free from impurities. Glass-makers differ in their recipes for
colour, of which there exists a great variety. The above will be
sufficient to give a general idea of the principle of colouring.
Suitable cullet, or broken glass,
may be mixed with the batch of the above glasses, in quantities at
The base of all Glasses is sand; and
suitable alkali is the chief solvent. In the Compound Glasses, oxide
of lead, whether in the form of litharge or red lead, as well as the
colouring metals, are active fluxes. These Glasses, therefore, require
less alkali, in proportion to the sand, than the Simple. Flint Glass,
which ought rather to be termed metallic Glass, needs, for its fusion,
less caloric than plate Glass, or any of the Simple Glasses. As the
Simple Glasses require intense heat, they are melted in open pots, so
that the flame comes at once in contact with the materials. Flint and
the Compound Glasses must be melted by the heat going through the pot,
which requires to be covered, or hooded; or the fumes and smoke of the
coal would carbonize or deoxydize the lead, and precipitate it to the
bottom, in its original metallic state.
or oak wood is used for fuel in some parts of France for Flint Glass,
with open pots; and so little carbon is produced by the smoke, as not to
affect materially the metal, although the flames play upon its surface.
Formerly, flints were calcined and ground, for Glass-making; but for many
of Wight, Lynn, or Reigate, sand have been substituted; these sands are
not only more free from iron, but less expensive in the preparation,
than flints when washed and calcined.
Some manufacturers cleanse sand by hand-washing, and others have machines: to render the Isle of Wight sand fit for