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these light tints, and gives a decided superiority to English over
foreign Glass. Dark colours are not used en masse as formerly,
owing to a great loss of light, and apparent shadowy blackness, which is
avoided by the modern practice of casing Flint Glass with one or more
thin coatings of intensely coloured Glass;*—whether of blue
from cobalt, green from iron and copper, ruby from gold; or any colour,
made from other suitable oxide of metal.
The density of the colour may be varied—
darker or lighter, according to the quantity of colouring oxide used,
but it is indispensable that the oxide should be perfectly pure; and the
various Glasses, whether transparent or opaque, requiring to be welded,
or cased, upon each other, in one homogeneous mass, should be of the
same specific gravity; therefore, in mixing, where a large quantity of
colouring oxide is used, less lead must be employed, so as to equalize
the composition, and render uniform the contraction and expansion of the
various layers of Flint and Coloured Glasses.†
The Glass-maker must be ever on the
watch for these essential conditions, or great losses will occur in
the operation
* See the manipulatory explanation of Cased Glass, termed by the French, Double-Triple, &c.
† The artists of the middle
ages leaded their beautiful tints of blue, red, yellow, amethyst, and
green, into windows, either thicker or thinner, of solid or cased Glass,
as the required effects suggested. They also cased a greenish glass
with transparent dark colours, occasionally cutting through the coloured
casing to show white alternately, and staining with yellow the white
parts thus cut out of the colour. Modern Glass-makers produce as fine
colours as those of mediæval manufacture, but time having slightly
dimmed or decomposed the surface of many of the fine old windows, a rich
subdued beauty of colorization is produced, that cannot be imitated in
new Glass.
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