Nikom Chimnok on mon 16 jun 97
In Harry Davis' book, THE POTTER'S ALTERNATIVE, he states a theory that
montmorillonite clays contain crystobalite that was formed in volcanoes,
and that it is not necessary to fire such clays to develop
crystobalite--it is already present in the clay.
The clay we are afflicted with here seems to meet the description of
montmorillonite, and it is almost impossible to fire. No matter how long
it has dried, it explodes readily if the temperature rise is greater
than about 20 C per hour, and sometimes even then. It cracks easily, and
mysteriously--one kilnload will be okay, then the next, fired in exactly
the same way, will be almost completely destroyed, every piece cracked.
And oddly enough, pieces that are fired above cone 6 tend not to crack
when cooled rapidly, whereas they do tend to crack when fired to cone
08-3. I once opened the door of a kiln fired to cone 04, and watched
every piece in the kiln crack before my eyes. Tight cooling cracks.
I'm curious whether Davis' theory has since been proven or disproven. We
have a volcano some 60 kilometers away. It leads me to believe that
while bisque-firing, pieces are going through a crystabolite expansion
on their way up, leading to firing cracks. And when fired hot, the
fluxes in the clay are eating the crystabolite, allowing quick cooling
with no problem. Of course this is only an idea; there may well be some
other cause that I haven't thought of.
I would be pleased if someone could comment on this issue, and
especially pleased if you could suggest a way around the firing
problems.
Nikom
Ron Roy on tue 17 jun 97
Big question Nikom but I cannot find anything in my experience to confirm
any part of Harry's theory. I do know there are cristobalite mines
(mountains) and I have seen some in crushed fire brick grog from bricks
that were repeatedly fired above 1100C. I have never seen any in
earthenware bodies fired to O4. Some of the cracking on opening a hot kiln
(above 600C) would result in dunting from the free quartz of which there is
plenty in any low fired body that I have tested. Same kind of dunting you
get in a bisque kiln.
If what Harry said is true then I would expect bentonite to have some.
Because the particles are so fine I also suspect it would aid in the
production of cristobalite.
>----------------------------Original message----------------------------
>In Harry Davis' book, THE POTTER'S ALTERNATIVE, he states a theory that
>
>montmorillonite clays contain cristobalite that was formed in volcanoes,
>
>and that it is not necessary to fire such clays to develop
>
>cristobalite--it is already present in the clay.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
Joseph Herbert on fri 20 jun 97
I will offer a couple of points about volcanos and clays. The major sources
of bentonite, a montmorillonite clay, is the weathered remains of ancient
volcanic ash falls. These ash beds are shards of glass mixed with micro
crystals or crystal fragments. Any crystal that was present in the molten
rock at the time of the eruption could be present in the ash bed. If a
silicon dioxide crystal were present in the high temperature partly molten
rock, you would expect that crystal to be cristobalite, a high temperature
form. If the glass shards are flying through the air and cooling and forming
crystals of silicon dioxide, again you would expcet the high temperature
form.
After the ash has fallen and is exposed to rain and chemical weathering, I
believe the change from glass to montmorillonite clay releases some excess
SiO2. How this material, in solution, would interact with pre-existing
"seed" crystals of cristobalite is an interesting question. The expectation
might be for the low temperature form, quartz, to predominate, but the
existence of an epitaxial surface that readily accepts the material being
deposited can have a strong influence on the eventual crystal form. It is
possible that the cristobalite that is in the ash fall is increased by
subsiquent geochemical processes. As a thought experiment, it seems
plausable. An actual experiment might not be as encouraging.
Joseph Herbert
JJHerb@aol.com
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