The very brilliant and useful substance, which forms the subject of
this article, is said to have been discovered by the Phoenicians.
The story goes that some Phoenician merchants, while cooking their
food on the sands near the seashore, noticed that the ashes of the
plant, with which they made their fire, caused some of the sand to
melt and form a vitreous substance; but whether this tale be true or
not, it is well known that for a long time these people made glass
from the materials which were abundant on their sea and river coasts.
Glass, however, was produced long before this by the Egyptians for the
beads and ornaments used in adorning their mummies, and many specimens
of these are in the British Museum. It is certain also that they well
knew how to make certain substances impart colour to glass for the
manufacture of most of these beads. The Romans made rich goblets of
ruby glass, some of which are to be seen in collections in this
country, as well as urns to receive the ashes of their dead, four of
which, of a green colour, are also in the British Museum. The
manufacture of these vessels proves that this nation was well skilled
in the arts of blowing and modelling glass; and their designs, which
we are now reproducing, show that they were at least not inferior in
artistic skill to those who have formed their taste in this highly
civilized age. We have no record of glass being used for glazing
purposes in ancient times. The Venerable Bede introduced it into this
country about 674 A.D., and employed it in the adornment of
church windows. Ordinary window glass was made at the works in
Crutched Friars in 1557, and plate glass at the large works of the
Ravenhead Plate Glass Company, near St. Helen's in Lancashire. About
1776, flint glass vessels were blown at the establishment in the Savoy
House; and the second Duke of Buckingham brought over Venetian
artists, at that time the most skilled, to make glass for mirrors,
carriage windows, and other useful purposes. Their workshop was in
Lambeth, and the date of their arrival in this country was 1673. The
French were before us in the art of casting glass plates; and in 1688,
Stewart commenced this branch of manufacture, which led to the
establishment of the very famous works of St. Gobain. England has now
large plate glass factories in different parts of the country, and
these together yield as their weekly production at least 140,000
superficial feet of the best polished plate, or seven and a quarter
millions of feet yearly. The value of plate glass made in England
annually, including the rough kinds used for glazing roofs, &c., is
estimated at 1,000,000_l._ France still stands very high, and her
plates are extremely perfect in manufacture. St. Marie d'Oignies, in
Belgium, also sends a considerable quantity of plate glass into the
market. This branch of manufacture has not yet extended to America,
which therefore is a large customer of Europe. Formerly, glass making
was very heavily taxed in this country, and in 1812 an additional duty
was placed on the manufacture of the raw material, which so greatly
depressed it, that the income which the State received fell from
328,000_l._ to 183,000_l._ per annum. Moreover, large quantities of
foreign glass were imported, and this too hindered the development of
the industry amongst us. On the repeal of the duty, however, the trade
began to increase, and has now reached very large dimensions.
Glass appears to be a mixture of silicates, the nature and chemical
composition of which will be explained in a later part of this
article.
The materials used are principally sand, with an alkaline substance,
either a salt of soda or potash and lime, though in some kinds of
glass, oxide of lead takes the place of lime. Other materials are
generally employed to correct impurities which may occur in the sand,
and which, if present, always impart an objectionable colour to the
glass.
There are two kinds of glass in ordinary use: common window glass,
which may be divided into sheet, crown, and plate; and flint glass,
which is used for decanters, wine-glasses, and tumblers; and, in some
special forms, for ornamental stones in imitation of jewels, and also
for lenses of telescopes and microscopes. The materials for making
these different kinds vary somewhat, although the principal
constituents are the same, viz. sand with some salt of soda or potash.
The scientific name for sand, or more properly for its principal
constituent, is silica. This compound silica, or oxide of silicon,
also called silicic acid, possesses properties similar to those which
belong to other acids, namely, it is able, when brought into contact
with bodies of an opposite character under suitable conditions, to
unite with them and to form salts. Everybody knows, that if tartaric
acid be added to carbonate of soda, an effervescence takes place;
carbonic acid passes off in the gaseous state, and the residue is
composed of a portion of the tartaric acid, which unites with the
soda, a double decomposition taking place. If silicic acid be mixed
with carbonate of soda, and if the mixture be heated to a high
temperature, that is, to a white heat, for some length of time, the
same kind of action occurs: carbonic acid goes off, the silica or
silicic acid uniting with the soda; and inasmuch as the soda salt was
originally called _carbonate_ of soda, after this action, in which
carbonic acid is replaced by silicic acid, it is called _silicate_ of
soda. Silicic acid at the ordinary temperature of the air and in the
dry state, has no action whatever upon carbonate of soda, but when
heated sufficiently, the action becomes vigorous. A very interesting
experiment may be performed in illustration of this fact in the
following manner: if a mixture of carbonate of soda and carbonate of
potash be heated in an ordinary fire-clay crucible, and if, when the
mixture is melted, some perfectly dry sand be poured into it,
effervescence will take place, owing to the expulsion of carbonic acid
from the carbonate of soda and potash by means of the silicic acid. If
the operation be performed in such a vessel that the carbonic acid can
be collected, its presence is readily indicated by the usual tests.
This experiment can be easily made by anyone who has ordinary chemical
apparatus at his command. If the mixture of carbonate of potash and
carbonate of soda be melted in a small platinum crucible; and if, when
melted, it be removed quickly while very hot into a tall beaker-glass,
and sand be then poured into it, the escaping carbonic acid will, on
account of its being heavier than air, be retained in the glass, and
its presence can be recognized by its turning lime-water milky (which
is, in fact, a solution of lime in water), owing to the formation of
carbonate of lime produced by the carbonic acid evolved uniting with
the lime dissolved in the water. A mixture of carbonate of soda and
carbonate of potash is here used, because either of these salts
requires a very high temperature to melt it; but when the two are
heated together, the fusibility of both is increased. When sand is
heated with oxide of lead (common litharge) they unite, forming a
compound similar to that produced by the silica uniting with the soda,
as described in the last paragraph. In the first case, a _soda_ glass
is formed; in the second, a _lead_ glass is the result. If these two
glasses be mixed together and melted in a crucible, and if the
proportions in which they are mixed be properly adjusted, and the
materials used be pure, a colourless and transparent glass will be
formed, similar in appearance to that which is employed in the
manufacture of decanters and tumblers. The same kind of glass may be
produced by mixing all the materials in due proportions and heating
them together. If, instead of oxide of lead, lime be mixed with
carbonate of soda and sand, and the mixture be heated to a high
temperature, a glass will be formed, in many respects similar to that
of which oxide of lead is a constituent, but differing from it in
several important particulars. First of all, the lead glass is highly
lustrous, and has a great power of refracting light, so that, when it
is cut, it presents a brilliant appearance, and by refraction readily
produces the prismatic colours. This property does not belong to the
glass containing lime, to anything like the same extent. Lead glass,
too, is much heavier than lime glass, and is therefore unsuited to
many of the purposes for which the latter is generally used, the
principal of which is for the glazing of windows.
If, instead of oxide of lead, which is a chemical compound of lead and
oxygen gas, or lime, which likewise is one of the metal calcium with
oxygen, _carbonate_ of lead or of lime be used, the silicic acid will
expel the carbonic acid from these substances at a high temperature,
just as it does the carbonic acid from the carbonate of soda and
carbonate of potash. It is necessary, for a proper understanding of
the scientific part of our subject, that this fact should be borne in
mind, and that the acid properties of silica should be thoroughly
recognized. Formerly, carbonate of soda was used in the manufacture of
ordinary window glass, but now it is found more economical to employ
_sulphate_ of soda, which is a much earlier product in the manufacture
of soda from common salt than the carbonate, and is therefore less
expensive. Carbonic acid is what chemists call a _weak_ acid, by which
is meant, that its compounds are not so firm and stable, as those
which are formed by other acids with the same substances. Sulphuric
acid is a strong and powerful acid, uniting very readily with the
oxides of certain metals to form very stable compounds. But although
this acid is chemically so powerful in its compounds, yet at a high
temperature it is expelled by silicic acid, showing that this
substance, so inert in its natural state and at the ordinary
temperature of the air, becomes exceedingly active in expelling other
acids and in forming compounds, when put under favourable conditions.
If a mixture of common sand and carbonate of soda, the carbonate of
soda being in excess, be heated, a glass will be obtained which is
slowly soluble in cold, readily soluble in hot water. To these
compounds the name of silicate is given, so that we speak of the soda
compound as silicate of soda, of the lead compound as silicate of
lead, and the lime compound as silicate of lime. Silicate of soda and
silicate of potash, when the alkali, that is to say, the soda or
potash, is in excess, are both soluble. If a solution of one of these
silicates be taken, and if carbonic acid be passed slowly through it,
after a time a gelatinous, white, flocculent substance will be formed
in the liquid, and eventually precipitated. This white flocculent
substance is silicic acid combined with the elements of water, and is
therefore called by chemists hydrate of silica. Now this hydrate of
silica is soluble in water and in hydrochloric acid; and the method by
which it can be brought into solution in water will be explained, when
treating fully of what are called soluble silicates and their
applications.
Soluble silicates are mentioned here, in order that a more perfect
understanding of the nature of silicious compounds may be obtained, by
those who do not possess a scientific knowledge of chemistry. The
silicic acid in the silicate of soda is precipitated or separated out
by carbonic acid, and hence it appears, that an action, exactly the
reverse of that which takes place at a high temperature, occurs, when
the silicic acid is removed from those conditions in which it has been
seen to be (chemically) so active.
Suppose that to a solution of silicate of soda or of potash a soluble
salt of calcium be added--the chloride, for example, which is a
compound of the metal calcium with chlorine--a double decomposition
will take place; the calcium will unite with oxygen in the silicate of
soda, forming lime; and this will again unite with the silicic acid,
forming silicate of lime; while the chlorine will unite with the
sodium, forming chloride of sodium, or common salt.
Here then, silicate of lime is obtained by a process very different
from that which has already been described, namely, by the heating of
lime with silica at a high temperature. The body formed in the latter
case is chemically the same as that produced in the former, there
being present the same weight of calcium, the same weight of oxygen,
and the same weight of silicic acid in each. Again, if to a solution
of silicate of soda, one containing a soluble lead salt, such as the
nitrate, be added, the silicic acid will unite with the oxide of lead
in the nitrate of lead, and the acid constituent of that body will
unite with the oxide of sodium or soda, forming nitrate of soda. It is
apparent, therefore, from these remarks, that in whatever way the
substances be made to unite, the effects produced as regards chemical
composition are the same. If some of the silicate of lime or silicate
of lead made by precipitation be dried and heated to a high
temperature in a crucible, it will melt or fuse, and form a vitreous
substance. In these last cases, as in many others which will have to
be alluded to, the silicates formed are not soluble in water, although
silicate of lime may be partially dissolved when heated in water under
extreme pressure, by which the temperature is considerably increased,
and even slightly in cold water.
To ensure the production of definite silicates by the agency of heat,
the materials must be mixed together in proper combining proportions;
for if more of the metallic oxide is introduced than can combine
chemically with the sand, it will be melted in the mass, but the
excess will not form a definite compound; whereas by precipitation,
the silicates formed always have, when thoroughly washed, a definite
composition. This subject will be again referred to, when the
manufacture of commercial glass is described.
It has been noticed that the glass found in the windows of old
churches and in other places where it has been exposed to the
prolonged action of the air and of moisture, has gradually become
rough on its surface, and has lost to a considerable extent its
transparency. This, which would be a defect in glass for the glazing
of ordinary windows, where transparency is desired, is rightly
regarded as a beauty in glass which is to be used for the
ornamentation of windows. Many reasons have been offered in
explanation of this apparently peculiar property of ancient glass; and
that which appears to be correct is, that glass is a mechanical
mixture of different silicates, some of which may be soluble in water,
and others insoluble. The old window glass, whose manufacture will be
more fully described by-and-by, was made in a less perfect manner than
modern appliances enable glass manufacturers now to produce the same
article, so that the silicates composing the old glass were not as
intimately mixed as those used in modern glass. By the slow action of
air and moisture, portions of the soluble silicates have been
dissolved out, and hence we frequently find a sort of honeycomb
appearance on the surface of ancient glass, as well as a thin film,
which, by refraction of light, causes an opalescence when viewed by
reflected light. Efforts have of late been made to produce a similar
effect by employing different methods in the process of manufacture,
but without complete success. The fact, however, that such changes
have taken place in this less perfectly fused glass, tends to show,
that if one silicate can be dissolved out, there cannot be _chemical_
union between all the silicates. If a piece of modern window glass be
heated in water under pressure in a closed vessel, it will present
somewhat the appearance of ancient glass, for a considerable quantity
of soluble silicate will be dissolved out from it. The object in
dwelling on this matter here, is to induce makers to attend more to
the chemical composition of their glass, for, doubtless, much more
satisfactory results would be obtained both as to the quality of the
material and the cost of its production, if thoroughly scientific
investigations were conducted by a competent chemist.