Chemistry Reference
In-Depth Information
3.2.
GLASS
If silica sand, a crystalline material (composed of silicon dioxide), is heated
until it melts and then cooled rapidly, it hardens into
silica glass
, whose com-
position is identical to that of the original sand, although it lacks crystalline
structure and properties. The temperature required for making silica glass is
however, very high: silica melts above 1700°C, more than 500°C above the
melting temperature of most types of ordinary glass and far beyond the tem-
peratures attainable in antiquity. Tools made from silica glass, such as pre-
historic hatchets and knives, and workshop debris found in the Sahara,
provide evidence of the early human interest in the use of this material. Since
chunks of natural silica glass do occur in the outer crust of the earth, it is
reasonable to recognize that such tools were made from gathered natural
silica glass (Clayton 1998; de Michele 1998).
Soda Glass
The addition of a relatively small amount of such substances as soda, potash,
or borax, all known as
fluxes
or
fluxing materials
, lowers the melting temper-
ature of silica sand from over 1700°C to below 1000°C, a temperature feasi-
ble in ancient furnaces (see Textbox 28).
TEXTBOX
28
FLUX
A
flux
(or
fluxing substance
) is a solid that, when added in minimal
amounts to another solid, promotes melting of the other solid. In other
words, a flux facilitates melting of solids by causing the melting tempera-
ture of a mixture of flux and a solid to be lower than that of the solid
alone. Even in the ancient past, it was realized that the addition of small
amounts of some materials to a large mass of a solid lowers the melting
point of the latter. Adding a flux to minerals or ores that regularly melt at
very high temperatures (above about 1300°C, too high to be reached in
the furnaces and ovens available at the time, for example), results in the
mixture melting at about 1000°C, readily attained in well-ventilated
ancient furnaces. The use of fluxes thus made it possible to perform man-
ufacturing processes that would otherwise have been impossible at the
time. Fluxes have been and still are widely used in such manufacturing
processes as
glassmaking
and metal
smelting
. Adding a flux to the raw
materials of glass or to ores being smelted reduces the melting tempera-
tures required for glassmaking or for metal smelting, making these
