The Venetians were the first in modern times who attained to any degree
of excellence in the art of working glass, but the French became
eventually so zealous of rivalling them, particularly in the
construction of mirrors, that a decree was issued by the court of
France, declaring not only that the manufacture of glass should not
derogate from the dignity of a nobleman, but that nobles alone should be
masters of glass-works. Within the last 30 or 40 years, Great Britain
has made rapid advances in this important art, and at the present day
her pre-eminence in every department hardly admits of dispute.
There are five different species of glass, each requiring a peculiar
mode of fabrication, and peculiar materials: 1. The coarsest and
simplest form of this manufacture is _bottle_ glass. 2. Next to it in
cheapness of material maybe ranked _broad_ or _spread window glass_. An
improved article of this kind is now made near Birmingham, under the
name of British or German plate. 3. Crown glass comes next, or window
glass, formed in large circular plates or discs. This glass is peculiar
to Great Britain. 4. Flint glass, crystal glass, or glass of lead. 5.
Plate or fine mirror glass.
The materials of every kind of glass are vitrified in pots made of a
pure refractory clay; the best kind of which is a species of shale or
slate clay dug out of the coal-formation near Stourbridge. It contains
hardly any lime or iron, and consists of silica and alumina in nearly
equal proportions. The masses are carefully picked, brushed, and ground
under edge iron wheels of considerable weight, and sifted through
sieves having 20 meshes in the square inch. This powder is moistened
with water (best hot), and kneaded by the feet or a loam-mill into an
uniform smooth paste. A large body of this dough should be made up at a
time, and laid by in a damp cellar to ripen. Previously to working it
into shapes, it should be mixed with about a fourth of its weight of
cement of old pots, ground to powder. This mixture is sufficiently
plastic, and being less contractile by heat, forms more solid and
durable vessels. Glass-house pots have the figure of a truncated cone,
with the narrow end undermost; those for bottle and window-glass, being
open at top, about 30 inches diameter at bottom, 40 inches at the mouth,
and 40 inches deep; but the flint-glass pots are covered in at top with
a dome-cap, having a mouth at the side, by which the materials are
introduced, and the glass is extracted. Bottle and crown-house pots are
from 3 to 4 inches thick; those for flint-houses are an inch thinner,
and of proportionally smaller capacity.
The well-mixed and kneaded dough is first worked upon a board into a
cake for the bottom; over this the sides are raised, by laying on its
edges rolls of clay above each other with much manual labour, and
careful condensation. The clay is made into lumps, is equalized, and
slapped much in the same way as for making POTTERY. The pots thus
fashioned must be dried very prudently, first in the atmospheric
temperature, and finally in a stove floor, which usually borrows its
heat directly from the glass-house. Before _setting the pots_ in the
furnace, they are annealed during 4 or 5 days, at a red heat in a small
reverberatory vault, made on purpose. When completely annealed, they are
transferred with the utmost expedition into their seat in the fire, by
means of powerful tongs supported on the axle of an iron-wheel carriage
frame, and terminating in a long lever for raising them and swinging
them round. The _pot-setting_ is a desperate service, and when
unskilfully conducted without due mechanical aids, is the forlorn hope
of the glass-founder.--_Quæque ipse miserrima vidi._ The celebrated
chemist, Dr. Irvine, caught his last illness by assisting imprudently at
this formidable operation. The working breast of the hot furnace must be
laid bare so as to open a breach for the extraction of the faulty pot,
and the insertion of the fresh one, both in a state of bright
incandescence. It is frightful to witness the eyes and fuming visages of
the workmen, with the blackening and smoking of their scorched woollen
clothes, exposed so long to the direct radiations of the flame. A light
mask and sack dress coated with tinfoil, would protect both their faces
and persons from any annoyance, at a very cheap rate.
The glass-houses are usually built in the form of a cone, from 60 to 100
feet high, and from 50 to 80 feet in diameter at the base. The furnace
is constructed in the centre of the area, above an arched or groined
gallery which extends across the whole space, and terminates without the
walls, in large folding doors. This cavern must be sufficiently high to
allow labourers to wheel out the cinders in their barrows. The middle of
the vaulted top is left open in the building, and is covered over with
the grate-bars of the furnace.
1. _Bottle glass._--The bottle-house and its furnace resemble nearly
_fig._ 505. The furnace is usually an oblong square chamber, built of
large fire-bricks, and arched over with fire-stone, a siliceous grit of
excellent quality extracted from the coal measures of Newcastle. This
furnace stands in the middle of the area; and has its base divided into
three compartments. The central space is occupied by the grate-bars; and
on either side is the platform or fire-brick _siege_, (seat,) raised
about 12 inches above the level of the ribs upon which the pots rest.
Each _siege_ is about 3 feet broad.
In the sides of the furnace, semi-circular holes of about a foot
diameter are left opposite to, and a little above the top of, each pot,
called working holes, by which the workmen shovel in the materials, and
take out the plastic glass. At each angle of the furnace there is
likewise a hole of about the same size, which communicates with the
calcining furnace of a cylindrical form, dome-shaped at top. The flame
that escapes from the founding or pot-furnace is thus economically
brought to reverberate on the raw materials of the bottle glass, so as
to dissipate their carbonaceous or volatile impurities, and convert them
into a frit. A bottle-house has generally eight other furnaces or
fire-arches; of which six are used for annealing the bottles after they
are blown, and two for annealing the pots, before setting them in the
furnace.
The laws of this country till lately prohibited the use for making
common bottles of any fine materials. Nothing but the common river sand,
and soap-boilers’ waste, was allowed. About 3 parts of waste, consisting
of the insoluble residuum of kelp, mixed with lime and a little saline
substance, were used for 1 part of sand. This waste was first of all
calcined in two of the fire arches or reverberatories reserved for that
purpose, called the coarse arches, where it was kept at a red heat, with
occasional stirring, from 24 to 30 hours, being the period of a journey
or _journée_, in which the materials could be melted and worked into
bottles. The roasted soap-waste was then withdrawn, under the name of
ashes, from its arch, coarsely ground, and mixed with its proper
proportion of sand. This mixture was now put into the fine arch, and
calcined during the working journey, which extended to 10 or 12 hours.
Whenever the pots were worked out, that frit was immediately
transferred into them in its ignited state, and the founding process
proceeded with such despatch that this first charge of materials was
completely melted down in 6 hours, so that the pots might admit to be
filled up again with the second charge of frit, which was founded in 4
hours more. The heat was briskly continued, and in the course of from 12
to 18 hours, according to the size of the pots, the quality of the fuel,
and the draught of the furnace, the vitrification was complete. Before
blowing the bottles, however, the glass must be left to settle, and to
cool down to the blowing consistency, by shutting the _cave_ doors and
feeding holes, so as to exclude the air from the fire-grate and the
bottom of the hearth. The glass or metal becomes more dense, and by its
subsidence throws up the foreign lighter earthy and saline matters in
the form of a scum on the surface, which is removed with skimming irons.
The furnace is now charged with coal, to enable it to afford a working
heat for 4 or 5 hours, at the end of which time more fuel is cautiously
added, to preserve adequate heat for finishing the _journey_.
It is hardly possible to convey in words alone a correct idea of the
manipulations necessary to the formation of a wine bottle; but as the
manufacturers make no mystery of this matter, any person may have an
opportunity of inspecting the operation. Six people are employed at this
task; one, called a gatherer, dips the end of an iron tube, about five
feet long, previously made red-hot, into the pot of melted _metal_,
turns the rod round so as to surround it with glass, lifts it out to
cool a little, and then dips and turns it round again; and so in
succession till a ball is formed on its end sufficient to make the
required bottle. He then hands it to the blower, who rolls the plastic
lump of glass on a smooth stone or cast-iron plate, till he brings it to
the very end of the tube; he next introduces the pear-shaped ball into
an open brass or cast-iron mould, shuts this together by pressing a
pedal with his foot, and holding his tube vertically, blows through it,
so as to expand the cooling glass into the form of the mould. Whenever
he takes his foot from the pedal-lever, the mould spontaneously opens
out into two halves, and falls asunder by its bottom hinge. He then
lifts the bottle up at the end of the rod, and transfers it to the
finisher, who, touching the glass-tube at the end of the pipe with a
cold iron, cracks off the bottle smoothly at its mouth-ring. The
finished bottles are immediately piled up in the hot annealing arch,
where they are afterwards allowed to cool slowly for 24 hours at least.
See BOTTLE MOULD.
2. _Broad or spread window glass._--This kind of glass is called
_inferior_ window glass, in this country, because coarse in texture, of
a wavy wrinkled surface, and very cheap, but on the Continent _spread_
window glass, being made with more care, is much better than ours,
though still far inferior in transparency and polish to crown glass,
which has, therefore, nearly superseded its use among us. But Messrs.
Chance and Hartley, of West Bromwich near Birmingham, have of late years
mounted a spread-glass work, where they make _British sheet glass_, upon
the best principles, and turn out an article quite equal, if not
superior to any thing of the kind made either in France or Belgium.
Their materials are those used in the crown-glass manufacture. The
vitrifying mixture is fritted for 20 or 30 hours in a reverberatory
arch, with considerable stirring and puddling with long-handled shovels
and rakes; and the frit is then transferred by shovels while red hot, to
the melting pots to be founded. When the glass is rightly vitrified,
settled, and brought to a working heat, it is lifted out by iron tubes,
as will be described under the article CROWN GLASS, blown into pears,
which being elongated into cylinders, are cracked up along one side,
parallel to the axis, by touching them with a cold iron dipped in water,
and are then opened out into sheets. Glass cylinders are spread in
France, and at West Bromwich, on a bed of smooth stone Paris-plaster, or
laid on the bottom of a reverberatory arch; the cylinder being placed on
its side horizontally, with the cracked line uppermost, gradually opens
out, and flattens on the hearth. At one time, thick plates were thus
prepared for subsequent polishing into mirrors; but the glass was never
of very good quality; and this mode of making mirror-plate has
accordingly been generally abandoned.
The spreading furnace or oven is that in which cylinders are expanded
into tables or plates. It ought to be maintained at a brisk red heat, to
facilitate the softening of the glass. The oven is placed in immediate
connection with the annealing arch, so that the tables may be readily
and safely transferred from the former to the latter. Sometimes the
cylinders are spread in a large muffle furnace, in order to protect them
from being tarnished by sulphureous and carbonaceous fumes.
[Illustration: 500 501]
_Fig._ 500. represents a ground plan of both the spreading and annealing
furnace; _fig._ 501. is an oblong profile in the direction of the dotted
line X X, _fig._ 500.
_a_ is the fire-place; _b b_ the canals or flues through which the flame
rises into both furnaces; _c_ the spreading furnace, upon whose sole is
the spreading slab. _d_ is the cooling and annealing oven; _e e_ iron
bars which extend obliquely across the annealing arch, and serve for
resting the glass tables against, during the cooling. _f f_ the channel
along which the previously cracked cylinders are slid, so as to be
gradually warmed; _g_ the opening in the spreading furnace, for
enabling the workmen to regulate the process; _h_ a door in the
annealing arch, for introducing the tools requisite for raising up and
removing the tables.
[Illustration: 502]
In forming glass-plates by the extension of a cylinder into a plane, the
workman first blows the lump of glass into the shape of an oblong pear,
the length of which must be nearly equal to the length of the intended
plate, and its diameter such, that the circumference when developed,
will be equal to the breadth of the plate. He now rests the blowing-iron
on a stool or iron bar, while an assistant with a pointed iron, pierces
a hole into the extreme end of the pear, in the line of the
blowing-pipe. This opening is then enlarged, by introducing the blade of
a pair of spring-tongs, while the glass is turned round; and by skilful
management, the end of the pear is eventually opened out into a
cylindrical mouth. The workman next mounts upon a stool, and holds the
blowing-iron perpendicularly. The blown cylinder is now cracked off, a
punto rod of iron having been previously stuck to its one end, to form a
spindle for working the other by. This rod has a flat disc on its end,
or three prongs, which being dipped in melted glass, are applied to the
mouth of the cylinder. By this as a handle, the glass cone is carried to
the fire, and the narrow end being heated, is next opened by spring
tongs, and formed into a cylinder of the same size as the other end. The
cylinder thus equalized, is next cracked or slit down in its side with a
pair of shears, laid on a smooth copper plate, detached from the iron
rod, spread out by heat into a plane surface, and finally annealed. This
series of transformations, is represented in _fig._ 502., at A, B, C, D,
E, F, G, H.
[Illustration: 503 504]
_Fig._ 503. and 504. represent a Bohemian furnace in which excellent
white window glass is founded. _Fig._ 503. is a longitudinal section of
the glass and annealing furnace. _Fig._ 504. is the ground plan. _a_ is
the ash pit vaulted under the sole of the furnace; the fireplace itself
is divided into three compartments; with a middle slab at _d_, which is
hollowed in the centre, for collecting any spilt glass, and two hearth
tiles or slabs _b b_. _c c_ are the draught or air holes; _e e_ are
arches upon which the bearing slabs _f f_ partly rest. In the middle
between these arches, the flame strikes upwards upon the pots _g g_,
placed as closely together as possible, for economy of room. _h_ is the
breast wall of the furnace; _i_, _fig._ 504., the opening through which
the pots are introduced; it is bricked up as soon as they are set. _k
k_, is the base of the cone or dome of the furnace; _l l l_, the working
orifices, which are made larger or smaller according to the size of the
glass articles to be made. _m_ is the flue which leads to the annealing
stove _n_, with an arched door. Exterior to this, there is usually a
drying kiln not shown in the figure; and there are adjoining stoves
called _arches_, for drying and annealing the new pots before they are
set.
The cooling or annealing arch, or leer, is often built independent of
the glass-house furnace, is then heated by a separate fire-place, and
constructed like a very long reverberatory furnace. See COPPER.
The leer pans or trays of sheet iron, are laid upon its bottom in an
oblong series, and hooked to each other.
[Illustration: 505]
3. _Crown-glass._--The crown-glass house with its furnace is represented
in _fig._ 505., where the _blowing_ operation is shewn on the one side
of the figure, and the _flashing_ on the other. The furnace is usually
constructed to receive 4 or 6 pots, of such dimensions as to make about
a ton of glass each at a time. There are, however, several subsidiary
furnaces to a crown-house. 1. A reverberatory furnace or _calcar_, for
calcining or fritting the materials; 2. a blowing furnace, for blowing
the pear-shaped balls made at the pot-holes, into large globes; 3. a
flashing furnace, and bottoming hole for communicating a softening heat,
in expanding the globe into a circular plate; 4. the annealing arch for
the finished tables; 5. the reverberatory oven for annealing the pots
prior to their being set upon the founding _siege_.
The materials of crown glass used to be, fine sand, by measure 5 parts,
or by weight 10; ground kelp by measure 11 parts, or by weight 16-1/2;
but instead of kelp, soda ash is now generally employed. From 6 to 8
cwt. of sand, lime, and soda-ash, mixed together in wooden boxes with a
shovel, are thrown on the sole of a large reverberatory, such as is
represented in the article COPPER. Here the mixture is well worked
together, with iron paddles, flat shovels, and rakes with long handles;
the area of this furnace being about 6 feet square, and the height 2
feet. The heat soon brings the materials to a pasty consistence, when
they must be diligently turned over, to favour the dissipation of the
carbon, sulphur, and other volatile matters of the kelp or soda ash, and
to incorporate the fixed ingredients uniformly with the sand. Towards
the end of 3 hours, the fire is considerably raised, and when the fourth
hour has expired, the fritting operation is finished. The mass is now
shovelled or raked out into shallow cast-iron square cases, smoothed
down, and divided before it hardens by cooling, into square lumps, by
cross sections with the spade. These frit-bricks are afterwards piled up
in a large apartment for use; and have been supposed to improve with
age, by the efflorescence of their saline constituents into carbonate of
soda on their surface.
The founding-pots are filled up with these blocks of frit, and the
furnace is powerfully urged by opening all the subterranean passages to
its grate, and closing all the doors and windows of the glass-house
itself. After 8 or 10 hours the vitrification has made such progress,
and the blocks first introduced are so far melted down, that another
charge of frit can be thrown in, and thus the pot is fed with frit till
the proper quantity is used. In about 16 hours the vitrification of the
frit has taken place, and a considerable quantity, amounting often to
the cwt. of liquid saline matter floats over the glass. This salt is
carefully skimmed off into iron pots with long ladles. It is called
Sandiver or Glass-gall, and consists usually of muriate of soda, with a
little sulphate. The pot is now ready for receiving the _topping of
cullet_, which is broken pieces of window glass, to the amount of 3 or 4
cwt. This is shovelled in at short intervals; and as its pressure forces
up the residuary saline matter, this is removed; for were it allowed to
remain, the body of the glass would be materially deteriorated.
The heat is still continued for several hours till the glass is perfect,
and the extrication of gas called the _boil_, which accompanies the
fusion of crown glass, has nearly terminated, when the fire is abated,
by shutting up the lower vault doors and every avenue to the grate, in
order that the glass may settle fine. At the end of about 40 hours
altogether, the fire being slightly raised by adding some coals, and
opening the doors, the glass is carefully skimmed, and the working of
the pots commences.
[Illustration: 506]
Before describing it, however, we may state that the marginal figure
506. shews the base of the crown-house cone, with the four open pots in
two ranges on opposite sides of the furnace, sitting on their raised
_sieges_, at each side of the grate. At one side of the base the door of
the vault is shewn, and its course is marked by the dotted lines.
[Illustration: 507 508]
_Detailed description of the crown-glass furnace_, _figs._ 507. 508.--It
is an oblong square, built in the centre of a brick cone, large enough
to contain within it, two or three pots at each side of the grate room,
which is either divided as shown in the plan, or runs the whole length
of the furnace, as the manufacturer chooses. _Fig._ 507. is a ground
plan, and _fig._ 508. a front elevation, of a six-pot furnace. 1, 2, 3,
_fig._ 507., are the working holes for the purposes of ventilation, of
putting in the materials, and of taking out the metal to be wrought. 4,
5, 6, 7, are pipe holes for warming the pipes before beginning to work
with them. 8, 9, 10, are foot holes for mending the pots and sieges. 11
is a bar of iron for binding the furnace, and keeping it from swelling.
The arch is of an elliptic form; though a barrel arch, that is, an arch
shaped like the half of a barrel cut longwise through the centre, is
sometimes used. But this soon gives way when used in the manufacture of
crown glass, although it does very well in the clay-furnace used for
bottle houses.
The best stone for building furnaces is fire-stone, from Coxgreen in the
neighbourhood of Newcastle. Its quality is a close grit, and it contains
a greater quantity of talc than the common fire-stone, which seems to be
the chief reason of its resisting the fire better. The great danger in
building furnaces is, lest the cement at the top should give way with
the excessive heat, and by dropping into the pots, spoil the metal. The
top should therefore be built with stones only, as loose as they can
hold together after the centres are removed, and without any cement
whatever. The stones expand and come quite close together when
annealing; an operation which takes from eight to fourteen days at most.
There is thus less risk of any thing dropping from the roof of the
furnace.
The inside of the square of the furnace is built either of Stourbridge
fire-clay annealed, or the Newcastle fire-stone, to the thickness of
sixteen inches. The outside is built of common brick about nine inches
in thickness.
The furnace is thrown over an ash-pit, or cave as it is called, which
admits the atmospheric air, and promotes the combustion of the furnace.
This cave is built of stone until it comes beneath the grate room, when
it is formed of fire-brick. The abutments are useful for binding and
keeping the furnace together, and are built of masonry. The furnaces are
stoutly clasped with iron all round, to keep them tight. In four-pot
furnaces this is unnecessary, provided there be four good abutments.
[Illustration: 509]
_Fig._ 509. is an elevation of the flashing furnace. The outside is
built of common brick, the inside of fire-brick, and the mouth or nose
of Stourbridge fire-clay.
[Illustration: 510]
_Fig._ 510. is the annealing kiln. It is built of common brick, except
round the grate room, where fire-brick is used.
[Illustration: 511]
Few tools are needed for blowing and flashing crown-glass. The requisite
ball of plastic glass is gathered, in successive layers as for bottles,
on the end of an iron tube, and rolled into a pear-shape, on a cast-iron
plate; the workman taking care that the air blown into its cavity is
surrounded with an equal body of glass, and if he perceives any side to
be thicker than another, he corrects the inequality by rolling it on the
sloping iron table called marver, (marbre). He now heats the bulb in the
fire, and rolls it so as to form the glass upon the end of the tube,
and by a dexterous swing or two he lengthens it, as shewn in I, _fig._
511. To extend the neck of that pear, he next rolls it over a smooth
iron rod, turned round in a horizontal direction, into the shape K,
_fig._ 511. By further expansion at the blowing-furnace, he now brings
it to the shape L, represented in _fig._ 511.
This spheroid having become cool and somewhat stiff, is next carried to
the bottoming hole (like _fig._ 509.), to be exposed to the action of
flame. A slight wall erected before one half of this hole, screens the
workman from the heat, but leaves room for the globe to pass between it
and the posterior wall. The blowing-pipe is made to rest a little way
from the neck of the globe, on a hook fixed in the front wall; and thus
may be made easily to revolve on its axis, and by giving centrifugal
force to the globe, while the bottom of it, or part opposite to the
pipe, is softened by the heat, it soon assumes the form exhibited in M,
_fig._ 511.
In this state the flattened globe is removed from the fire, and its rod
being rested on the _casher box_ covered with coal cinders, another
workman now applies the end of a solid iron rod tipped with melted
glass, called a _punto_, to the nipple or prominence in the middle; and
thus attaches it to the centre of the globe, while the first workman
cracks off the globe by touching its tubular neck with an iron chisel
dipped in cold water. The workman having thereby taken possession of the
globe by its bottom or knobbed pole attached to his punty rod, he now
carries it to another circular opening, where he exposes it to the
action of moderate flame with regular rotation, and thus slowly heats
the thick projecting remains of the former neck, and opens it slightly
out, as shewn at N, in _fig._ 511. He next hands it to the _flasher_,
who resting the iron rod in a hook placed near the side of the orifice
A, _fig._ 509., wheels it rapidly round opposite to a powerful flame,
till it assumes first the figure O, and finally that of a flat circular
table.
The flasher then walks off with the table, keeping up a slight rotation
as he moves along, and when it is sufficiently cool, he turns down his
rod into a vertical position, and lays the table flat on a dry block of
fire-clay, or bed of sand, when an assistant nips it off from the
_punto_ with a pair of long iron shears, or cracks it off with a touch
of cold iron. The loose table or plate is lastly lifted up horizontally
on a double pronged iron fork, introduced into the annealing arch _fig._
510. and raised on edge; an assistant with a long-kneed fork preventing
it from falling too rapidly backwards. In this arch a great many tables
of glass are piled up in iron frames, and slowly cooled from a heat of
about 600° to 100° F., which takes about 24 hours; when they are
removed. A circular plate or table of about 5 feet diameter weighs on an
average 9 pounds.
4. _Flint glass._--This kind of glass is so called because originally
made with calcined flints, as the siliceous ingredient. The materials at
present employed in this country for the finest flint glass or crystal,
are first, Lynn sand, calcined, sifted, and washed; second, an oxide of
lead, either red lead or litharge; and third, pearl ash. The pearl ash
of commerce must however be purified by digesting it in a very little
hot water, which dissolves the carbonate of potash, and leaves the
foreign salts, chiefly sulphate of potash, muriate of potash, and
muriate of soda. The solution of the carbonate being allowed to cool and
become clear in lead pans, is then run off into a shallow iron boiler,
and evaporated to dryness. Nitre is generally added as a fourth
ingredient of the body of the glass; and it serves to correct any
imperfections which might arise from accidental combustible particles,
or from the lead being not duly oxidized. The above four substances
constitute the main articles; to which we may add arsenic and manganese,
introduced in very small quantities, to purify the colour and clear up
the transparency of the glass. The black oxide of manganese, when used
in such quantity only as to peroxidize the iron of the sand, simply
removes the green tinge caused by the iron; but if more manganese be
added than accomplishes that purpose, it will give a purple tinge to the
glass; and in fact, most manufacturers prefer to have an excess rather
than a defect of manganese, since cut glass has its brilliancy increased
by a faint lilac hue. The arsenic is supposed to counteract the injury
arising from excess of manganese, but is itself very apt on the other
hand to communicate some degree of opalescence, or at least, to impair
the lustre of the glass. When too much manganese has been added, the
purple tinge may indeed be removed by any carbonaceous matter, as by
thrusting a wooden rod down into the liquid glass; but this cannot be
done with good effect in practice, since the final purple tinge is not
decided till the glass is perfectly formed, and then the introduction of
charcoal would destroy the uniformity of the whole contents of the pot.
The raw materials of flint glass, are always mixed with about a third or
a fourth of their weight of broken crystal of like quality; this mixture
is thrown into the pot with a shovel; and more is added whenever the
preceding portions by melting subside; the object being to obtain a pot
full of glass, to facilitate the skimming off the impurities, and
sandiver. The mouth of the pot is now shut, by applying clay-lute round
the stopper, with the exception of a small orifice below, for the escape
of the liquid saline matter. Flint glass requires about 48 hours for its
complete vitrification, though the materials be more fusible than those
of crown glass; in consequence of the contents of the pot being
partially screened by its cover from the action of the fire, as also
from the lower intensity of the heat.
[Illustration: 512 513]
_Fig._ 512. represents a flint glass house for 6 pots, with the arch or
leer on one side for annealing the crystal ware. In _fig._ 513., the
base of the cone is seen, and the glass pots _in situ_ on their platform
ranged round the central fire grate. The dotted line denotes the contour
of the furnace, _fig._ 512.
Whenever the glass appears fine, and is freed from its air bubbles,
which it usually is in about 36 hours, the heat is suffered to fall a
little by closing the bottom valves, &c., that the pot may settle; but
prior to working the metal, the heat is _somewhat raised_ again.
It would be useless to describe the manual operations of fashioning the
various articles of the flint-glass manufacture, because they are
indefinitely varied to suit the conveniences and caprices of human
society.
Every different flint-house has a peculiar proportion of glass
materials. The following have been offered as good practical mixtures.
1. Fine white sand 300 parts.
Red lead or litharge 200
Refined pearl ashes 80
Nitre 20
Arsenic and manganese, a minute quantity.
In my opinion, the proportion of lead is too great in the above recipe,
which is given on the authority of Mr. James Geddes, of Leith. The glass
made with it would be probably yellowish, and dull.
2. Fine sand 50·5
Litharge 27·2
Refined pearl ashes (carbonate of potash, with 5
per cent. of water) 17·5
Nitre 4·8
-----
100·0
To these quantities from 30 to 50 parts of broken glass or cullet are
added; with about a two-thousandth part of manganese, and a
three-thousandth part of arsenic. But manganese varies so extremely in
its purity, and contains often so much oxide of iron, that nothing can
be predicated as to its quantity previously to trial.
M. Payen, an eminent manufacturing chemist in France, says that the
composition of crystal does not deviate much from the following
proportions:--
Wood fire. Coal fire.
Siliceous sand 3 3
Minium 2 2-1/4
Carbonate of potash 1-1/2 1-2/3
I conceive that this glass contains too much lead and potash. Such a
mixture will produce a dull metal, very attractive of moisture: defects
to which the French crown-glass also is subject.
The flint-glass _leer_ for annealing glass, is an arched gallery or
large flue, about 36 feet long, 3 feet high, 4 wide; having its floor
raised above 2 feet above the ground of the glass-house. The hot air and
smoke of a fire-place at one end pass along this gallery, and are
discharged by a chimney 8 or 10 feet short of the other end. On the
floor of the vault, large iron trays are laid and hooked to each other
in a series, which are drawn from the fire end towards the other by a
chain, wound about a cylinder by a winch-handle projecting through the
side. The flint-glass articles are placed in their hot state into the
tray next the fire, which is moved onwards to a cooler station whenever
it is filled, and an empty tray is set in its place. Thus, in the course
of about 20 hours, the glass advances to the cool end thoroughly
annealed.
Besides colourless transparent glass, which forms the most important
part of this manufacture, various coloured glasses are made to suit the
taste of the public. The taste at Paris was lately for opaline crystal;
which may be prepared by adding to the above composition (No. 2.)
phosphate of lime, or well burnt bone-ash in fine powder, washed, and
dried. The article must be as uniform in thickness as possible, and
speedily worked into shape, with a moderate heat. Oxide of tin, _putty_,
was formerly used for making opalescent glass, but the lustre of the
body was always impaired by its means.
Crystal vessels have been made recently of which the inner surface is
colourless, and all the external facets coloured. Such works are easily
executed. The end of the blowing-rod must be dipped first in the pot
containing colourless glass, to form a bulb of a certain size, which
being cooled a little is then dipped for an instant into the pot of
coloured glass. The two layers are associated without intermixture; and
when the article is finished in its form, it is white within and
coloured without. Fluted lines somewhat deeply cut, pass through the
coloured coat, and enter the colourless one; so that when they cross,
their ends alone are coloured.
For some time past, likewise, various crystal articles have been
exhibited in the market with coloured enamel-figures on their surface,
or with white incrustations of a silvery lustre in their interior. The
former are prepared by placing the enamel object in the brass mould, at
the place where it is sought to be attached. The bulb of glass being put
into the mould, and blown while very hot, the small plate of enamel gets
cemented to the surface. For making the white argentine incrustations,
small figures are prepared with an impalpable powder of dry porcelain
paste, cemented into a solid by means of a little gypsum plaster. When
these pieces are thoroughly dried, they are laid on the glass while it
is red hot, and a large patch of very liquid glass is placed above it,
so as to encase it and form one body with the whole. In this way the
incrustation is completely enclosed; and the polished surface of the
crystal which scarcely touches it, gives a brilliant aspect, pleasing to
the eye.
An uniform flint-glass, free from striæ, or _wreath_, is much in demand
for the optician. It would appear that such an article was much more
commonly made by the English manufacturers many years ago, than at
present; and that in improving the brilliancy of crystal-glass they have
injured its fitness for constructing optical lenses, which depends not
so much on its whiteness and lustre as on the layers of different
densities being parallel to each other. The oxide of lead existing in
certain parts of a potful of glass in greater proportion than in other
parts, increases the density unequally in the same mass, so that the
adjoining strata are often very different in this respect. Even a potful
of pretty uniform glass, when it stands some time liquid, becomes
eventually unequable by the subsidence of the denser portions; so that
striæ and gelatinous appearances begin to manifest themselves, and the
glass becomes of little value. Glass allowed to cool slowly in mass in
the pot is particularly full of wreath; and if quickly refrigerated,
that is in two or three hours, it is apt to split into a multitude of
minute splinters, of which no use can be made. For optical purposes, the
glass must be taken out in its liquid state, being gathered on the end
of the iron rod from the central portion of a recently skimmed pot,
after the upper layers have been worked off in general articles.
M. Guinand, of Brennets near Geneva, appears to have hit upon processes
that furnished almost certainly pieces of flint-glass capable of forming
good lenses of remarkable dimensions, even of 11 inches diameter; of
adequate density and transparency, and nearly free from _striæ_. M.
Cauchoix, the eminent French optician, says, that out of ten object
glasses, 4 inches in diameter, made with M. Guinand’s flint-glass, eight
or nine turned out very good, while out of an equal number of object
glasses made of the flint-glass of the English and French manufactories,
only one, or two at most, were found serviceable. The means by which M.
Guinand arrived at these results have not been published. He has lately
died, and it is not known whether his son be in possession of his
secret.
An achromatic object glass for telescopes and microscopes consists of at
least two lenses; the one made with glass of lead, or flint glass, and
the other with crown glass; the former possessing a power of dispersing
the coloured rays relatively to its mean refractive power, much greater
than the latter; upon which principle, the achromatism of the image is
produced, by re-uniting the different coloured rays into one focus.
Flint glass to be fit for this delicate purpose must be perfectly
homogeneous, or of uniform density throughout its substance, and free
from wavy veins or wreathes; for every such inequality would occasion a
corresponding inequality in the refraction and dispersion of the light;
like what is perceived in looking through a thick and thin solution of
gum Arabic imperfectly mixed. Three plans have been prescribed for
obtaining homogeneous pieces of optical glass: 1. to lift a mass of it
in large ladles, and let it cool in them; 2. to pour it out from the
pots into moulds; 3. to allow it to cool in the pots, and afterwards to
cut it off in horizontal strata. The last method, which is the most
plausible, seldom affords pieces of uniform density, unless peculiar
precautions have been adopted to settle the flint glass in uniform
strata; because its materials are of such unequal density, the oxide of
lead having a specific gravity of 8, and silica of 2·7, that they are
apt to stand at irregular heights in the pots.
One main cause of these inequalities lies in the construction of the
furnace, whereby the bottom of the pot is usually much less heated than
the upper part. In a plate glass furnace the temperature of the top of
the pot has been found to be 130° Wedgew., while that of the bottom was
only 110°, constituting a difference of no less than 2610° F. The
necessary consequence is that the denser particles which subside to the
bottom, during the fusion of the materials, and after the first
extrication of the gases, must remain there, not being duly agitated by
the expansive force of caloric, acting from below upwards.
The preparation of the best optical glass is now made a great mystery by
one or two proficients. The following suggestions, deduced from a
consideration of principles, may probably lead to some improvements, if
judiciously applied. The great object is to counteract the tendency of
the glass of lead to distribute itself into strata of different
densities; which may be effected either by mechanical agitation or by
applying the greatest heat to the bottom of the pot. But however
homogeneous the glass may be thereby made, its subsequent separation
into strata of different densities must be prevented by rapid cooling
and solidification. As the deeper the pots, the greater is the chance of
unequal specific gravity in their contents, it would be advisable to
make them wider and shallower than those in use for making ordinary
glass. The intermixture may be effected either by lading the glass out
of one pot into another in the furnace, and back again, with copper
ladles, or by stirring it up with a rouser, then allowing it to settle
for a short time, till it becomes clear and free from air bubbles. The
pot may now be removed from the furnace, in order to solidify its
contents in their homogeneous state; after which the glass may be broken
in pieces, and be perfected by subjecting it to a second fusion; or what
is easier and quicker, we may form suitable discs of glass without
breaking down the potful, by lifting it out in flat copper ladles with
iron shanks, and transferring the lumps after a little while into the
annealing leer.
To render a potful of glass homogeneous by agitation, is a more
difficult task, as an iron rod would discolour it, and a copper rod
would be apt to melt. An iron rod sheathed in laminated platinum would
answer well, but for its expense. A stone-ware tube supported within by
a rod of iron, might also be employed for the purpose in careful hands;
the stirring being repeated several times, till at last the glass is
suffered to stiffen a little by decrease of temperature. It must be then
allowed to settle and cool, after which the pot, being of small
dimensions, may be drawn out of the fire.
[Illustration: 514]
2. The second method of producing the desired uniformity of mixture,
consists in applying a greater heat to the bottom than to the upper part
of the melting pot. _Fig._ 514. represents in section a furnace
contrived to effect this object. It is cylindrical, and of a diameter no
greater than to allow the flames to play round the pot, containing from
three to four cwts. of vitreous materials. A is the pot, resting upon
the arched grid _b a_, built of fire-bricks, whose apertures are wide
enough to let the flames rise freely, and strike the bottom and sides of
the vessel. From 1-1/2 to 2 feet under that arch, the fuel grate _c d_
is placed. B C are the two working openings for introducing the
materials, and inspecting the progress of the fusion; they must be
closed with fire-tiles and luted with fire-clay at the beginning of the
process. At the back of the furnace, opposite the mouth of the
fire-place, there is a door-way, which is bricked up, except upon
occasion of putting in and taking out the pot. The draught is regulated
by means of a slide-plate upon the mouth of the ash-pit _f_. The pot
being heated to the proper pitch, some purified pearl ash, mixed with
fully twice its weight of colourless quartz sand, is to be thrown into
it, and after the complete fusion of this mixture, the remaining part of
the sand along with the oxide of lead (fine litharge) is to be strown
upon the surface. These siliceous particles in their descent serve to
extricate the air from the mass. Whenever the whole is fused, the heat
must be strongly urged to ensure a complete uniformity of combination by
the internal motions of the particles. As soon as the glass has been
found by making test phials to be perfectly fine, the fire must be
withdrawn, the two working holes must be opened, as well as the mouths
of the fire-place and ash-pit, to admit free ingress to cooling currents
of air, so as to congeal the liquid mass as quickly as possible; a
condition essential to the uniformity of the glass. It may be worth
while to stir it a little with the pottery rod at the commencement of
the cooling process. The solidified glass may be afterwards detached by
a hammer in conchoidal discs, which after chipping off their edges, are
to be placed in proper porcelain or stone-ware dishes, and exposed to a
softening heat, in order to give them a lenticular shape. Great care
must be taken that the heat thus applied by the muffle furnace be very
equable, for otherwise wreathes might be very readily re-produced in the
discs. A small oven upon the plan of a baker’s, is best fitted for this
purpose, which being heated to dull redness, and then extinguished, is
ready to soften and afterwards anneal the conchoidal pieces.
Guinand’s dense optical flint glass, of specific gravity 3·616, consists
by analysis, of oxide of lead 43·05; silica 44·3; and potash 11·75; but
requires for its formation the following ingredients: 100 pounds of
ground quartz; 100 pounds of fine red lead; 35 pounds of purified
potash; and from 2 to 4 pounds of saltpetre. As this species of glass is
injured by an excess of potash, it should be compounded with rather a
defect of it, and melted by a proportionally higher or longer heat. A
good optical glass has been made in Germany with 7 parts of pure red
lead, 3 parts of finely ground quartz, and 2 parts of calcined borax.