1. Glass is a substance produced from a combination of silicious
earths with alkalies, and, in many cases, with metallic oxydes. The
basis of every species of glass is silex, which is found in a state
nearly pure in the sands of many situations. It is also found in the
common flints and quartz pebbles.
2. When quartz pebbles or flints are employed, they must be first
reduced to powder. This is done by grinding them in a mill, after they
have been partially reduced, by heating them in the fire, and plunging
them into cold water. Sand has the advantage of being already in a
state of division sufficiently minute for the purpose. To prepare it
for application, it only requires to be washed and sifted, in order to
free it from the argillaceous and other substances unfit for use. A
great proportion of the sand employed in the manufacture of the better
kinds of glass in the United States, is taken from the banks of the
Delaware River.
3. The alkaline substances used are potash and soda. For the finer
kinds of glass, pearlash, or soda procured by decomposing sea-salt, is
used; but, for the inferior sorts, impure alkalies, such as barilla,
Scotch and Irish kelp, and even wood-ashes, as well as the refuse of
the soap-boiler's kettle, are made to answer the purpose. Lime, borax,
and common salt, are also frequently used as a flux in aid of some of
the other substances just mentioned.
4. Of the metallic oxydes which make a part of the materials of some
glass, the deutoxyde of lead, or, as it is usually denominated, red
lead, is the most common. This substance is employed in making flint
glass, which is rendered by it more fusible, heavy and tough, and more
easy to be ground or cut, while, at the same time, it increases its
brilliancy and refractive power.
5. Black oxyde of manganese is also used in small quantities, with the
view of rendering the glass more colorless and transparent. Common
nitre produces the same effect. White arsenic is also added to the
materials of this kind of glass, to promote its clearness; but, if too
much is used, it communicates a milky whiteness. The use of this
substance in drinking vessels is not free from danger, when the glass
contains so much alkali as to render any part of it soluble in acids.
6. The furnace in which the materials are melted is a large conical
stack, such as is represented at the head of this article. In some
cases, it is surrounded by a large chimney, which extends above the
roof of the building. In the sides are several apertures, near which
are placed the crucibles, or melting-pots, containing the materials.
The fuel is applied in an arch, which is considerably lower than the
surface of the ground on which the operators stand, while at work.
7. The melting-pots are made chiefly of the most refractory clays and
sand. Much of the clay used for this purpose, in many of the
glass-houses in the United States, is imported from Germany. The
materials, having been sifted, and mixed with a suitable quantity of
water, the homogeneous mass is formed into crucibles, by spreading it
on the inside of vessels which are much in the shape of a common
wash-tub. After the clay has become sufficiently solid to sustain
itself, the hoops are removed from the vessel, and the several staves
taken apart.
8. The crucibles are suffered to dry in the atmosphere for two or
three months, after which they are applied to use as they may be
needed. Before they are placed in the main furnace, they are gradually
raised to an intense heat in one of smaller dimensions, built for this
express purpose. The fuel employed in fusing the _metal_ is chiefly
pine wood, which, in all cases, is previously dried in a large oven.
Four of the five furnaces near Philadelphia, which belonged to Doctor
Dyott, were heated with rosin.
9. The materials having been mixed, in the proposed proportions, which
are determined by weight, they are thrown into the melting-pots, and,
by a gradually increasing heat, reduced to a paste, suitable for
application by the blower. This part of the process is commonly
performed at night, while the blowers are absent from the works.
10. The applications of glass are so exceedingly extensive, that it is
inconvenient, if not impossible, to manufacture every species of it at
one glass-house or at one establishment. Some, therefore, confine
their attention to the production of window glass, and such articles
of hollow ware as may be made, with profit, from the same kind of
paste. Others make vials and other species of ware, employed by the
druggist, apothecary, and chemist. And again, the efforts, at some
factories, are confined entirely to the manufacture of flint glass, or
to that of plate glass for mirrors.
11. The principal operations connected with the manufacture of
different species of glass, after the paste has been prepared, may be
included under the following heads; viz., blowing, casting, moulding,
pressing and grinding; although all these are never performed in one
and the same establishment.
12. _Blowing._--The operation of blowing is nearly or quite the same
in the production of every species of glass ware, in which it is
employed. The manipulations, however, connected with making different
articles, are considerably varied, to suit their particular
conformation. This circumstance renders it impossible for us to give
more than a general outline of the process of this manufacture.
13. In the formation of window glass, the workman gathers upon the end
of an iron tube a sufficient amount of the metal, which he brings to a
cylindrical form by rolling it upon a cast iron or stone table. He
then blows through the tube with considerable force, and thus expands
the glass to the form of an inflated bladder. The inflation is
assisted by the heat, which causes the air and moisture of the breath
to expand with great power.
14. Whenever the glass has become too stiff, by cooling, for
inflation, it is again softened by holding it in the blaze of the
fuel, and the blowing is repeated, until the globe has been expanded
to the requisite thinness. Another workman next receives it at the
other end, upon an iron rod, called a _punt_, or _punting iron_, when
the blowing iron is detached. It is now opened, and spread into a
smooth sheet, by the centrifugal force acquired by the rapid whirl
given to it, in the manner exhibited in the preceding cut. The sheet
thus produced is of a uniform thickness, except at the centre, where
the iron rod had been attached.
15. An inferior kind of window glass, the materials of which are sand,
kelp, and soap-boilers' waste, is made by blowing the _metal_ into
cones, about a foot in diameter at their base; and these, while hot,
are touched on one side with a cold iron dipped in water. This
produces a crack, which runs through the whole length of the cone. The
glass then expands into a sheet somewhat resembling a fan. This is
supposed to be the oldest method of manufacturing window or plate
glass.
16. The window glass produced in the manner first described, is called
_crown glass_; and the other, _broad glass_. But by neither of these
methods can the largest panes be produced. The blowing for these
differs from the methods just described, in that the material is blown
into an irregular cylinder, open at its further end. When a sufficient
number of these cylinders have accumulated, the end to which the
blowing iron had been attached, is _capped off_ by drawing round it a
circle of melted glass, and the cylinder is divided longitudinally by
touching it through its whole length with a hot iron. The cylinders,
in this state, are put into the annealing oven, where, by aid of a
heat which raises the glass to redness, it is expanded into sheets.
These sheets are then broken into panes of several sizes by the aid of
a diamond and a straight edge, as in the case of glass blown by other
methods.
17. _Casting._--Plate glass formed by the method last mentioned, is
denominated _cylinder glass_; and it is used not only for windows, but
also for mirrors not exceeding four feet in length. Plates of greater
dimensions are produced by a process called _casting_. The casting is
performed by pouring the material, in a high state of fusion, upon a
table of polished copper of large size, and having a rim elevated
above its general surface, as high as the proposed plate is to be
thick. To spread the glass perfectly, and to render the two surfaces
parallel, a heavy roller of polished copper, resting upon the rim at
the edges, is passed over it.
18. Plates thus cast are always dull and uneven. To render them good
reflectors, it is necessary to grind and polish them. The plate to be
polished is first cemented with plaster of Paris to a table of wood or
stone. A quantity of wet sand, emery, or pulverized flints, is spread
upon it, and another glass plate, similarly cemented to a wooden or
stone surface, is placed upon it. The two plates are then rubbed
together, until their surfaces have become plane and smooth. The last
polish is given by colcothar and putty. Both sides are polished in the
same manner.
19. _Moulding._--Ornamental forms and letters are produced on the
external surface of vessels, by means of metallic moulds; and the
process by which this kind of work is performed is called moulding. In
the execution, the workman gathers upon the end of his iron tube, a
proper amount of the material, which he extends, and brings to a
cylindrical form, by rolling it upon his table. He then expands it a
little by a slight blast, and afterwards lets it down into the mould,
which is immediately filled by blowing still stronger through the
tube.
20. The vessel is then taken from the mould, and disengaged from the
tube. The same tube, or a punting iron having been attached to the
bottom, the other end is softened in the fire, and brought to the
proposed form with appropriate tools, while the iron is rolled up and
down upon the long arms of the glass-blower's chair. The ornamental
moulds are made of cast iron, brass, or copper, and are composed of
two parts, which open and shut upon hinges. The moulds for plain
vials, castor oil bottles, small demijohns, &c., are made of the kind
of clay used for the crucibles. These consist merely of a mass of the
clay, with a cylindrical hole in it of proper diameter and depth.
21. _Pressing._--This process is applied in the production of vessels
or articles which are very thick, and which are not contracted at the
top. The operation is performed in iron moulds, which consist of two
parts, and which have upon their internal surfaces the figures to be
impressed upon the glass. The material, while in an elastic condition,
is put into the lower part of the mould; and the other part, called
the _follower_, is immediately brought upon it with considerable
force.
22. Every species of glass, before it can be used with safety, must be
_annealed_, to diminish its brittleness. The annealing consists merely
in letting down the temperature by degrees. Small boys, therefore,
convey the articles, whatever they may be, as fast as they are made,
to a moderately heated oven, which, when filled, is suffered to cool
by degrees.
23. _Cutting._--The name of _cut glass_ is given to the kind which is
ground and polished in figures, appearing as if cut with a sharp
instrument. This operation is confined chiefly to flint glass, which,
being more tough and soft than the other kinds, is more easily
wrought. In addition to this, it is considerably more brilliant,
producing specimens of greater lustre.
24. An establishment for grinding glass contains a great number of
wheels of cast iron, stone, and wood, of different sizes; and the
process consists entirely in holding the glass against these, while
they are revolving with rapidity. When a considerable portion of the
material is to be removed, the grinding is commonly commenced on the
iron wheel, on which is constantly pouring water and sharp sand, from
a vessel above, which, from its shape, is called a _hopper_.
25. The period of the invention of glass is quite unknown; but the
following is the usual story of its origin. Some merchants, driven by
a storm upon the coasts of Phoenicia, near the River Belus, kindled a
fire on the sand to cook their victuals, using as fuel some weeds
which grew near. The ashes produced by the incineration of these
plants, coming in contact with the sand, united with its particles,
and, by the influence of the heat, produced glass.
26. This production was accidentally picked up by a Tyrian merchant,
who, from its beauty and probable utility, was led to investigate the
causes of its formation, and who, after many attempts, succeeded in
the manufacture of glass. The legend probably originated in the fact,
that glass was very anciently made at Tyre; and that the sand on the
seashore in the immediate neighbourhood of the Belus, was well adapted
to glass-making.
27. It is certainly probable, that an accidental vitrification might
have given rise to the discovery; but the circumstance would have been
more likely to take place in some operation requiring greater heat
than that necessary for dressing food in the open air. The invention
of glass must have been effected as early as fifteen hundred years
before our era. It was manufactured very anciently in Egypt; but
whether that country or Phoenicia is entitled to the preference, as
regards priority in the practice of this art, cannot be determined.
28. Glass was made in considerable perfection at Alexandria, and was
thence supplied to the Romans as late as the first quarter of the
second century. Before this time, however, Rome had her glass
manufactories, to which a particular street was assigned. The
attention of the workmen was directed chiefly to the production of
bottles and ornamental vases, specimens of which still remain, as
monuments of their extraordinary skill.
29. In modern times, the manufacture of glass was confined
principally to Italy and Germany. Venice became particularly
celebrated for the beauty of the material, and the skill of its
workmen; and as early as the thirteenth century, it supplied the
greatest part of the glass used in Europe. The artists of Bohemia,
also, came to be held in considerable reputation.
30. The art was first practised in England, in the year 1557, when a
manufactory was erected at Crutched Friars, in the city of London, and
shortly afterwards, another at the Savoy, in the Strand. In these
establishments, however, were made little else than common window
glass, and coarse bottles, all the finer articles being still imported
from Venice. In 1673, the celebrated Duke of Buckingham brought
workmen from Italy, and established a manufactory for casting plate
glass for mirrors and coach windows. The art, in all its branches, is
now extensively practised in great perfection, not only in Great
Britain, but in many of the other kingdoms of Europe.
31. Before the commencement of the late war with England, very little,
if any, glass was manufactured in the United States, except the most
common window glass, and the most ordinary kinds of hollow ware.
Apothecaries' vials and bottles, as well as every other variety of the
better kinds of glass wares, had been imported from Europe, and
chiefly from England.
32. Our necessities, created by the event just mentioned, produced
several manufactories, which, however, did not soon become
flourishing, owing, at first, to inexperience, and, after the peace,
to excessive importations. But adequate protection having been
extended to this branch of our national industry, by the tariff of
1828, it is now in a highly prosperous condition--so much so, that
importations of glass ware have nearly ceased.
[Illustration: OPTICIAN.]