Recipe for Glass...
- Sand-silica/fine sand or pulverized sandstone.
- Soda (wood ashes) or
- Potash (alkali) lowers the melting point. Used in making finer glass
- Lime (a stabilizer)
The properties of glass are varied by adding other substances commonly
in the form of oxides, i.e., lead for brilliance and weight, Boron for
thermal and electrical resistance, Barium to increase the refractive index
(optical), and Cerium to absorb infrared rays. Metallic oxides to impart color
or decolorize.
All sand on earth has some traces of "iron oxides" and the presence of
this iron produces glass of an aqua color in hues ranging greenish to
blue. Around 1673, the English used ground flint to produce "clear" glass.
In 1674, "lead glass" was invented which made glass clear by adding 25%
red lead to the batch. We call this now "lead crystal" and how the term
"crystal clear" came about. Both these methods were expensive, so due to
economic reasons, most old glass is found in aqua. Iron-slag was used in
glass recipes from the 1600's to approximately 1860 or so. This made the
glass a dark olive green or "black" glass.
1880 saw a rise in the demand for "clear" food preserving containers. At
this time, manganese was added to the batch to make glass clear. The
amount of manganese in a given batch, along with the amount of
ultra-violet rays the glass was exposed to, will cause the glass to become
purple in various degrees of density. This color was termed (sca, sun
colored amethyst.) Manganese was most commonly used from 1880 through
1914, WW1 saw our (U.S.) supply cut off by Germany. It was at this time,
selenium was discovered and used as a "decolorizing" agent to make glass
clear. Glass made with selenium that is exposed to ultra-violet light rays
will become either straw, wheat or honey colored. Clear glass produced
from 1914 through 1940 is most apt to change to these colors due to the
selenium content. Modern technology can now separate the iron-oxide out of
the sand.
True colors of blue, green, yellow, brown, purple etc., are produced by
the addition of metallic oxides to the glass batch.
