SAND GLASS, GLAZE, AND ENAMEL - Archaeological Chemistry - page 116

processes easier to perform and more economical (since less fuel is required
for their completion).
Fluxes are also particularly important in soldering and brazing opera-
tions, where they lower the melting point of the metals or alloys at the
joining site. Moreover, they are effective in cleaning the surface of the
joined parts, rendering them receptive to the solder or brazing metal.
Rosin and borax are examples of fluxes used since antiquity for metal
joining operations. Rosin is generally used mainly for soldering and borax ,
for brazing.
The activity of a flux is due to the cations (positively charged ions) that
it bears. When heating a mixture of a solid and a flux, the cations of the
flux penetrate the atomic lattice of the solid, breaking up bonds (links)
and thus lowering the temperature at which the solid melts. In glassmak-
ing, for example, the cations of the flux break up bonds between silicon
and oxygen atoms in silica sand, the main component of glass; in extrac-
tive metallurgy they break up bonds in the silicate minerals, the main com-
ponents of many metal ores, lowering the temperature at which the
mixture melts. Soda , potash , and borax are examples of fluxes that have
been widely used as fluxes (it is worth noting that these substances have
also been used as cleansers, inhibitors of decay processes (e.g., during
mummification), and mordants for dyeing.
Soda
Soda (composed of sodium carbonate) was acquired in antiquity either in
the form of natron , or, when prepared, as soda ash. Natron is a natural
mixture of sodium bicarbonate, sodium carbonate, lesser amounts of
common salt, and sodium sulfate, and some organic matter. It occurs in a
few places in the world, such as in dry lakebeds in desert regions, in Egypt
and Siberia, for example. From these few sources, natron was traded and
transported to many others in the ancient world, where it was used (von
Lipmann 1937; Lucas 1932).
Potash
Potash (composed of potassium oxide), also a flux, was mainly used as a
glass modifier. It was generally introduced into the glass melt in the form
of either pearl ash , composed of potassium carbonate, vegetable ash , one
of the main constituents of which is potassium carbonate, or saltpeter , a
mineral composed of potassium nitrate.
Soda as well as potash have also been made, since early antiquity, by
burning weeds until only their ash remains - thus known as either soda
ash or potash ash . The ash may also contain as much as 5% of sodium or
potassium carbonate. Kelp , a large seaweed of the order Laminaria , and
barilla plants, of the genus Salsola , which grow on many seashores, have