8. Enamel; silicate and stannate or antimoniate of potash or soda, and
lead.
The glasses which contain several bases are liable to suffer different
changes when they are melted or cooled slowly. The silica is divided
among these bases, forming new compounds in definite proportions, which
by crystallizing, separate from each other, so that the general mixture
of the ingredients which constituted glass is destroyed. It becomes then
very hard, fibrous, opaque, much less fusible, a better conductor of
electricity and of heat; forming what Reaumur styled _devitrified_
glass; and what is called after him, Reaumur’s porcelain.
This altered glass can always be produced in a more or less perfect
state, by melting the glass and allowing it to cool very slowly; or
merely by heating it to the softening pitch, and keeping it at this heat
for some time. The process succeeds best with the most complex vitreous
compounds, such as bottle glass; next with ordinary window glass; and
lastly with glass of potash and lead.
This property ought to be kept constantly in view in manufacturing
glass. It shows why in making bottles we should fashion them as quickly
as possible with the aid of a mould, and reheat them as seldom as may be
absolutely necessary. If it be often heated and cooled, the glass loses
its ductility, becomes refractory, and exhibits a multitude of stony
granulations throughout its substance. When coarse glass is worked at
the enameller’s lamp, it is apt to change its nature in the same way, if
the workman be not quick and expert at his business.
From these facts we perceive the importance of making a careful choice
of the glass intended to be worked in considerable masses, such as the
large object glasses of telescopes; as their annealing requires a very
slow process of refrigeration, which is apt to cause devitrified specks
and clouds. For such purposes, therefore, no other species of glass is
well adapted except that with basis of potash and lead; or that with
basis of potash and lime. These two form the best flint glass, and crown
glass; and they should be exclusively employed for the construction of
the object glasses of achromatic telescopes.
GLASS-MAKING, _general principles of_. Glass may be defined in technical
phraseology, to be a transparent homogeneous compound formed by the
fusion of silica with oxides of the alkaline, earthy, or common metals.
It is usually colourless, and then resembles rock crystal, but is
occasionally stained by accident or design with coloured metallic
oxides. At common temperatures it is hard and brittle, in thick pieces;
in thin plates or threads, flexible and elastic; sonorous when struck;
fracture conchoidal, and of that peculiar lustre called vitreous; at a
red heat, becoming soft, ductile and plastic. Besides glass properly so
called, other bodies are capable of entering into vitreous fusion, as
phosphoric acid, boracic acid, arsenic acid, as also certain metallic
oxides, as of lead, and antimony, and several chlorides; some of which
are denominated glasses. Impure and opaque vitriform masses are called
slags; such are the productions of blast iron furnaces and many
metallurgic operations.
Silica, formerly styled the earth of flints, which constitutes the basis
of all commercial glass, is infusible by itself in the strongest fire of
our furnaces; but its vitreous fusion is easily effected by a competent
addition of potash or soda, either alone or mixed with lime or litharge.
The silica, which may be regarded as belonging to the class of acids,
combines at the heat of fusion with these bases, into saline compounds;
and hence glass may be viewed as a silicate of certain oxides, in which
the acid and the bases exist in equivalent proportions. Were these
proportions, or the quantities of the bases which silica requires for
its saturation at the melting point, exactly ascertained, we might
readily determine beforehand the best proportions of materials for the
glass manufacture. But as this is far from being the case, and as it is,
moreover, not improbable that the capacity of saturation of the silica
varies with the temperature, and that the properties of glass also vary
with the bases, we must, in the present state of our knowledge, regulate
the proportions rather by practice than by theory, though the latter may
throw an indirect light upon the subject. For example, a good
colourless glass has been found by analysis to consist of 72 parts of
silica, 13 parts of potash, and 10 parts of lime, in 95 parts. If we
reduce these numbers to the equivalent ratios, we shall have the
following results; taking the atomic weights as given by Berzelius.
1 atom potash = 590 14·67
1 lime 356 8·84
3 silica 1722 42·79 } 71·49
2 silica 1155 28·70 }
---- -----
3823 95·00
This glass would therefore have been probably better compounded with the
just atomic proportions, to which it nearly approaches, viz. 71·49
silica, 14·67 potash, and 8·84 lime, instead of those given above as its
actual constituents.
The proportions in which silica unites with the alkaline and other
oxides are modified by the temperature as above stated; the lower the
heat, the less silica will enter into the glass, and the more of the
base will in general be required. If a glass which contains an excess of
alkali be exposed to a much higher temperature than that of its
formation, a portion of the base will be set free to act upon the
materials of the earthen pot, or to be dissipated in fumes, until such a
silicate remains as to constitute a permanent glass corresponding to
that temperature. Hence the same mixture of vitrifiable materials will
yield very different results, according to the heats in which it is
fused and worked in the glass-house; and therefore the composition
should always be referrible to the going of the furnace. When a species
of glass which at a high temperature formed a transparent combination
with a considerable quantity of lime, is kept for some time in fusion at
a lower temperature, a portion of the lime unites with the silica into
another combination of a semi-vitreous or even of a stony aspect, so as
to spoil the transparency of the glass altogether. There is probably a
supersilicate, and a subsilicate formed in such cases; the latter being
much the more fusible of the two compounds. The Reaumur’s porcelain
produced by exposing bottle glass to a red heat for 24 hours, is an
example of this species of vitreous change in which new affinities are
exercised at a lower temperature. An excess of silica, caused by the
volatilization of alkaline matter with too strong firing, will bring on
similar appearances.
The specific gravity of glass varies from 2·3 to 3·6. That of least
specific gravity consists of merely silica and potash fused together;
that with lime is somewhat denser, and with oxide of lead denser still.
Plate glass made from silica, soda, and lime, has a specific gravity
which varies from 2·50 to 2·6; crystal or flint glass from 3·0 to 3·6.
The power of glass to resist the action of water, alkalis, acids, air,
and light, is in general the greater, the higher the temperature
employed in its manufacture, the smaller the proportion of its fluxes,
and the more exact the equivalent ratios of its constituents. When glass
contains too much alkali, it is partially soluble in water. Most crystal
glass is affected by having water boiled in it for a considerable time;
but crown glass being poorer in alkali, and containing no lead, resists
that action much longer, and is therefore better adapted to chemical
operations. The affinity of glass for water, or its hygrometric
attraction, is also proportional to the quantity of alkali which it
contains. In general also potash glass is more apt to become damp than
soda glass, agreeably to the respective hygrometric properties of these
two alkalis, and also to the smaller proportion of soda than of potash
requisite to form glass.
Air and light operate upon glass probably by their oxidizing property.
Bluish or greenish coloured glasses become by exposure colourless, in
consequence undoubtedly of the peroxidizement of the iron, to whose
protoxide they owe their tint; other glasses become purple red from the
peroxidizement of the manganese. The glasses which contain lead, suffer
another kind of change in the air, if sulphuretted hydrogen be present;
the oxide of lead is converted into a sulphuret, with the effect of
rendering the surface of the glass opaque and iridescent. The more lead
is in the glass, the quicker does this iridescence supervene. By boiling
concentrated sulphuric acid in a glass vessel, or upon glass, we can
ascertain its power of resisting ordinary menstrua. Good glass will
remain smooth and transparent; bad glass will become rough and dim.
The brittleness of unannealed glass by change of temperature is
sometimes very great. I have known a thick vessel to fly by vicissitudes
of the atmosphere alone. This defect may be corrected by slowly heating
the vessel in salt-water or oil to the highest pitch consistent with the
nature of these liquids, and letting it cool very slowly. Within the
limits of that range of heat, it will, in consequence of this treatment,
bear alternations of temperature without cracking as before.
It has been said that glass made from silica and alkalis alone, will not
resist the action of water, but that the addition of a little lime is
necessary for this effect. In general 100 parts of quartzose sand
require 33 parts of dry carbonate of soda for their vitrification, and
45 parts of dry carbonate of potash. But to make unchangeable alkaline
glass, especially with potash, a smaller quantity of this than the above
should be used, with a very violent heat. A small proportion of lime
increases the density, hardness, and lustre of glass; and it aids in
decomposing the alkaline sulphates and muriates always present in the
pearl ash of commerce. From 7 to 20 parts of dry slaked lime have been
added for 100 of silica, with advantage, it is said, in some German
glass manufactories, where the alkaline matter is soda; for potass does
not assimilate well with the calcareous earth.
In many glass works on the Continent, sulphate of soda is the form under
which alkaline matter is introduced into glass. This salt requires the
addition of 8 per cent. of charcoal to decompose and dissipate its acid;
a result which takes place at a high heat, without the addition of any
lime. 88 pounds of quartz-sand, 44 pounds of dry glauber salt, and 3
pounds of charcoal, properly mixed and fused, afford a limpid, fluent,
and workable glass; with the addition of 17 pounds of lime, these
materials fuse more readily into a plastic mass. If less carbon be
added, the fusion becomes more tedious. The two following formulæ afford
good glauber salt glass.
1. 2.
------ ----
Sand 100 60·3
Calcined sulphate of soda 50 26·8
Lime 20 10·8
Charcoal 2·65 2·1
The first mixture has been proved in the looking-glass manufactory of
Neuhaus near Vienna, and the second by the experiments of Kirn. The
fusion of the first requires 18, of the second 21 hours. The
bluish-green tinge which these otherwise beautiful and brilliant glasses
possess, is not removable by the ordinary means, such as manganese or
arsenic, which decolour alkaline glass. When the sulphate of soda and
charcoal are used in smaller proportions, the glass becomes more
colourless. The tinge is no doubt owing to the sulphur combining with
the oxide of sodium, in some such way as in the pigment _ultramarine_.
By a proper addition of galena (the native sulphuret of lead), to
glauber salt and quartz sand, without charcoal, it is said a tolerably
good crystal glass may be formed. The sulphuric acid of the salt is
probably converted by the reaction of the sulphuret of lead into
sulphurous acid gas, which is disengaged.
One atom of sulphuret of lead = 1495·67, is requisite to decompose 3
atoms of sulphate of soda = 2676. It is stated, on good authority, that
a good colourless glass may be obtained by using glauber salt without
charcoal, as by the following formula.
Quartz-sand 100 pounds
Calcined glauber salt 24
Lime 20
Cullet of soda glass 12
The melting heat must be continued for 26-1/2 hours. A small quantity of
the sand is reserved to be thrown in towards the conclusion of the
process, in order to facilitate the expulsion of air bubbles. The above
mixture will bear to be blanched by the addition of manganese and
arsenic. The decomposition of the salt is in this case effected by the
lime, with which the sulphuric acid first combines, is then converted
into sulphurous acid, and dissipated. Glass made in this way was found
by analysis to consist of 79 parts of silica, 12 lime, and 9·6 soda,
without any trace of gypsum or sulphuric acid.
Glauber salt is partially volatilized by the heat of the furnace, and
acts upon the arch of the oven and the tops of the pots. This is best
prevented by introducing at first into the pots the whole of the salt
mixed with the charcoal, the lime, and one fourth part of the sand;
fusing this mixture at a moderate heat, and adding gradually afterwards
the remainder of the sand, increasing the temperature at the same time.
If we put in the whole ingredients together, as is done with potash
glass, the sand and lime soon fall to the bottom, while the salt rises
to the surface, and the combination becomes difficult and unequal.
Sulphate of potash acts in the same way as sulphate of soda.
Muriate of soda also, according to Kirn, may be used as a glass flux
with advantage. The most suitable proportions are 4 parts of potash, 2
of common salt, and 3 of lime, agreeably to the following
compositions:--
1. 2.
---- ----
Quartz-sand 60·0 75·1
Calcined carbonate of potash 17·8 19·1
Common salt 8·9 9·5
Lime 13·3 14·3
For No. 1., the melting heat must be 10 hours, which turns out a very
pure, solid, good glass; for No. 2., 23 hours of the furnace are
required. Instead of the potash, glauber salt may be substituted; the
proportions being then 19·1 glauber salt, 9·5 muriate of soda, 14·3
lime, 75·1 sand, and 1·3 charcoal.
The oxide of lead is an essential constituent of the denser glasses, and
may be regarded as replacing the lime, so as to form with the
quartz-sand a silicate of lead. It assimilates best with purified pearl
ash, on account of the freedom of this alkali from iron, which is
present in most sodas.