Michael Banks on sat 22 may 99
------------------
Evan, I tried the lime-alumina-silica eutectic mixture too, early in my
ceramic experimenting, probably as have many people on the list. For quite
a while I couldn't figure out why it failed to melt, anywhere as low as 1100
degrees.
What I hadn't appreciated at the time was that eutectics apply to intimately
homogenous mixtures. Stirred-up 200 mesh whiting, alumina and quartz flour
, do not constitute a sufficiently fine-grained or intimate mix. This
mixture is too poor to allow sufficient molecular diffusion in the
characteristically viscous incipient silicate melt to facilitate true
melting. The most intimate mix is a well mixed liquid, or its solid
equivalent, a glass. Therefore frits of whiting, alumina and quartz of the
right eutectic ratio of CaO/Al2O3/SiO2 do melt (or freeze) close to the
temperature on the phase diagram. That's how the eutectic points were
determined in the first place. So you gotta frit it first.
Another factor operating is that some of the potters raw ingredients have
very stable crystalline structures and resist dissolution by the incipient
melt. Their melting rate is too slow.
For example the 200 mesh (or 350 mesh, or 400 mesh, or any mesh) silica you
get from your supplier is NOT silica=21 What=21=21 I hear you say, =
Preposterous=21
But in fact, most commercial =22silica=22 is in fact the mineral alpha =
quartz.
The composition of this is silica of course.... But quartz is a very stable,
cross-linked arrangement of silica-oxygen tetrahedra, which simple resists
melting to a remarkable degree. It does melt eventually at the normal
silica MP, but takes an awful long time. The least melt-resistant form of
silica is silica glass, which has the least crosslinked Si-O bonds. Calling
quartz =22silica=22 is equivalent to calling a diamond =22carbon=22.
E.G. Both diamonds and charcoal are composed of carbon, but in a practical
sense they're as different as chalk and cheese. Diamonds crystalline
structure is a very stable, cross-linked array of tetrahedra, while charcoal
has an amorphous disordered structure. A good example of their relative
stability, is the fact that diamonds can resist being suspended in magma at
1100-1200 degrees, long enough to be transported from +200km deep in the
earth to the surface. Pieces of charcoal do not survive long in magma.
Therefore phase diagrams assume total homogeneity and prolonged heating to
allow equilibrium conditions to be achieved. This obviates the differences
between raw ingredients. Otherwise standardisation would be impossible.
Even diamonds succumb to magma eventually=21
As for anorthite, tridymite and pseudowollastonite, these are the
crystalline products appearing on solidification on slow-cooling of the
eutectic melt. A glaze of this composition, if cooled slowly enough, would
be a opaque matt composed of a crystalline mesh of these minerals.
Anorthite and pseudowollastonite do in fact occur in some lime-matt and
alumna matt glazes.
Michael Banks,
Nelson,
New Zealand
=3E =3EFrom Ivor Lewis ivorredhill=40yahoo.com.au
=3E Monday, 17 May 1999 Australian Central Standard Time
=3E Subject: Eutectic Quandary
=3E =3EFrom time to time people dealing with ceramic problems use the term
=22Eutectic=22.
=3E This was done recently in a posting to Clayart and a recipe was given in
=3E molecular oxide terms with the statement that =22 this Seger forms a
eutectic at
=3E cone 03=22.
=3E
=3E I accept the intent of the person who posted this information. Mr. =
Seger=92s
=3E recipes have always been a good starting point for a glaze composition,
look
at
=3E the popularity of the Seger Cone 8 recipe. It underscores almost eighty
percent
=3E of all published cone 8 glaze recipes.
=3E
=3E Recently I became interested in the confusion which surrounds the use of
the
=3E term =22Eutectic=22. I use the definitions found in the American =
Ceramics
Society
=3E book of Phase Diagrams for Ceramists. The large diagrams which are on =
the
wall
=3E in front of me convey information which shows that a eutectic is a
relationship
=3E between precise mineral compositions. For example, there is a point at
1100
=3E degrees C in the CaO-SiO2-Al2O3 system which seems pretty useful to =
stone
ware
=3E glazers. Fancy having a glaze which melts at such a low temperature, =
think
of
=3E the power and fuel savings. Well chums, I formulated it out of whiting,
flint
=3E and alumina hydrate which were the colosest I could get to the pure =
oxides
and
=3E fired it to cone 8 flat 9 over. Not the prettiest glaze I have ever =
made.
No,
=3E just a friable white powder. Now had I made it out of the mineral
compositions
=3E shown in the chart it would have melted. What are those minerals. On the
chart
=3E is says Tridymite, Anorthite and Pseudowollastonite. You ever heard of
these?
=3E Can you buy them? In fact I ran a whole series of tests combining =
minerals
which
=3E you can buy at the ceramic supply house. The best was from Wollastonite,
Kaolin
=3E and Whiting. This fused and gave an indication that with the addition of=
a
=3E supplementary flux (I would use Soda Felspar) it would be an excellent
glaze.
=3E
=3E Now methinks there is some confusion about what is intended and what can
be
=3E interpreted.
=3E
=3E My confusion arises because there is another term, not often seen, which
is
=3E =22DEFORMATION EUTECTIC=22 (sorry, not shouting) I emphasise because it =
is
hard to
=3E find anything about the concept and most books ignore it. At this point =
I
refuse
=3E to give a reference. A Deformation Eutectic is that point at which any
mixture
=3E of minerals or compounds will start to change shape under influence of =
its
own
=3E weight as it is heated. It could be assigned a temperature if the hot
junction
=3E of the thermocouple were embedded in the specimen.
=3E
=3E So every time we use a cone or a bar which bends or droops we are
employing
the
=3E Deformation Eutectic. We are not using the Thermodynamic Eutectic, =
defined
as
=3E that
=3E
=3E =22=85.invariant (unique temperature, pressure, composition) point for a
system at
=3E which the phase reaction on the addition or removal of heat results in =
an
=3E increase or decrease respectively of the proportion of liquid to solid
phases,
=3E without change of temperature......=22
=3E
=3E Ernest M. Levin et al, =22Phase Diagrams for Ceramists=22 . P 6, note =
(17).
Compiled
=3E by the National Bureau of Standards, 1964 . American Ceramics Society.
Columbus,
=3E Ohio.
=3E
=3E Sadly, though people talk and lecture about Glaze Chemistry the allied
subject
=3E of Glaze Physics seems a forgotten topic. Perhaps it should be revived.
=3E
=3E Best wishes to all. Now I must go and load the kiln for another firing.
=3E
=3E Ivor Lewis.
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