The chemistry of colours depends upon a very nice adjustment of
relative quantities of carbon and oxygen, as well as of colouring oxide,
and of alkali or lead.
is specially affected by an excess of alkali, the colour varying from
deep gold topaz to light amber-like opalescent green, as the alkali
predominates. The proportion of lead is diminished in either case;
and although an excess of alkali extracts most colour from the oxide,
it renders the Glass liable to become unhomogeneous, by the exudation
of its alkali. It is to be regretted that the specific gravities are
not given by Klaproth in his analyses.
Notwithstanding the use of oxide of lead by
the ancients in their coloured Glasses and artificial gems and enamels,
the lightness of the fragments of their white cut Glass indicates the
absence of lead from the constituents of much of the ancient artificial
crystal. A fragment in the possession of Mr, Roach Smith, alluded to in
another part of this work, and introduced into the coloured Plate (3,
fig. 3) can have no lead in its constituents, as its specific gravity
is but 2.049; Flint Glass of the ordinary gravity being 3.200, Plate
Glass about 2.500, and real crystal about 2.500. No doubt, this relic
possessed a large quantity of alkali, probably carbonate of soda or
potash, part of which has exuded by lapse of time, leaving the entire
atoms or particles of undecomposed silex; thus rendering the glass
less compact, of diminished specific gravity, and less conservative.
These fragments probably formed part of a Roman white drinking-glass,
which, in the days of ancient Rome was scarce, and highly valuable.
Had lead formed one of its constituents, the Glass would have been
more permanent; the surface would not have been so much decomposed and
"crazed," as it is technically termed, leaving more or less