The first object in glass making is to obtain suitable materials. The
sand which is employed for window glass differs from that which is
required for flint glass, in that the latter should be as pure as
possible. The maker can correct the impurities in the window glass
sand, provided they be not present in too great quantities; but it is
far more difficult, in the case of flint glass, to chemically
counteract the influence of those substances which might impair its
tint. So that the manufacturer would rather pay large prices for his
sand, than trust to expedients which in their application might fail,
and thus cause a greater loss.
One of the principal and most troublesome impurities met with in sand,
is iron in the form of oxide. There are two oxides of iron: one, the
protoxide, which imparts a green colour to glass; and the other the
peroxide, whose staining property is yellow. A very small quantity of
the former will give an appreciably green tint, whereas it requires a
large quantity of the peroxide to produce even a delicate yellow. In
all glass making, it is found necessary to use something which will
counteract the colouring properties of these two oxides. The material
employed was black oxide of manganese. This is still used in certain
glass-works, but from its injurious action on the fire-clay pots,
arsenious acid or common white arsenic is employed to effect the same
object. The chemical action in the two cases is different: the black
oxide of manganese is what is termed an oxidizing agent, and gives up,
at a high temperature, a portion of its oxygen to the protoxide of
iron, thereby converting it into the peroxide. It thus becomes
comparatively harmless, by converting a quantity of that oxide, which
gives a green colour, into the other oxide, which has little or no
power of colouring, except it be present in large quantities. The
difficulty in using black oxide of manganese is, the exact
proportioning of it to the quantity of iron present in the sand, a
quantity which cannot be easily determined. If the black oxide of
manganese be used in excess, some of the oxide of manganese remains
unreduced, and, when this is the case, it gives a purple colour to
glass. If used in exact proportions, it is reduced to an oxide which
does not impart colour to glass. This may be seen in many of the old
plate glass windows which were employed for glazing purposes some
sixty or seventy years ago, the colour of the panes being generally
purple.
Since this article was written, I have been consulted by a glass firm
of eminence, as to the use of pure black oxide of manganese in the
manufacture of flint glass, instead of that ordinarily supplied in
commerce. The black oxide of manganese usually sold contains many
other constituents besides black oxide of manganese; amongst these are
iron, copper, cobalt, and alumina.
The iron, as will be seen from what has before been stated, is a
decidedly objectionable ingredient to use along with the manganese.
Copper and cobalt both stain glass, the former of a bluish-green
colour, while the latter makes it blue; and a small quantity of the
latter has great staining power. I have thought it advisable to give
analyses of the black oxides of manganese, and they are as follows:
Binoxide of manganese (Molecule, Mn.O_2), is found native as
pyrolusite or polyanite. Appended are two analyses of pyrolusite
containing sesquioxide of iron.
Red oxide of Manganese 87A.0 72A.5
Oxygen 11A.6 9A.8
Sesquioxide of Iron 1A.3 4A.2
Alumina 0A.3
Baryta 1A.2
Lime 0A.3
Silica 0A.8 1A.4
Water 5A.8 1A.6
----- ----
108A.3 99A.5
The native binoxide often contains both copper and cobalt in addition
to iron; frequently to the amount of as much as 1 per cent. of copper
and about A.54 per cent. of cobalt.
Wad, a native binoxide of manganese, sometimes contains 54A.34 per
cent. of iron, while nearly all the manganese ores contain more or
less alumina, varying from A.5 per cent. to as much as 20 per cent.
From the composition of ordinary commercial black oxide of manganese,
as shown by these analyses, it is evident that it is better to use the
pure article, and this has been found to be the case by the firm who
have adopted it in lieu of commercial black oxide of manganese. I
therefore strongly recommend all glass makers to try and experiment
with it, for the results obtained will largely counterbalance the
extra cost of the pure material; and I also much doubt whether the
same injurious effects will be produced on the pots, as is the case
where commercial manganese is employed.
Arsenious acid also acts as an oxidizing agent, in that it gives up
its oxygen to the protoxide of iron, converting it into the peroxide;
but the arsenic itself, which has lost its oxygen, is reduced to the
metallic state, and being volatile, does not remain with the glass,
but passes off by the flues of the furnace. If too much arsenic is
used, it sometimes renders the glass milky or cloudy.
Before describing in detail the method of mixing and founding glass,
it will be necessary to mention the composition of the vessels in
which the glass is made. They are called glass-pots, and differ in
shape according to the different kinds of glass to be made in them.
Glass-pots are made of fire-clay (generally the best Stourbridge),
which is a silicate of alumina, and here great care is taken to select
that which contains least lime or iron. It is ground, then moistened
and well kneaded together, and left to ripen, while a certain quantity
of old glass-pot is ground fine and mixed with the fresh fire-clay.
Masses about the size of two hands are kneaded separately, the object
being to exclude all air bubbles, and to obtain a perfectly
homogeneous lump. The bottom of the glass-pot is then laid, the masses
of fire-clay being pressed in with the greatest care, so as to avoid
all cracks or places where air might enter during the slow process of
drying.
The modern shape is round; though formerly certain glass-pots, called
_cuvettes_, used in the purifying of plate glass, were square. Pots
used in the manufacture of common crown and sheet window glass,
generally speaking, are larger at the top than at the bottom; but
whatever may be the shape of the pot, the method of its building is
the same. The sides are carefully made of fire-clay, each piece being
laid on by itself and kneaded like the bottom of the pot, so that it
is slowly built up until it reaches the desired height. It is then
dried very gradually, and the process is finished in artificially
warmed chambers. Before putting it in its place in the glass-furnace,
it is allowed to remain for some time in what is called a pot-arch,
that is, an archway built of fire-clay bricks, along the side of which
is a fireplace, by means of which the arch is brought up to a red
heat; and after it has been heated sufficiently, is removed while
red-hot and put into the furnace. Glass-pots are never allowed to
cool, and with care they may last for several months. From this
description of their manufacture, it will be clear that it is attended
with considerable cost, varying from 5_l._ to 10_l._
There are three different kinds of ordinary pots for crown, plate, and
flint glass; and of these the last is decidedly the most expensive, as
its top is covered over, and presents the appearance of a dome with an
opening in front, through which the materials can be introduced when
the pot is charged, and from which, when made, the glass may be drawn,
in order to be blown into shape by the workman. In glass-furnaces the
pots are sometimes arranged in a circle, with their mouths opening
into the glass-house; but now a different construction is sometimes
employed, since other methods of heating the furnaces have been
introduced. It is hardly within the scope of this article to enter
into a description of glass-furnaces; suffice it to state, that they
should be of such a construction as to yield the greatest amount of
well-regulated heat for the smallest consumption of fuel, and this
object seems to be best effected by the adoption of Mr. Siemens'
excellent principle of heating furnaces. For some years his process
has been in use at the Thames Plate Glass Company's Works, where the
saving of fuel has been very considerable, and the glass greatly
improved, owing to the fact that impurities from the fuel employed
cannot possibly find such easy entrance into the glass-pot. In any
case, the construction of the furnace is such, as to be best adapted
to the convenience of the workmen, according to the kinds of glass
which they have to make. Differently arranged furnaces are used for
bottles from those employed for crown and sheet glass.
It has lately come to my knowledge that flint glass, that is to say,
the glass used for tumblers, decanters, and such like, is occasionally
injured by the appearance in it of little opaque white spots. Some
portions of glass of this character have been analyzed by me, when I
found that these white spots were owing to the presence of a glass
containing alumina. Now alumina raises the melting point of any glass
of which it is a constituent. So, then, these white spots were due to
the presence in the flint glass, which was perfectly clear, of a much
less fusible glass which was only partly made when the flint glass was
ready for working. On investigating the matter, it was found that the
alumina came from the glass-pots, for when by my advice the faulty pot
was withdrawn from the furnace and carefully examined, although it had
been in work only six weeks, the bottom was honey-combed to a very
considerable extent, showing that portions of the pot had been
dissolved; and inasmuch as the fire-clay, of which the pots are made,
contains a large quantity of alumina, it was not difficult to trace
the source of these white spots which had rendered useless much very
valuable glass. On inquiry it was found that the pots had been made
entirely of new clay, and on reference to the book of workings, which
was kept in the glass-house, it was also found that for some time, the
glass-pots used in that establishment had been made of new clay, and
that on a previous occasion a similar calamity had before happened.
In the records kept where pots were made, as has already been
described, with a portion of old pot as well as new clay, no white
spots had ever appeared in the glass. It is therefore manifest, that
it is much safer to use a portion of old pot than to trust to pots
made entirely of new clay.
Having considered briefly the manufacture of glass-pots, I shall
proceed to the treatment of the materials to be employed. In making
common window glass, ordinary sand, which does not contain any very
large quantity of iron, may be used, the alkali employed being
sulphate of soda, while the purifying material is either arsenic or
black oxide of manganese. A small quantity of anthracite coal is added
to the mixture, in order to assist in the reduction of the sulphate of
soda, together with some lime. The materials are carefully mixed and
placed in the furnace, where they are heated for some time, a process
which is called "fritting." Its object is to perfectly dry the
materials, so as to expel carbonic acid gas, which would otherwise
cause swelling in the glass; but no combination must take place, to
allow of silicates being formed, otherwise the alkali would melt first
and attack the substance of the glass-pots, and part of it would be
volatilized and lost. When this operation is completed, the fritt is
put into the hot glass-pot, and submitted to the action of the heat of
the furnace, until the glass is made, or "founded," as it is
technically termed. In the case of sheet and crown glass, this process
lasts from sixteen to seventeen hours, for it will be remembered that
the top of the pot is open to the furnace, so that the flames pass
over the surface of its contents. In this way the materials get heated
more rapidly than when a covered glass-pot is used.