Hank Murrow on sun 19 feb 06
On Feb 19, 2006, at 11:02 AM, nsmheralds@netzero.net wrote:
> Southern Ice is made from halloysite. The chemical formula of both
> halloysite and kaolinite is Al2Si2O5(OH)4. Thus, an oxide analysis
> will reveal that the two minerals have identical oxide content. So
> what's the difference? I asked myself the same question. According
> to webmineral.com, kaolinite is a secondary mineral derived from the
> weathering of alumino-silicate minerals and halloysite is a product of
> hydrothermal alteration or surface weathering of aluminosilicate
> minerals, such as feldspars and rhyolitic deposits. Since halloysite
> is hydrothermally altered, its internal structure is different than
> that of ordinary kaolinite, which makes it more likely to be
> transluscent.
Hank weighs in;
This is also true of rhyolite, whose alkalis are within the clay
lattice, rather than next to the clay lattice in the case of
kaolin/feldspar-based clays. This allows for a much more continuous and
gradual melt for these materials. The greatest chance of translucency
is when you eliminate the titania and iron (to name the most common
impurities).
> If you consult your ceramics supply catalog, you should find (if
> they're being helpful enouch) an oxide content analysis of the various
> kaolins, ball clays, felspars, etc, that they carry. You'll find that
> many ball clays are chemically similar to kaolins and that most
> kaolins, like the well-known EPK, contain a good handful of
> impurities. You will also note that Grolleg kaolin is pure kaolinite.
> The absence of impurities also allows for greater transluscency,
> which I think has something to do with the effect those impurities
> have on the final crystalline structure of the fired body. I'm told
> that straight kaolin is almost impossible to throw, so there must be
> more involved. I recently threw a small bowl out of Redding Terrane
> laterite, a material derrived by the weathering of basalt, that is
> mostly kaolinitic clay, alumina and iron oxide. It didn't handle at
> all well, although I'm not sure how much of that had to do with the
> size of the particles (it was pretty lumpy) and how much of it had to
> do with the plasticity of the minerals themselves.
I have prospected and mined a weathered Andesite (similar to basalt,
but cleaner) from the Middle fork of the Santiam River. This material
is very plastic and makes a superb Cone 5 Ox. body. If you add 20%
limestone to the body material.....you have a lovely waxy medium dark
glaze at cone 5 Ox. At cone 10+ it is a lovely jet black when pulled
from the kiln for quick cooling. I have pics and will email them to
anyone interested.
Those who 'know' materials only from their description in a ceramics
catalog or textbook resemble folks looking for a date from the
prospective candidates' description of themselves. Nothing less than
experience 'in the hand' and 'out of the kiln' will suffice to
understand them well.
Cheers, Hank in Eugene,
looking forward to sharing my rocks and fired pots at the Clayart room
at the Red Lion. Anyone else so inclined?
www.murrow.biz/hank
skiasonaranthropos@FSMAIL.NET on mon 20 feb 06
Hello Nathan,
You stated that =93halloysite ... internal structure is different than that=
of ordinary kaolinite, which makes it more likely to be translucent=92 One
problem with that surely is that during the process of becoming
translucent, firing, the structure of both kaolinite and halloysite
breakdown. Consequently the porcelain contains neither kaolinite nor
halloysite rather will be a mixture of glass, mullite, pores with only a
few relicts of the precursor minerals
And without wishing to sound too picky Grolleg is not pure kaolinite as it
contains in excess of 10% feldspar, mica, quartz and a few other minerals.
Its worth remembering that no clay potters use are pure, and hence do not
conform to the theoretical formulas often shown in books. This is true for
many materials used, for example one popular feldspar contains over 30% fee
quartz
Regards,
Antony
| |
|
