Joseph Herbert on mon 10 aug 98
In the post of Fri, 7 Aug 1998 08:18:51 EDT, Tracy Dotson indicates that
"Most sand is silica." That is generally true but the exceptions are worth
thinking about. The word "sand" describes a size of particle and indicates
nothing about the composition of the particles. Often, but not always, the
grains that survive as sand size particles are quartz. For a non-silica
example, the "sand" on the beaches of the Yucatan coast is all calcium
carbonate. Feels really funny. Many beaches along the bays of the Texas
coast are mostly oyster shell particles, calcium carbonate again. It is
possible, in high mountain lakes, to find feldspar sand. Pink and blocky
grains. Looks funny. Los Alamos, White Sands, in New Mexico is known for
blindingly white gypsum sand. So, some sands will melt easily at kiln
temperatures and some will change form drastically.
The melting point of silica is indeed high, depending on its crystal structure
at the time of melting, at around 3000 degrees F. However, if the "sand" is
not all silica, the melting temperature can be lower, sometimes much lower.
Most pottery glazes, for example, are impure silica powders.
If, instead of saying "sand" one were to say "20 mesh flint", there should be
little problem. That material is nearly pure silica in the form of crypto-
crystalline quartz and the size is in the "sand" range.
One approach to preventing sticking to a shelf is a mixture of alumina hydrate
and wax emulsion applied to a pot foot. When the material is heated, the wax
vaporizes and the hydrate converts to alumina. This leaves a layer of very
refractory material, alumina, where the mixture used to be. Such a layer of
alumina at the bottom of a pot (above the foot) can act as a "brake for runny
glazes. The alumina can increase the viscosity of the glaze enough to keep it
from running further. Mixing relatively coarse flint and wax emulsion can be
used to keep a lid from sticking to a pot. The large grains of flint act as
stilts after the wax vaporizes.
Probably being too picky but it is a sticky subject.
Joseph Herbert
JJHerb@aol.com
| |
|
