's
behind what we do as potters -- someone or multiple people somewhere
have already done the work for us, and done and documented it well
using fantastic equipment in well funded labs. We need only seek and
find unless one is doing something totally strange and novel -- as in
very, very, very weird. In this day and age we need only be clever
potters and learn how to research if we want more understanding about
the hows and whys behind what we're seeing and producing.
Going to the moon, making huge advances in material sciences, the
advent of computers and all the research behind that whole deal, our
military establishments and our governments in whatever country we
live in have given us potter of the world much that we can use.
Older writings by potters are excellent -- for their time and place as
concerns clay science. Our basic practices have not changed much, just
how we think about it all. I like Daniel Rhodes, for example -- super
smart guy in touch with the scientific world at large. He takes us
right up to the top of the class with what he wrote, hints at quite a
bit more, but never wrote beyond what was understandable in his time
and what was practical for potters to use, understand, and employ. No
fancy bs to make himself appear better than he was -- he was smart and
worked to give us that best. He left the field open for growth and
advancement. Not withstanding the seeming divorce between arts and
science I have to wonder sometimes if we are laying down true paths we
can use now and leave for the future.
We seem to have basic definitions down. Fundamentals are good. Dave
Finkelnburg just gave a good summary. So then work smart. I'd much
rather study a half dozen modern articles where someone has worked
with the system I'm interested in than try to figure it all out for
myself given the limits and constraints inherent in ordinary
potter-life. Kind of like would I rather do complicated math by pencil
and paper or use a calculator and an applicable math program or avail
myself of the work of a reputable publication where someone has
already solved the equation and mapped out the steps. I don't know
about you, but my time and resources are limited so I work smart where
I can.
Or running on intuition I'll try something just for the heck of it and
see what happens with a little kiln magic and a testing of the limits.
There's nothing wrong with that if you don't mind a few failures along
the way...some have kept me laughing and shaking my head for days
while keeping me thinking and studying and interested and having fun
in what I do.
Marian
Neon-Cat
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