Bonnema on thu 29 oct 98
Dear Clayarters,
Another question from the Jim Robinson Body Building article in the 6/88
Studio Potter. He calls for silica(amorphous) in most of his glaze tests
and silica(quartz) in only one. Can anyone shed some light on the
difference. We have always used Sil-Co-Sil #52 in our glazes, which is
that?
Thanks,
Garret Bonnema
Bethel Maine
--
Garret Bonnema
bonnema@megalink.net
Craig Martell on sun 1 nov 98
Hi Garret:
I talked to Jim on the phone today and asked him about the silica. He says
that the amorphous silica called for in the tests was actually a very fine
mesh crystalline silica from Illinois Minerals and was a company generated
misnomer. He says you should use Unimin #1240 Silica in the tests. Unimin
is the new name for Illinois Minerals according to Jim.
Also, Jim says that when you get glazes that are real close to fitting, as
in one or two craze lines showing, exchange Unimin Ultra Fine Silica (400
mesh) for 200 or 300 mesh silica to get the glaze to fit. The theory here
is that a finer mesh silica will fuse earlier and more completely, creating
a stronger clay-glaze interface which will stop the crazing. Jim says he's
done this with good success.
He also suggests that people get a product catalog from Unimin and check out
some of the micron particle size silicas that they have. These ultra fines
are great for glazes, where complete fusion is wanted but not for claybodies
where some crystalline silica is desired in the body glass. So, 200 mesh
crystalline is what is recommended for clays.
regards, Craig Martell in Oregon
Joseph Herbert on mon 2 nov 98
Garret Bonnema writes: "He calls for silica (amorphous) in most of his glaze
tests and silica (quartz) in only one. Can anyone shed some light on the
difference"
Ode to a Rock Forming Mineral
Oh! Framework of our glazes, glassformer of the elite and plebe alike, bones
of the crust of the earth, you are silica. We bow at your feet in studious
worship of all your many forms. Silica!! Denizen of beaches and dunes,
Filler of sand boxes and pool filters, Maker of windows, clother of pots,
Silica we salute you. Author of new age crystal dreams, old age carved
skulls, dazzling geodes, and controller of radio, Silica, we adore you!
But I digress.
Silicon Dioxide - the chemical combination we call silica occurs in many
different crystal forms. These different crystal arrangements of the
connections between the silicon and oxygen atoms have separate names. The
most common arrangement of these atoms is in a low temperature three
dimensional connected network that is called quartz. This is the silica of
the clear hexagonal crystals with the six sided pyramid terminations that we
often see in mineral stores or nostrum shops. This material is also the
source of radio frequency control crystals and stove lighters. The curious
property of the crystal produces a spark when squeezed or vibrates when
electrified.
Other similar crystal arrangements of the silicon and oxygen atoms are
cristobalite, coesite, and stischovite. These are higher temperature, higher
pressure forms of silica. Cristobalite is important in glazes and the other
two are found in meteor craters. Really high pressure.
A little different arrangement of the atoms is called cryptocrystalline.
Crypto means hidden and this material has the microscopic structure of quartz
but shows none of the pretty faces of the quartz crystals to the naked eye.
The names given to this kind of material are: flint, jasper, chalcedony, and
agate. The material is chemically and structurally identical to quartz but
the individual crystal units are so small there is no visible expression of
crystal structure. By X-ray identification, the crystal structure is
identical to quartz.
A sort of strange non-crystalline form of silica does exist, the semiprecious
stone Opal. In an opal, there is a kind of glassy waterladen material that
suspends many tiny little balls of crystalline silica. The tiny balls are
close to the wavelength of light in size and they produce the interesting
optical effects seen in opal. The watery component accounts for the ease with
which opal can be damaged by heat.
The answer to the question is that the person referenced uses flint in his
glaze tests and has confused the term cryptocrystalline with the term
amorphous. Flint is easy to get and has the same chemical composition as
megascopically crystalline quartz. There may be some difference in melting
behavior but after the melting has taken place, which it should in a mature
glaze, I would be surprised if there is a difference between the two.
In the world of minerals both the chemical composition and the crystal
structure are important in deciding if the material deserves a name. A
further complication for potters is the fact that all these things end up as
oxides in a melted mass. The fugitive parts of the mineral have evaporated or
burned off and just the oxides join in the melt. Whatever mineral you throw
into your glaze, only the nonvolatile oxides remain to make the glaze. In
some cases, which of two minerals you use doesn't matter since the oxides are
the same in the end.
Joseph Herbert
JJHerb@aol.com
Shelley S on wed 4 nov 98
Joseph,
Re your answer, I would like to add to the question, since it has come up
at our studio lately:
Laguna sells "Amorphous Silica," which it lists in its catalog as such, and
is Imsil A-25. Do you think it may be called for in recipes because of its
finer mesh? What I don't understand is that they also sell a 325 mesh
silica, as a separate product. Is it the same thing?
thanks,
Shelley
Craig Martell on fri 6 nov 98
----------------------------Original message----------------------------
>Laguna sells "Amorphous Silica," which it lists in its catalog as such,
>and is Imsil A-25. Do you think it may be called for in recipes >because
of its finer mesh? What I don't understand is that they also >sell a 325
mesh silica, as a separate product. Is it the same thing?
Hi:
Imsil A-25 is an Illinois Minerals product, or it used to be. I've been
told that the Company name is now Unimin. A-25 is an ultra fine silica with
a 400 mesh or finer rating. I think that Unimin #1240 is 325 mesh and I
don't know if it's actually amorphous or very fine crystalline silica. I
would say that the ultra fines such as A-25 are used in receipes where an
earlier fusion is wanted. I would think that A-25 would be very useful in
the cone 5-6 and lower range in high silica glazes and maybe in some matts.
Finer mesh silicas are said to penetrate the clay glaze interface more and
promote a stronger bond at that point.
I don't know if some of the above is factual, or wishful thinking but the
logic is sound.
regards, Craig Martell in Oregon
Bonnema on fri 6 nov 98
Thanks to the people who wrote such thought out answers to my question on
amorphous vs. quartz silica. Does anyone have a phone number for Illinois
Minerals (Unimin)?
Garret Bonnema
--
Garret Bonnema
bonnema@megalink.net
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