Enamels and enamelling; an introduction to the preparation and ...

THE COMPOSITION AND PROPERTIES OF GLASS. 9
'sideration in comparison with power to withstand external
influences.
The property of resistance in both glass and enamel
entirely depends on their chemical composition, properly con-
stituted glasses being able to withstand the action of the
atmosphere for centuries without appreciable injury ; whilst
if defective, only a very few years will elapse before peculiar
symptoms of decomposition will become manifest the glass
becomes dull, even opaque, and thin scales peel off, this occur-
rence being preceded by iridescence. The last phenomenon
is due not to the formation of coloured compounds but to a
special property possessed by extremely thin strata of colour-
less bodies, and one that is particularly well exemplified in
the case of soap bubbles. After the separation of these scales
the glass appears dull, and exhibits an uneven surface.
This decomposition is the result of the action of atmo-
spheric carbonic acid (carbon-dioxide) which dissolves a portion
of the alkali base and liberates silica. The subjoined analyses
of the external layers and inner portion of an antique Roman
glass vessel plainly indicate the similarity
composition and the weathering
of silicate rocks :
between this de-
External Internal
layer.
mass.
Silica 48-8 59-2
Alumina 3-4 5*6
Lime 11-3 7'0
Magnesia 6'8 1-0
Ferrous oxide 11-3 2-5
Manganous oxide trace trace
Soda . . 21-7
Potash 3-0
Moisture 19-3
Many kinds of glass become dull (" blind ") suddenly, for
the most part after exposure to moderate warmth. This is
frequently observable in the case of potash glass,
and is due
to the formation of an aqueous silicate. The prolonged action
of hot water seems to corrode every kind of glass to a greater