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potter density, cristobalite formation,

updated fri 28 feb 97 on tue 4 feb 97

"We know that the same wonderful potters were out there; they must have been
very lonely. Joyce Beautiful Mojave"

Not necessarily true Since I have been thinking about pottery, in the sense
of wanting to make it, there seems to me to have been a real, and large,
increase in the number of people working as studio potters. When I started
thinking about this, in 1968, the books easily available were by Kenney,
Nelson, and Rhodes. The Potters Book was out there, but I didn t find it
easily. At that time, I knew of one person working as a
educator/potter/ceramic artist. I was then married to an art teacher and
feel I would have been at least vaguely aware of others, if lots of them
existed. I think that the seeds of possibility planted in the 50 s and
60 s by pioneers like Leach on his U.S. tours, Mackenzie, Rhodes, Soldner,
and others by their teaching and working found sufficient fertile soil in the
back-to-the-land/self-sufficient/hippie-dippy/artsy-crafty/ ain t it groovy
years in the late 60 s early 70 s. All us boomers were looking for things to
do and quiet a few tried clay and some of them stayed with it. As the
number, and number of kinds, of people involved increased, the possibilities
for commercial interaction increased greatly so we have many more books,
devices, and products aimed at us. This is just an intuitive opinion, but I
think a look at the content and page count of Ceramics Monthly since its
inception supports the opinion to some extent.

Cristobalite formation - 2750 degrees required?

Hammer and Hammer, in the Potter s Dictionary , offer a relatively long
article about the phases of silica. They state that cristobalite formation
from quartz requires days at high temperature to complete. The stated time
is 10 days at 2374 degrees F. for complete conversion. The phase change
starts when ever quartz is heated to 1598 degrees F. (or hotter) and
continues at some speed, faster when hotter, until complete conversion is
achieved. There is an additional silica phase called tridymite that may also
form at normal pressures and elevated temperatures, if some catalysts are
present. There are other phases of silica, like stischovite, that form under
extreme pressure and temperature (meteor impact).

We may remember from our physical chemistry course that a phase of a material
is chemically identically and physically different from some other phase of
the material. The most recognizable phase situation is that of water which
exists in a combination of gas, liquid, or solid phases depending on the
temperature. Not normally recognized are additional ice phases, having
differing crystal structures, that appear as temperature drops and pressure
increases. For some reason, a slight change in crystal structure can be
called by the same mineral name, as in alpha quartz and beta quartz, while a
more substantial change in crystal structure gets a new mineral name, e. g.
Cristobalite. This may have something to do with the ego of scientists since
naming the Joe phase of quartz is not nearly as satisfying as naming Joeite
or Herbertanium.

Phases of a material have a certain set of temperature and pressure (and
maybe other) conditions under which they are stable. Generally, if the
material is exposed to the set of conditions, the particular phase is
produced. In the current example of quartz, if the material is exposed to a
temperature above 573 degrees C., it changes phase from alpha to beta quartz.
This change is easily reversible, like the freezing and melting of water, so
that when the temperature falls, the alpha quartz phase is again present.
This easy reversal is not always possible. In some materials, carbon is an
example, a high temperature-high pressure phase may persist for long periods
of time at earth surface conditions. Diamonds are a form (phase) of carbon
that is stable at conditions found about 20 miles below the surface of the
earth. Graphite is the form (Phase) of carbon that is formed near the
earth s surface. However, diamonds persist for millions of years at earth
surface conditions without any observable change. It is much the same with
crystobalite, once formed it tends to persist. Persist enough to be a
material of commerce that is sold as a material to be added to earthenware
clay bodies.

One thing to note here is that, since these phases exist because of the
recognition of physical, not chemical, differences, they can only be detected
by physical means. In the case of minerals or ceramic objects this means
visual examination, optical microscopic examination of thin sections of the
material, or crystal structural investigation by X-ray crystallography.

Back to cristobalite formation. The stated experimental result, days at high
temperature for complete conversion, is the result of an observation of the
reaction of pure quartz to exposure to elevated temperature. One would
prepare several samples of quartz powder and place them all in the oven at
high temperature. After one day, a sample is removed, cooled, and x-rayed.
The resulting pattern indicates the amount of sample that is quartz and the
amount that is cristobalite. The following day, the next sample is removed,
cooled and x-rayed. The pattern shows less quartz and more cristobalite. If
you have enough samples, the last one will be all cristobalite. If you don t
have enough sample, the graph you plot of percent quartz against time gives
you a time to 100% cristobalite. This is a very tidy experiment and is
almost unrelated to the many unknown things that happen in a clay body or

In the real world there are things that can encourage the formation of a
particular phase of a material. One is a catalyst which I think I would
rather call an epitaxial agent. A catalyst encourages a chemical reaction
and phase changes are not chemical. Since a solid state phase change
involves a change in crystal structure, perhaps something that supplies a
pattern for the new phase can encourage the change. It is a little hard to
visualize how presence of a solid of one structure could influence the
structure of another solid. How would that be communicated? Is a single
point of contact sufficient to relay the "desirability" of the other phase?
The thermodynamic explanation for phase changes or crystal shape formation
is a reduction in a quantity called free energy. One could imagine that the
atoms of a crystal are influenced by the energy fields associated with the
arrangement of atoms in another material around a point of contact. This
influence initiates the alteration of bond angles and lengths which, when
propagated through the crystal becomes the new phase.

This all seems rather mysterious and perhaps improbable. However, there is
empirical evidence that something happens. Adding talc encourages
cristobalite formation. This is a recognized phenomenon. How it happens is
a different question.

Molybdenum Oxide

Conrad - Advanced Ceramics Manual - indicates MoO3 is used as to encourage
crystal development, break up colorants (?), and opacify (at over 2%). He
indicates it gives a slight yellow color and that it is toxic in large doses.

Hammer and Hammer (Potter s Dictionary) indicate that MoO3 is used in yellow
stains and as a color stabilizer. They also state it is soluble and toxic.
(No indication of dose levels)

The combination of weak color, solubility, and toxicity makes MoO3 a less
than ideal glaze material. The colorant disperser idea is interesting but I
wonder if that isn t more from the glass industry side of ceramics where all
things are frits.