This, like English crown-glass, has a soda flux, whereas flint-glass
requires potash, and is never of good quality when made with soda. We
shall distribute our account of this manufacture under two heads.
1. The different furnaces and principal machines, without whose
knowledge it would be impossible to understand the several processes of
a plate-glass factory.
2. The materials which enter into the composition of this kind of glass,
and the series of operations which they undergo; devoting our chief
attention to the changes and improvements which long experience,
enlightened by modern chemistry, has introduced into the great
manufactory of Saint-Gobin in France, under the direction of M.
Tassaert. It may however be remarked that the English plate-glass
manufacture derives peculiar advantages from the excellence of its
grinding and polishing machinery.
The clay for making the bricks and pots should be free from lime and
iron, and very refractory. It is mixed with the powder of old pots
passed through a silk sieve. If the clay be very plastic it will bear
its own weight of the powder, but if shorter in quality, it will take
only three-fifths. But before mingling it with the cement of old pots,
it must be dried, bruised, then picked, ground, and finally elutriated
by agitation with water, decantation through a hair sieve, and
subsidence. The clay fluid after passing the sieve is called _slip_
(coulis.)
The furnace is built of dry bricks, cemented with slip, and has at each
of its four angles a peculiar annealing arch, which communicates with
the furnace interiorly, and thence derives sufficient heat to effect in
part, if not wholly, the annealing of the pots, which are always
deposited there a long time before they are used. Three of these arches
exclusively appropriated to this purpose, are called pot-arches. The
fourth is called the _arch of the materials_, because it serves for
drying them before they are founded. Each arch has, moreover, a
principal opening called the throat, another called _bonnard_, by the
French workmen, through which fire may be kindled in the arch itself,
when it was thought to be necessary for the annealing of the pots; a
practice now abandoned. The duration of a furnace is commonly a year, or
at most 14 months; that of the arches is 30 years or upwards, as they
are not exposed to so strong a heat.
In the manufacture of plate-glass two sorts of crucibles are employed,
called the pots and the basins, (_cuvettes_). The first serve for
containing the materials to be founded, and for keeping them a long time
in the melted state. The _cuvettes_ receive the melted glass after it is
refined, and decant it out on the table to be rolled into a plate. Three
pots hold liquid glass for six small basins, or for three large ones,
the latter being employed for making mirrors of great dimensions, that
is, 100 inches long and upwards. Furnaces have been lately constructed
with 6 pots, and 12 cuvettes, 8 of which are small, and 4 large; and
cuvettes of three sizes are made, called _small_, _middling_, and
_large_. The small are perfect cubes, the middling and the large ones
are oblong parallelopipeds. Towards the middle of their height, a notch
or groove, two or three inches broad, and an inch deep, is left, called
the girdle of the cuvette, by which part they are grasped with the
tongs, or rather are clamped in the iron frame. This frame goes round
the four sides of the small cuvettes, and may be placed indifferently
upon all their sides; in the other cuvettes, the girdle extends only
over the two large sides, because they cannot be turned up. See _m_ T,
_fig._ 515., p. 590.
The pot is an inverted truncated cone, like a crown glass pot. It is
about 30 inches high, and from 30 to 32 inches wide, including its
thickness. There is only a few inches of difference between the diameter
of the top and that of the bottom. The bottom is 3 inches thick, and the
body turns gradually thinner till it is an inch at the mouth of the pot.
The large building or factory, of which the melting furnace occupies the
middle space, is called the _halle_ in French. At Ravenhead in
Lancashire it is called the foundry, and is of magnificent dimensions,
being probably the largest apartment under one roof in Great Britain,
since its length is 339 feet, and its breadth 155. The famous _halle_ of
St. Gobin is 174 feet by 120. Along the two side walls of the _halle_,
which are solidly constructed of hewn stone, there are openings like
those of common ovens. These ovens, destined for the annealing of the
newly cast plates, bear the name of _carquaises_. Their soles are raised
two feet and a half above the level of the ground, in order to bring
them into the same horizontal plane with the casting tables. Their
length, amounting sometimes to 30 feet, and their breadth to 20, are
required in order to accommodate 6, 8, or even 10 plates of glass,
alongside of each other. The front aperture is called the throat, and
the back door the little throat (_gueulette_). The carquaise is heated
by means of a fire-place of a square form called a _tisar_, which
extends along its side.
The founding or melting furnace is a square brick building laid on solid
foundations, being from 8 to 10 feet in each of its fronts, and rising
inside into a vault or crown about 10 feet high. At each angle of this
square, a small oven or arch is constructed, likewise vaulted within,
and communicating with the melting furnace by square flues, called
_lunettes_, through which it receives a powerful heat, though much
inferior to that round the pots. The arches are so distributed as that
two of the exterior sides of the furnace stand wholly free, while the
two other sides, on which the arches encroach, offer a free space of
only 3 feet. In this interjacent space, two principal openings of the
furnace, of equal size in each side, are left in the building. These are
called tunnels. They are destined for the introduction of the pots and
the fuel.
On looking through the tunnels into the inside of the furnace, we
perceive to the right hand and the left, along the two _free_ sides, two
low platforms or _sieges_, at least 30 inches in height and breadth. See
_figs._ 506. 508.
These _sieges_ (seats) being intended to support the pots and the
cuvettes filled with heavy materials, are terminated by a slope, which
ensures the solidity of the fire-clay mound. The slopes of the two
sieges extend towards the middle of the furnace so near as to leave a
space of only from 6 to 10 inches between them for the hearth. The end
of this is perforated with a hole sufficiently large to give passage to
the liquid glass of a broken pot, while the rest is preserved by lading
it from the mouth into the adjoining cuvette.
In the two large parallel sides of the furnace, other apertures are left
much smaller than the tunnels, which are called _ouvreaux_ (peep holes).
The lower ones, or the _ouvreaux en bas_, called _cuvette_ openings,
because being allotted to the admission of these vessels, they are
exactly on a level with the surface of the _sieges_, and with the floor
of the _halle._ Plates of cast iron form the thresholds of these
openings, and facilitate the ingress and egress of the cuvettes. The
apertures are arched at top, with hewn stone like the tunnels, and are
18 inches wide when the cuvettes are 16 inches broad.
The upper and smaller apertures, or the higher _ouvreaux_ called the
_lading_ holes, because they serve for transvasing the liquid glass, are
three in number, and are placed 31 or 32 inches above the surface of the
_sieges_. As the pots are only 30 inches high, it becomes easy to work
through these openings either in the pots or the _cuvettes_. The pots
stand opposite to the two pillars which separate the openings, so that a
space is left between them for one or more _cuvettes_ according to the
size of the latter. It is obvious that if the tunnels and _ouvreaux_
were left open, the furnace would not draw or take the requisite
founding heat. Hence the openings are shut by means of fire-tiles. These
are put in their places, and removed by means of two holes left in them,
in correspondence with the two prongs of a large iron fork supported by
an axle and two iron wheels, and terminated by two handles which the
workmen lay hold of when they wish to move the tile.
The closing of the tunnel is more complex. When it is shut or ready for
the firing, the aperture appears built up with bricks and mortar from
the top of the arch to the middle of the tunnel. The remainder of the
door-way is closed; 1. on the two sides down to the bottom, by a small
upright wall, likewise of bricks, and 8 inches broad, called walls of
the _glaye_; 2. by an assemblage of pieces called pieces of the _glaye_,
because the whole of the closure of the tunnel bears the name of
_glaye_. The upper hole, 4 inches square, is called the _tisar_, through
which billets of wood are tossed into the fire. Fuel is also introduced
into the posterior openings. The fire is always kept up on the hearth of
the tunnel, which is, on this account, 4 inches higher than the
furnace-hearth, in order that the glass which may accidentally fall down
on it, and which does not flow off by the bottom hole, may not impede
the combustion. Should a body of glass, however, at any time obstruct
the grate, it must be removed with rakes, by opening the tunnel and
dismounting the fire-tile stoppers of the _glaye_.
Formerly wood fuel alone was employed for heating the melting-furnaces
of the mirror-plate manufactory of Saint-Gobin; but within these few
years, the Director of the works makes use with nearly equal advantage
of pit-coal. In the same establishment, two melting furnaces may be
seen, one of which is fixed with wood, and the other with coals, without
any difference being perceptible in the quality of the glass furnished
by either. It is not true, as has been stated, that the introduction of
pit-coal has made it necessary to work with covered pots in order to
avoid the discoloration of the materials, or that more alkali was
required to compensate for the diminished heat in the covered pots. They
are not now covered when pit-coal is used, and the same success is
obtained as heretofore by leaving the materials two or three hours
longer in the pots and the cuvettes. The construction of the furnaces in
which coal is burned, is the same as that with wood, with slight
modifications. Instead of the close bottomed hearth of the wood furnace,
there is an iron grate in the coal-hearth through which the air enters,
and the waste ashes descend.
When billets of wood were used as fuel, they were well dried beforehand,
by being placed a few days on a frame work of wood called the wheel,
placed two feet above the furnace and its arches, and supported on four
pillars at some distance from the angles of the building.
_Composition of plate-glass._--This is not made now, as formerly, by
random trials. The progress of chemistry, the discovery of a good
process for the manufacture of soda from sea salt, which furnishes a
pure alkali of uniform power, and the certain methods of ascertaining
its purity, have rendered this department of glass-making almost
entirely new, in France. At Saint-Gobin no alkali is employed at present
except artificial crystals of soda, prepared at the manufactory of
Chauny, subsidiary to that establishment. Leaden chambers are also
erected there for the production of sulphuric acid from sulphur. The
first crop of soda crystals is reserved for the plate-glass manufacture,
the other crystals and the mother-water salts are sold to the makers of
inferior glass.
At the mirror-plate works of Ravenhead, near St. Helen’s in Lancashire,
soda crystals, from the decomposition of the sulphate of soda by chalk
and coal, have been also tried, but without equal success as at
Saint-Gobin; the failure being unquestionably due to the impurity of the
alkali. Hence, in the English establishment the soda is obtained by
treating sea-salt with pearl-ash, whence carbonate of soda and muriate
of potash result. The latter salt is crystallized out of the mingled
solution, by evaporation at a moderate heat, for the carbonate of soda
does not readily crystallize till the temperature of the solution fall
below 60° Fahr. When the muriate of potash is thus removed, the alkaline
carbonate is evaporated to dryness.
Long experience at Saint-Gobin has proved that one part of dry carbonate
of soda is adequate to vitrify perfectly three parts of fine siliceous
sand, as that of the mound of Aumont near Senlis, of Alum Bay in the
Isle of Wight, or of Lynn in Norfolk. It is also known that the degree
of heat has a great influence upon the vitrification, and that increase
of temperature will compensate for a certain deficiency of alkali; for
it is certain that a very strong fire always dissipates a good deal of
the soda, and yet the glass is not less beautiful. The most perfect
mirror-plate has constantly afforded to M. Vauquelin in analysis, a
portion of soda inferior to what had been employed in its formation.
Hence, it has become the practice to add for every 100 parts of cullet
or broken plate that is mixed with the glass composition, one part of
alkali, to make up for the loss that the old glass must have
experienced.
To the above mentioned proportions of sand and alkali independently of
the cullet which may be used, dry slaked lime carefully sifted is to be
added to the amount of one seventh of the sand; or the proportion will
be, sand 7 cwt.; quicklime 1 cwt.; dry carbonate of soda 2 cwt. and 37
lbs.; besides cullet. The lime improves the quality of the glass,
rendering it less brittle and less liable to change. The preceding
quantities of materials suitably blended, have been uniformly found to
afford most advantageous results. The practice formerly was to dry that
mixture, as soon as it was made, in the arch for the materials, but it
has been ascertained that this step may be dispensed with, and the small
portion of humidity present is dissipated almost instantly after they
are thrown into the furnace. The coat of glaze previously applied to the
inside of the pot, prevents the moisture from doing them any harm. For
this reason, when the demand for glass at Saint-Gobin is very great, the
materials are neither fritted nor even dried, but shovelled directly
into the pot; this is called founding _raw_. Six workmen are employed in
shovelling-in the materials either fritted or otherwise, for the sake of
expedition, and to prevent the furnace getting cooled. One-third of the
mixture is introduced at first; whenever this is melted, the second
third is thrown in, and then the last. These three stages are called the
first, second, and third fusion or founding.
According to the ancient practice, the founding and refining were both
executed in the pots, and it was not till the glass was refined, that it
was laded into the cuvettes, where it remained only 3 hours, the time
necessary for the disengagement of the air bubbles introduced by the
transvasion, and for giving the _metal_ the proper consistence for
casting. At present, the period requisite for founding and refining, is
equally divided between the pots and the _cuvettes_. The materials are
left 16 hours in the pots, and as many in the _cuvettes_; so that in 32
hours, the glass is ready to be cast. During the last two or three
hours, the fireman or _tiseur_ ceases to add fuel; all the openings are
shut, and the glass is allowed to assume the requisite fluidity; an
operation called _stopping_ the glass, or _performing the ceremony_.
The transfer of the glass into the _cuvettes_, is called _lading_,
(_tréjetage_). Before this is done, the cuvettes are cleared out, that
is, the glass remaining on their bottom, is removed, and the ashes of
the firing. They are lifted red hot out of the furnace by the method
presently to be described, and placed on an iron plate, near a tub
filled with water. The workmen, by means of iron paddles 6 feet long,
flattened at one end and hammered to an edge, scoop out the fluid glass
expeditiously, and throw it into water; the _cuvettes_ are now returned
to the furnace, and a few minutes afterwards the lading begins.
In this operation, ladles of wrought iron are employed, furnished with
long handles, which are plunged into the pots through the upper openings
or lading holes, and immediately transfer their charge of glass into the
buckets. Each workman dips his ladle only three times, and empties its
contents into the cuvette. By these three immersions (whence the term
_tréjeter_ is derived), the large iron spoon is heated so much that when
plunged into a tub full of water, it makes a noise like the roaring of a
lion, which may be heard to a very great distance.
The founding, refining, and _ceremony_, being finished, they next try
whether the glass be ready for casting. With this view, the end of a rod
is dipped into the bucket, which is called _drawing the glass_, the
portion taken up being allowed to run off, naturally assumes a
pear-shape, from the appearance of which, they can judge if the
consistence be proper, and if any air bubbles remain. If all be right,
the _cuvettes_ are taken out of the furnace, and conveyed to the part of
the _halle_ where their contents are to be poured out. This process
requires peculiar instruments and manipulations.
_Casting._--While the glass is refining, that is, coming to its highest
point of perfection, preparation is made for the most important process,
the casting of the plate, whose success crowns all the preliminary
labours and cares. The oven or _carquaise_ destined to receive and
anneal the plate, is now heated by its small fire or _tisar_, to such a
pitch that its sole may have the same temperature as that of the plates,
being nearly red-hot at the moment of their being introduced. An unequal
degree of heat in the _carquaise_ would cause breakage of the glass. The
casting table is then rolled towards the front door or throat, by means
of levers, and its surface is brought exactly to the level of the sole
of the oven.
The table T, _fig._ 515., is a mass of bronze, or now preferably
cast-iron, about 10 feet long, 5 feet broad, and from 6 to 7 inches
thick, supported by a frame of carpentry, which rests on three cast-iron
wheels. At the end of the table opposite to that next to the front of
the oven, is a very strong frame of timber-work, called the puppet or
standard, upon which the bronze roller which spreads the glass is laid,
before and after the casting. This is 5 feet long by 1 foot in diameter;
it is thick in the metal but hollow in the axis. The same roller can
serve only for two plates at one casting, when another is put in its
place, and the first is laid aside to cool; for otherwise the hot roller
would at a third casting, make the plate expand unequally, and cause it
to crack. When the rollers are not in action, they are laid aside in
strong wooden trestles, like those employed by sawyers. On the two sides
of the table in the line of its length, are two parallel bars of bronze,
_t_, _t_, destined to support the roller during its passage from end to
end; the thickness of these bars determines that of the plate. The table
being thus arranged, a crane is had recourse to for lifting the cuvette,
and keeping it suspended, till it be emptied upon the table. This
raising and suspension are effected by means of an iron gib, furnished
with pullies, held horizontally, and which turns with them.
[Illustration: 515]
The tongs T, _fig._ 515., are made of four iron bars, bent into a square
frame in their middle, for embracing the bucket. Four chains proceeding
from the corners of the frame V, are united at their other ends into a
ring which fits into the hook of the crane.
Things being thus arranged, all the workmen of the foundry co-operate in
the manipulations of the casting. Two of them fetch, and place quickly
in front of one of the lower openings, the small cuvette-carriage, which
bears a forked bar of iron, having two prongs corresponding to the two
holes left in the fire-tile door. This fork mounted on the axle of two
cast-iron wheels, extends at its other end into two branches terminated
by handles, by which the workmen move the fork, lift out the tile
stopper, and set it down against the outer wall of the furnace.
The instant these men retire, two others push forward into the opening
the extremity of the tongs-carriage, so as to seize the bucket by the
girdle, or rather to clamp it. At the same time, a third workman is busy
with an iron pinch or long chisel, detaching the bucket from its seat,
to which it often adheres by some spilt glass; whenever it is free, he
withdraws it from the furnace. Two powerful branches of iron united by a
bolt, like two scissor blades, which open, come together, and join by a
quadrant near the other end, form the tongs-carriage, which is mounted
upon two wheels like a truck.
The same description will apply almost wholly to the iron-plate
carriage, on which the bucket is laid the moment it is taken out of the
furnace; the only difference in its construction is, that on the bent
iron bars which form the tail or lower steps of this carriage (in place
of the tongs) is permanently fastened an iron plate, on which the bucket
is placed and carried for the casting.
Whenever the _cuvette_ is set upon its carriage, it must be rapidly
wheeled to its station near the crane. The tongs T above described are
now applied to the girdle, and are then hooked upon the crane by the
suspension chains. In this position the bucket is skimmed by means of a
copper tool called a sabre, because it has nearly the shape of that
weapon. Every portion of the matter removed by the sabre is thrown into
a copper ladle (_poche de gamin_), which is emptied from time to time
into a cistern of water. After being skimmed, the bucket is lifted up,
and brushed very clean on its sides and bottom; then by the double
handles of the suspension-tongs it is swung round to the table, where it
is seized by the workmen appointed to turn it over; the roller having
been previously laid on its ruler-bars, near the end of the table which
is in contact with the annealing oven. The _cuvette_-men begin to pour
out towards the right extremity E of the roller, and terminate when it
has arrived at the left extremity D. While preparing to do so, and at
the instant of casting, two men place within the ruler-bar on each side,
that is between the bar and the liquid glass, two iron instruments
called _hands_, _m_, _m_, _m_, _m_, which prevent the glass from
spreading beyond the rulers, whilst another draws along the table the
wiping bar _c_, _c_, wrapped in linen, to remove dust, or any small
objects which may interpose between the table and the liquid glass.
Whenever the melted glass is poured out, two men spread it over the
table, guiding the roller slowly and steadily along, beyond the limits
of the glass, and then run it smartly into the wooden standard prepared
for its reception, in place of the trestles V, V.
The empty bucket, while still red-hot, is hung again upon the crane, set
on its plate-iron carriage, freed from its tongs, and replaced in the
furnace, to be speedily cleared out anew, and charged with fresh fluid
from the pots. If while the roller glides along, the two workmen who
stand by with picking tools, perceive _tears_ in the matter in advance
of the roller, and can dexterously snatch them out, they are suitably
rewarded, according to the spot where the blemish lay, whether in the
centre, where it would have proved most detrimental, or near the edge.
These tears proceed usually from small portions of semi-vitrified
matter which fall from the vault of the furnace, and from their density
occupy the bottom of the _cuvettes_.
While the plate is still red-hot and ductile, about 2 inches of its end
opposite to the _carquaise_ door is turned up with a tool; this portion
is called the _head of the mirror_; against the outside of this head,
the shovel, in the shape of a rake without teeth, is applied, with which
the plate is eventually pushed into the oven, while two other workmen
press upon the upper part of the head with a wooden pole, eight feet
long, to preserve the plate in its horizontal position, and prevent its
being warped. The plate is now left for a few moments near the throat of
the _carquaise_, to give it solidity; after which it is pushed further
in by means of a very long iron tool, whose extremity is forked like the
letter y, and hence bears that name; and is thereby arranged in the most
suitable spot for allowing other plates to be introduced.
However numerous the manipulations executed from the moment of
withdrawing the _cuvette_ from the furnace, till the cast-plate is
pushed into the annealing oven, I have seen them all performed in less
than five minutes; such silence, order, regularity, and despatch prevail
in the establishment of Saint-Gobin.
When all the plates of the same casting have been placed in the
_carquaise_, it is sealed up, that is to say, all its orifices are
closed with sheets of iron, surrounded and made tight with plastic loam.
With this precaution, the cooling goes on slowly and equably in every
part, for no cooling current can have access to the interior of the
oven.
After they are perfectly cooled, the plates are carefully withdrawn one
after another, keeping them all the while in a horizontal position, till
they are entirely out of the _carquaise._ As soon as each plate is taken
out, one set of workmen lower quickly and steadily the edge which they
hold, while another set raise the opposite edge, till the glass be
placed upright on two cushions stuffed with straw, and covered with
canvas. In this vertical position they pass through, beneath the lower
edge of the plate, three girths or straps each four feet long, thickened
with leather in their middle, and ending in wooden handles; so that one
embraces the middle of the plate, and the other two, the ends. The
workmen, six in number, now seize the handles of the straps, lift up the
glass closely to their bodies, and convey it with a regular step to the
warehouse. Here the head of the plate is first cut off with a diamond
square, and then the whole is attentively examined, in reference to its
defects and imperfections, to determine the sections which must be made
of it, and the eventual size of the pieces. The pairings and small
cuttings detached are set aside, in order to be ground and mixed with
the raw materials of another glass-pot.
The apartment in which the roughing-down and smoothing of the plates is
performed, is furnished with a considerable number of stone tables,
truly hewn and placed apart like billiard tables, in a horizontal
position, about 2 feet above the ground. They are rectangular, and of
different sizes proportional to the dimensions of the plates, which they
ought always to exceed a little. These tables are supported either on
stone pillars or wooden frames, and are surrounded with a wooden board
whose upper edge stands somewhat below their level, and leaves in the
space between it and the stone all round an interval of 3 or 4 inches,
of which we shall presently see the use.
A cast plate, unless formed on a table quite new, has always one of its
faces, the one next the table, rougher than the other; and with this
face the roughing-down begins. With this view, the smoother face is
cemented on the stone table with Paris-plaster. But often instead of one
plate, several are cemented alongside of each other, those of the same
thickness being carefully selected. They then take one or more crude
plates of about one-third or one-fourth the surface of the plate fixed
to the table, and fix it on them with liquid gypsum to the large base of
a quadrangular truncated pyramid of stone; of a weight proportioned to
its extent, or about a pound to the square inch. This pyramidal muller,
if small sized, bears at each of its angles of the upper face a peg or
ball, which the grinders lay hold of in working it; but when of greater
dimension, there is adapted to it horizontally a wheel of slight
construction, 8 or 10 feet in diameter, whose circumference is made of
wood rounded so as to be seized with the hand. The upper plate is now
rubbed over the lower ones, with moistened sand applied between.
This operation is however performed by machinery. The under plate being
fixed or imbedded in stucco, on a solid table, the upper one likewise
imbedded by the same cement in a cast-iron frame, has a motion of
circum-rotation given to it closely resembling that communicated by the
human hand and arm, moist sand being supplied between them. While an
excentric mechanism imparts this double rotatory movement to the upper
plate round its own centre, and of that centre round a point in the
lower plate, this plate placed on a moveable platform changes its
position by a slow horizontal motion, both in the direction of its
length and its breadth. By this ingenious contrivance, which pervades
the whole of the grinding and polishing machinery, a remarkable
regularity of friction and truth of surface is produced. When the plates
are sufficiently worked on one face, they are reversed in the frames,
and worked together on the other. The Paris plaster is usually coloured
red, in order to shew any defects in the glass.
The smoothing of the plates is effected on the same principles by the
use of moist emery washed to successive degrees of fineness, for the
successive stages of the operation; and the polishing process is
performed by rubbers of hat-felt and a thin paste of colcothar and
water. The colcothar, called also crocus, is red oxide of iron prepared
by the ignition of copperas, with grinding and elutriation of the
residuum.
The last part of the polishing process is performed by hand. This is
managed by females, who slide one plate over another, while a little
moistened putty of tin finely levigated is thrown between.
Large mirror-plates are now the indispensable ornaments of every large
and sumptuous apartment; they diffuse lustre and gaiety round them, by
reflecting the rays of light in a thousand lines, and by multiplying
indefinitely the images of objects placed between opposite parallel
planes.
The _silvering_ of _plane_ mirrors consists in applying a layer of
tin-foil alloyed with mercury to their posterior surface. The workshop
for executing this operation is provided with a great many smooth tables
of fine freestone or marble, truly levelled, having round their contour
a rising ledge, within which there is a gutter or groove which
terminates by a slight slope in a spout at one of the corners. These
tables rest upon an axis of wood or iron which runs along the middle of
their length; so that they may be inclined easily into an angle with the
horizon of 12 or 13 degrees, by means of a hand-screw fixed below. They
are also furnished with brushes, with glass rules, with rolls of woollen
stuff, several pieces of flannel, and a great many weights of stone or
cast-iron.
The glass-tinner, standing towards one angle of his table, sweeps and
wipes its surface with the greatest care, along the whole surface to be
occupied by the mirror-plate; then taking a sheet of tin-foil adapted to
his purpose, he spreads it on the table, and applies it closely with a
brush, which removes any folds or wrinkles. The table being horizontal,
he pours over the tin a small quantity of quicksilver, and spreads it
with a roll of woollen stuff; so that the tin-foil is penetrated and
apparently dissolved by the mercury. Placing now two rules, to the right
and to the left, on the borders of the sheet, he pours on the middle a
quantity of mercury sufficient to form every where a layer about the
thickness of a crown piece; then removing with a linen rag the oxide or
other impurities, he applies to it the edge of a sheet of paper, and
advances it about half an inch. Meanwhile another workman is occupied in
drying very nicely the surface of the glass that is to be silvered, and
then hands it to the master workman, who, laying it flat, places its
anterior edge first on the table, and then on the slip of paper; now
pushing the glass forwards, he takes care to slide it along so that
neither air nor any coat of oxide on the mercury can remain beneath the
plate. When this has reached its position, he fixes it there by a weight
applied on its side, and gives the table a gentle slope, to run off all
the loose quicksilver by the gutter and spout. At the end of five
minutes he covers the mirror with a piece of flannel, and loads it with
a great many weights, which are left upon it for 24 hours, under a
gradually increased inclination of the table. By this time the plate is
ready to be taken off the marble table, and laid on a wooden one sloped
like a reading desk, with its under edge resting on the ground, while
the upper is raised successively to different elevations by means of a
cord passing over a pulley in the ceiling of the room. Thus the mirror
has its slope graduated from day to day, till it finally arrives at a
vertical position. About a month is required for draining out the
superfluous mercury from large mirrors; and from 18 to 20 days from
those of moderate size. The sheets of tin-foil being always somewhat
larger than the glass-plate, their edges must be pared smooth off,
before the plate is lifted off the marble table.
_Process for silvering concave mirrors._--Having prepared some very fine
Paris plaster by passing it through a silk sieve, and some a little
coarser passed through hair-cloth, the first is to be made into a creamy
liquor with water, and after smearing the concave surface of the glass
with a film of olive oil, the fine plaster is to be poured into it, and
spread by turning about, till a layer of plaster be formed about a tenth
of an inch thick. The second or coarse plaster, being now made into a
thin paste, poured over the first, and moved about, readily incorporates
with it, in its imperfectly hardened state. Thus an exact mould is
obtained of the concave surface of the glass, which lies about
three-quarters of an inch thick upon it, but is not allowed to rise
above its outer edge.
The mould being perfectly dried, must be marked with a point of
coincidence on the glass, in order to permit of its being exactly
replaced in the same position, after it has been lifted out. The mould
is now removed, and a round sheet of tin-foil is applied to it, so large
that an inch of its edge may project beyond the plaster all round; this
border being necessary for fixing the tin to the contour of the mould by
pellets of white wax softened a little with some Venice turpentine.
Before fixing the tin-foil, however, it must be properly spread over the
mould, so as to remove every wrinkle; which the pliancy of the foil
easily admits of, by uniform and well-directed pressure with the
fingers.
The glass being placed in the hollow bed of a tight sack filled with
fine sand, set in a well-jointed box capable of retaining quicksilver,
its concave surface must be dusted with sifted wood-ashes, or Spanish
white contained in a small cotton bag, and then well wiped with clean
linen rags, to free it from all adhering impurity, and particularly the
moisture of the breath. The concavity must be now filled with
quicksilver to the very lip, and the mould being dipped a little way
into it, is withdrawn, and the adhering mercury is spread over the tin
with a soft flannel roll, so as to amalgamate and brighten its whole
surface, taking every precaution against breathing on it. Whenever this
brightening seems complete, the mould is to be immersed, not vertically,
but one edge at first, and thus obliquely downwards till the centres
coincide; the mercury meanwhile being slowly displaced, and the mark on
the mould being brought finally into coincidence with the mark on the
glass. The mould is now left to operate by its own weight, in expelling
the superfluous mercury, which runs out upon the sand-bag and thence
into a groove in the bottom of the box, whence it overflows by a spout
into a leather bag of reception. After half an hour’s repose, the whole
is cautiously inverted, to drain off the quicksilver more completely.
For this purpose, a box like the first is provided with a central
support rising an inch above its edges; the upper surface of the support
being nearly equal in diameter to that of the mould. Two workmen are
required to execute the following operation. Each steadies the mould
with the one hand, and raises the box with the other, taking care not to
let the mould be deranged, which they rest on the (convex) support of
the second box. Before inverting the first apparatus, however, the
reception bag must be removed, for fear of spilling its mercury. The
redundant quicksilver now drains off; and if the weight of the sand-bag
is not thought sufficient, supplementary weights are added at pleasure.
The whole is left in this position for two or three days. Before
separating the mirror from its mould, the border of tin-foil, fixed to
it with wax, must be pared off with a knife. Then the weight and
sand-bag being removed, the glass is lifted up with its interior coating
of tin-amalgam.
_For silvering a convex surface._--A concave plaster mould is made on
the convex glass, and their points of coincidence are defined by marks.
This mould is to be lined with tin-foil, with the precautions above
described; and the tin surface being first brightened with a little
mercury, the mould is then filled with the liquid metal. The glass is to
be well cleaned, and immersed in the quicksilver bath, which will expel
the greater part of the metal. A sand-bag is now to be laid on the
glass, and the whole is to be inverted as in the former case on a
support; when weights are to be applied to the mould, and the mercury is
left to drain off for several days.
If the glass be of large dimensions, 30 or 40 inches, for example,
another method is adopted. A circular frame or hollow ring of wood or
iron is prepared, of twice the diameter of the mirror, supported on
three feet. A circular piece of new linen cloth of close texture is cut
out, of equal diameter to the ring, which is hemmed stoutly at the
border, and furnished round the edge with a row of small holes, for
lacing the cloth to the ring, so as to leave no folds in it, but without
bracing it so tightly as to deprive it of the elasticity necessary for
making it into a mould. This apparatus being set horizontally, a leaf of
tin-foil is spread over it, of sufficient size to cover the surface of
the glass; the tin is first brightened with mercury, and then as much of
the liquid metal is poured on as a plane mirror requires. The convex
glass, well cleaned, is now set down on the cloth, and its own weight,
joined to some additional weights, gradually presses down the cloth, and
causes it to assume the form of the glass which thus comes into close
contact with the tin submersed under the quicksilver. The redundant
quicksilver is afterwards drained off by inversion, as in common cases.
The following recipe has been given for silvering the inside of glass
globes. Melt in an iron ladle or a crucible, equal parts of tin and
lead, adding to the fused alloy one part of bruised bismuth. Stir the
mixture well and pour into it as it cools two parts of dry mercury;
agitating anew and skimming off the drossy film from the surface of the
amalgam. The inside of the glass globe being freed from all adhering
dust and humidity, is to be gently heated, while a little of the
semi-fluid amalgam is introduced. The liquidity being increased by the
slight degree of heat, the metallic coating is applied to all the points
of the glass, by turning round the globe in every direction, but so
slowly as to favour the adhesion of the alloy. This silvering is not so
substantial as that of plane mirrors: but the form of the vessel,
whether a globe, an ovoid, or a cylinder, conceals or palliates the
defects by counter reflection from the opposite surfaces.
_Coloured Glasses and Artificial Gems._--The general vitreous body
preferred by Fontanieu in his treatise on this subject, which he calls
the Mayence base, is prepared in the following manner. Eight ounces of
pure rock-crystal or flint in powder, mixed with 24 ounces of salt of
tartar, are baked and left to cool. This is afterwards poured into a
basin of hot water, and treated with dilute nitric acid till it ceases
to effervesce; when the frit is to be washed till the water comes off
tasteless. The frit is now dried and mixed with 12 ounces of fine white
lead, and the mixture is to be levigated and elutriated with a little
distilled water. An ounce of calcined borax is to be added to about 12
ounces of the preceding mixture in a dry state, the whole rubbed
together in a porcelain mortar, then melted in a clean crucible, and
poured out into cold water. This vitreous matter must be dried, and
melted a second and a third time, always in a new crucible, and after
each melting poured into cold water as at first, taking care to separate
the lead that may be revived. To the last glass ground to powder, five
drachms of nitre are to be added, and the mixture being melted for the
last time, a mass of crystal will be found in the crucible with a
beautiful lustre. The diamond is well imitated by this Mayence base.
Another very fine white crystal may be obtained, according to M.
Fontanieu, from eight ounces of white lead, two ounces of powdered
borax, half a grain of manganese, and three ounces of rock-crystal
treated as above.
The colours of artificial gems are obtained from metallic oxides. The
_oriental topaz_ is prepared by adding oxide of antimony to the base;
the amethyst from manganese with a little purple precipitate of Cassius;
the beryl from antimony and a very little cobalt; yellow artificial
diamond and opal, from horn-silver (chloride of silver); blue stone from
cobalt. See PASTES and PIGMENTS VITRIFIABLE.
The following are recipes for making the different kinds of glass.
1. _Bottle glass._--11 pounds of dry glauber salts; 12 pounds of soaper
salts; a half bushel of waste soap ashes; 56 pounds of sand; 22 pounds
of glass skimmings; 1 cwt. of green broken glass; 25 pounds of basalt.
This mixture affords a dark green glass.
2. Yellow or white sand 100 parts; kelp 30 to 40; lixiviated wood ashes
from 160 to 170 parts; fresh wood ashes 30 to 40 parts; potter’s clay 80
to 100 parts; cullet or broken glass 100. If basalt be used, the
proportion of kelp may be diminished.
In two bottle-glass houses in the neighbourhood of Valenciennes, an
unknown ingredient, sold by a Belgian, was employed, which he called
_spar_. This was discovered by chemical analysis to be sulphate of
baryta. The glass-makers observed that the bottles which contained some
of this substance were denser, more homogeneous, more fusible, and
worked more kindly, than those formed of the common materials. When one
prime equivalent of the silicate of baryta = 123, is mixed with three
primes of the silicate of soda = (3 × 77·6) = 232·8, and exposed in a
proper furnace, vitrification readily ensues, and the glass may be
worked a little under a cherry-red heat, with as much ease as a glass of
lead, and has nearly the same lustre. Since the ordinary run of
glass-makers are not familiar with atomic proportions, they should have
recourse to a scientific chemist, to guide them in using such a
proportion of sulphate of baryta as may suit their other vitreous
ingredients; for an excess or defect of any of them will injure the
quality of the glass.
3. _Green window glass, or broad glass._--11 pounds of dry glauber salt;
10 pounds of soaper salts; half a bushel of lixiviated soap waste; 50
pounds of sand; 22 pounds of glass pot skimmings; 1 cwt. of broken green
glass.
4. _Crown glass._--300 parts of fine sand; 200 of good soda ash; 33 of
lime; from 250 to 300 of broken glass; 60 of white sand; 30 of purified
potash; 15 of saltpetre (1 of borax), 1/2 of arsenious acid.
5. _Nearly white table glass._--20 pounds of potashes; 11 pounds of dry
glauber salts; 16 of soaper salt; 55 of sand; 140 of cullet of the same
kind. Another.--100 of sand; 235 of kelp; 60 of wood ashes; 1-1/3 of
manganese; 100 of broken glass.
6. _White table glass._--40 pounds of potashes; 11 of chalk; 76 of sand;
1/2 of manganese; 95 of white cullet.
Another.--50 of purified potashes; 100 of sand; 20 of chalk; and 2 of
saltpetre.
Bohemian table or plate glass is made with 63 parts of quartz; 26 of
purified potashes; 11 of sifted slaked lime, and some cullet.
7. _Crystal glass._--60 parts of purified potashes; 120 of sand; 24 of
chalk; 2 of saltpetre; 2 of arsenious acid; 1/16 of manganese.
Another.--70 of purified pearl ashes; 120 of white sand; 10 of
saltpetre; 1/2 of arsenious acid; 1/3 of manganese.
A third.--67 of sand; 23 of purified pearl ashes; 10 of sifted slaked
lime; 1/4 of manganese; (5 to 8 of red lead).
A fourth.--120 of white sand; 50 of red lead; 40 of purified pearl ash;
20 of saltpetre; 1/3 of manganese.
A fifth.--120 of white sand; 40 of pearl ash purified; 35 of red lead;
13 of saltpetre; 1/12 of manganese.
A sixth.--30 of the finest sand; 20 of red lead; 8 of pearl ash
purified; 2 of saltpetre; a little arsenious acid and manganese.
A seventh.--100 of sand; 45 of red lead; 35 of purified pearl ashes; 1/7
of manganese; 1/3 of arsenious acid.
8. _Plate glass._--Very white sand 300 parts; dry purified soda 100
parts; carbonate of lime 43 parts; manganese 1; cullet 300.
Another.--Finest sand 720; purified soda 450; quicklime 80 parts;
saltpetre 25 parts; cullet 425.
A little borax has also been prescribed; much of it communicates an
exfoliating property to glass.
Tabular view of the composition of several kinds of Glass.
+------------------+----+----+-----+----+----+-----+----+-----+----+
| | No.| No.| No. | No.| No.| No. | No.| No. | No.|
| | 1.| 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. |
| +----+----+-----+----+----+-----+----+-----+----+
|Silica |71·7|69·2| 62·8|69·2|60·4|53·55|59·2|51·93|42·5|
|Potash |12·7|15·8| 22·1| 8·0| 3·2| 5·48| 9·0|13·77|11·7|
|Soda | 2·5| 3·0| | 3·0| S. | | | | |
| | | | | |pot.| | | | |
|Lime |10·3| 7·6| 12·5|13·0|20·7|29·22| | | 0·5|
|Alumina | 0·4| 1·2| | 3·6|10·4| 6·01| | | 1·8|
|Magnesia | | 2·0|} | 0·6| 0·6| | | | |
|Oxide of iron | 0·3| 0·5|} 2·6| 1·6| 3·8| 5·74| 0·4| | |
| -- manganese| 0·2| |} | | | | 1·0| | |
| -- lead | | | | | | |28·2|33·28|43·5|
|Baryta | | | | | 0·9| | | | |
+------------------+----+----+-----+----+----+-----+----+-----+----+
No. 1. is a very beautiful white wine glass of Neuwelt in Bohemia.
No. 2. Glass tubes, much more fusible than common wine glasses.
No. 3. Crown glass of Bohemia.
No. 4. Green glass, for medicinal phials and retorts.
No. 5. Flask glass of St. Etienne, for which some heavy spar is used.
No. 6. Glass of Sèvres.
No. 7. London glass employed for chemical and physical purposes.
No. 8. English flint glass.
No. 9. Guinand’s flint glass.
The manufacture of _Glass beads_ at Murano near Venice, is most
ingeniously simple. Tubes of glass of every colour, are drawn out to
great lengths in a gallery adjoining the glass-house pots, in the same
way as the more moderate lengths of thermometer and barometer tubes are
drawn in our glass-houses. These tubes are chopped into very small
pieces of nearly uniform length on the upright edge of a fixed chisel.
These elementary cylinders being then put in a heap into a mixture of
fine sand and wood ashes, are stirred about with an iron spatula till
their cavities get filled. This curious mixture is now transferred to an
iron pan suspended over a moderate fire, and continually stirred about
as before, whereby the cylindrical bits assume a smooth rounded form; so
that when removed from the fire and cleared out in the bore, they
constitute beads, which are packed in casks, and exported in prodigious
quantities to almost every country, especially to Africa and Spain.
GLASS CUTTING AND GRINDING, for common and optical purposes. By this
mechanical process the surface of glass may be modified into almost any
ornamental or useful form.
1. The grinding of crystal ware. This kind of glass is best adapted to
receive polished facets, both on account of its relative softness, and
its higher refractive power, which gives lustre to its surface. The
cutting shop should be a spacious long apartment, furnished with
numerous sky-lights, having the grinding and polishing lathes arranged
right under them, which are set in motion by a steam-engine or
water-wheel at one end of the building. A shaft is fixed as usual in
gallowses along the ceiling; and from the pulleys of the shaft, bands
descend to turn the different lathes, by passing round the driving
pulleys near their ends.
[Illustration: 516]
The turning lathe is of the simplest construction. _Fig._ 516. D is an
iron spindle with two well-turned prolongations, running in the iron
puppets _a a_, between two concave bushes of tin or type metal, which
may be pressed more or less together by the thumb-screws shown in the
figure. These two puppets are made fast to the wooden support B, which
is attached by a strong screw and bolt to the longitudinal beam of the
workshop A. E is the fast and loose pulley for putting the lathe into
and out of geer with the driving shaft. The projecting end of the
spindle is furnished with a hollow head-piece, into which the rod _c_ is
pushed tight. This rod carries the cutting or grinding disc plate. For
heavy work, this rod is fixed into the head by a screw. When a conical
fit is preferred, the cone is covered with lead to increase the
friction.
Upon projecting rods or spindles of that kind the different discs for
cutting the glass are made fast. Some of these are made of fine
sandstone or polishing slate, from 8 to 10 inches in diameter, and from
3/4 to 1/2 inch thick. They must be carefully turned and polished at the
lathe, not only upon their rounded but upon their flat face, in order to
grind and polish in their turn the flat and curved surfaces of glass
vessels. Other discs of the same diameter, but only 3/4 of an inch
thick, are made of cast tin truly turned, and serve for polishing the
vessels previously ground; a third set consist of sheet iron from 1/6 to
1/2 an inch thick, and 12 inches in diameter, and are destined to cut
grooves in glass by the aid of sand and water. Small discs of
well-hammered copper from 1/2 to 3 inches in diameter, whose
circumference is sometimes flat, and sometimes concave or convex, serve
to make all sorts of delineations upon glass by means of emery and oil.
Lastly, there are rods of copper or brass furnished with small
hemispheres from 1/24 to 1/4 of an inch in diameter, to excavate round
hollows in glass. Wooden discs are also employed for polishing, made of
white wood cut across the grain, as also of cork.
[Illustration: 517]
The cutting of deep indentations, and of grooves, is usually performed
by the iron disc, with sand and water, which are allowed constantly to
trickle down from a wooden hopper placed right over it, and furnished
with a wooden stopple or plug at the apex, to regulate by its greater or
less looseness the flow of the grinding materials. The same effect may
be produced by using buckets as shown in _fig._ 517. The sand which is
contained in the bucket F, above the lathe, has a spigot and faucet
inserted near its bottom, and is supplied with a stream of water from
the stopcock in the vessel G, which, together running down the inclined
board, are conducted to the periphery of the disc as shown in the
figure, to whose lowest point the glass vessel is applied with pressure
by the hand. The sand and water are afterwards collected in the tub H.
Finer markings which are to remain without lustre, are made with the
small copper discs, emery, and oil. The polishing is effected by the
edge of the tin disc, which is from time to time moistened with putty
(white oxide of tin) and water. The wooden disc is also employed for
this purpose with putty, colcothar, or washed tripoli. For fine
delineations, the glass is first traced over with some coloured varnish,
to guide the hand of the cutter.
In grinding and facetting crystal glass, the deep grooves are first cut,
for example the cross lines, with the iron disc and rounded edge, by
means of sand and water. That disc is one sixth of an inch thick and 12
inches in diameter. With another iron disc about half an inch thick, and
more or less in diameter, according to the curvature of the surface, the
grooves may be widened. These roughly cut parts must be next smoothed
down with the sandstone disc and water, and then polished with the
wooden disc about half an inch thick, to whose edge the workman applies,
from time to time, a bag of fine linen containing some ground pumice
moistened with water. When the cork or wooden disc edged with hat felt
is used for polishing, putty or colcothar is applied to it. The above
several processes in a large manufactory, are usually committed to
several workmen on the principle of the division of labour, so that each
may become expert in his department.
2. _The grinding of optical glasses._--The glasses intended for optical
purposes being spherically ground, are called lenses; and are used
either as simple magnifiers and spectacles, or for telescopes and
microscopes. The curvature is always a portion of a sphere, and either
convex or concave. This form ensures the convergence or divergence of
the rays of light that pass through them, as the polishing does the
brightness of the image.
The grinding of the lenses is performed in brass moulds, either concave
or convex, formed to the same curvature as that desired in the lenses;
and may be worked either by hand or by machinery. A gauge is first cut
out out of brass or copper plate to suit the curvature of the lens, the
circular arc being traced by a pair of compasses. In this way both a
convex and concave circular gauge are obtained. To these gauges the
brass moulds are turned. Sometimes, also, lead moulds are used. After
the two moulds are made, they are ground face to face with fine emery.
The piece of glass is now roughed into a circular form by a pair of
pincers, leaving it a little larger than the finished lens ought to be,
and then smoothed round upon the stone disc, or in an old mould with
emery and water, and is next made fast to a holdfast. This consists of a
round brass plate having a screw in its back; and is somewhat smaller in
diameter than the lens, and two thirds as thick. This as turned concave
upon the lathe, and then attached to the piece of glass by drops of
pitch applied to several points of its surface, taking care while the
pitch is warm, that the centre of the glass coincides with the centre of
the brass plate. This serves not merely as a holdfast, by enabling a
person to seize its edge with the fingers, but it prevents the glass
from bending by the necessary pressure in grinding.
The glass must now be ground with coarse emery upon its appropriate
mould, whether convex or concave, the emery being all the time kept
moist with water. To prevent the heat of the hand from affecting the
glass, a rod for holding the brass plate is screwed to its back. For
every six turns of circular motion, it must receive two or three rubs
across the diameter in different directions, and so on alternately. The
middle point of the glass must never pass beyond the edge of the mould;
nor should strong pressure be at any time applied. Whenever the glass
has assumed the shape of the mould, and touches it in every point, the
coarse emery must be washed away, finer be substituted in its place, and
the grinding be continued as before, till all the scratches disappear,
and a uniform dead surface be produced. A commencement of polishing is
now to be given with pumice-stone powder. During all this time the
convex mould should be occasionally worked in the concave, in order that
both may preserve their correspondence of shape between them. After the
one surface has been thus finished, the glass must be turned over, and
treated in the same way upon the other side.
Both surfaces are now to be polished. With this view equal parts of
pitch and rosin must be melted together, and strained through a cloth to
separate all impurities. The concave mould is next to be heated, and
covered with that mixture in a fluid state to the thickness uniformly of
one quarter of an inch. The cold convex mould is now to be pressed down
into the yielding pitch, its surface being quite clean and dry, in order
to give the pitch the exact form of the ground lens; and both are to be
plunged into cold water till they be chilled. This pitch impression is
now the mould upon which the glass is to be polished, according to the
methods above described with finely washed colcothar and water, till the
surface become perfectly clear and brilliant. To prevent the pitch from
changing its figure by the friction, cross lines must be cut in it about
1/2 an inch asunder, and 1-12th of an inch broad and deep. These grooves
remove all the superfluous parts of the polishing powder, and tend to
preserve the polishing surface of the pitch clean and unaltered. No
additional colcothar after the first is required in this part of the
process; but only a drop of water from time to time. The pitch gets warm
as the polishing advances, and renders the friction more laborious from
the adhesion between the surfaces. No interruption must now be suffered
in the work, nor must either water or colcothar be added; but should the
pitch become too adhesive, it must be merely breathed upon, till the
polish be complete. The nearer the lens is brought to a true and fine
surface in the first grinding, the better and more easy does the
polishing become. It should never be submitted to this process with any
scratches perceptible in it, even when examined by a magnifier.
As to small lenses and spectacle eyes, several are ground and polished
together in a mould about 6 inches in diameter, made fast to a
stiffening plate of brass or iron of a shape corresponding with the
mould. The pieces of glass are affixed by means of drops of pitch as
above described, to the mould, close to each other, and are then all
treated as if they formed but one large lens. Plane glasses are ground
upon a surface of pitch rendered plane by the pressure of a piece of
plate glass upon it in its softened state.
Lenses are also ground and polished by means of machinery, into the
details of which the limits of this work will not allow me to enter.
A Return to an Order of the Honourable the House of Commons, dated 1st
March, 1838, of the Amount of Duty charged on Glass; distinguishing
the Amount charged on Flint, Plate, Broad, Crown, Bottle and German
Sheet, in the Year ending the 5th day of January, 1838; together with
the Amount of Drawback on each description of Glass; the produce of
the Duties in England, Scotland, and Ireland stated separately.
+----------+--------------+-------------+-------------+--------------+
|Amount of | | | | |
|Duty | England | Scotland | Ireland | Total |
|charged on| | | | |
+----------+--------------+-------------+-------------+--------------+
| | _£. s. d._| _£. s. d._| _£. s. d._| _£. s. d._|
|Flint | | | | |
|Glass. | 76,052 1 0 | 7,530 9 4 | 6,736 12 11 | 90,319 3 3 |
|Plate. | 68,902 10 | | | 68,902 10 |
|Broad. | 10,789 10 | | | 10,789 10 |
|Crown. |533,404 6 7 |16,423 11 6 | |549,827 18 1 |
|Bottle. |122,617 10 2 |32,246 4 1 | 3,642 0 3 |158,505 14 6 |
|German | | | | |
|Sheet. | 25,511 17 | | | 25,511 17 |
+----------+--------------+-------------+-------------+--------------+
|Total. |837,277 14 9 |56,200 4 11 |10,378 13 2 |903,856 12 10 |
+----------+--------------+-------------+-------------+--------------+
+--------+--------------+-------------+----------+--------------+
|Amount | | | | |
|of Draw-| | | | |
|back on | England | Scotland | Ireland | Total |
|Exporta-| | | | |
|tion. | | | | |
+--------+--------------+-------------+----------+--------------+
| | _£. s. d._| _£. s. d._|_£. s. d._| _£. s. d._|
|Flint | | | | |
|Glass. | 15,597 2 7 | 1,726 15 5 |107 14 8 | 17,431 12 8 |
|Plate. | 3,983 17 9 | | | 3,983 17 9 |
|Broad. | 4 10 | | | 4 10 |
|Crown. |168,892 10 2 | 8,626 9 0 | 10 9 1 |177,529 8 3 |
|Bottle. | 56,770 10 5 |14,819 8 1 |274 10 5 | 71,864 8 11 |
|German | | | | |
|Sheet. | 22,889 17 9 | 32 15 6 | | 22,922 13 3 |
+--------+--------------+-------------+----------+--------------+
|Total. |268,138 8 8 |25,205 8 0 |392 14 2 |293,736 10 10 |
+--------+--------------+-------------+----------+--------------+
The duties payable in the United Kingdom, upon the different
descriptions of glass are, for--
_£. s. d._
Flint glass, the finished article 0 0 2 per lb.
British plate or German sheet, and crown
glass, ditto 3 13 6 per cwt.
Broad glass, ditto 1 10 0 --
Bottles, ditto 0 7 0 --
Plate glass, the fused material in pot 3 0 0 --
GLAZES. See POTTERY.
GLAZIER, is the workman who cuts plates, or panes of glass, with the
diamond, and fastens them by means of putty in frames or window
casements. See DIAMOND, for an explanation of its glass-cutting
property.
GLAUBER SALT; is the old name of sulphate of soda.
[Illustration: 518 519 520]
GLOVE MANUFACTURE. In February, 1822, Mr. James Winter of
Stoke-under-Hambdon, in the county of Somerset, obtained a patent for an
improvement upon a former patent machine of his for sewing and pointing
leather gloves. _Fig._ 518. represents a pedestal, upon which the
instrument called the jaws is to be placed. _Fig._ 519. shows the jaws,
which instead of opening and closing by a circular movement upon a
joint, as described in the former specification, are now made to open
and shut by a parallel horizontal movement, effected by a slide and
screw; _a a_ is the fixed jaw, made of one piece, on the under side of
which is a tenon, to be inserted into the top of the pedestal. By means
of this tenon the jaws may be readily removed, and another similar pair
of jaws placed in their stead, which affords the advantage of expediting
the operation by enabling one person to prepare the work whilst another
is sewing; _b b_ is the movable jaw, made of one piece. The two jaws
being placed together in the manner shown at _fig._ 519., the movable
jaw traverses backwards and forwards upon two guide-bars, _c_, which are
made to pass through holes exactly fitted to them, in the lower parts of
the jaws. At the upper parts of the jaws are, what are called the
indexes, _d d_, which are pressed tightly together by a spring, shown at
_fig._ 520., and intended to be introduced between the perpendicular
ribs of the jaws at _e_. At _f_, is a thumb-screw, passing through the
ribs for the purpose of tightening the jaws, and holding the leather
fast between the indexes while being sewn; this screw, however, will
seldom, if ever, be necessary if the spring is sufficiently strong; _g_
is an eye or ring fixed to the movable jaw, through which the end of a
lever _h_, in _fig._ 518., passes; this lever is connected by a spring
to a treadle _i_, at the base of the pedestal, and by the pressure of
the right foot upon this treadle, the movable jaw is withdrawn; so that
the person employed in sewing may shift the leather, and place another
part of the glove between the jaws. The pieces called indexes, are
connected to the upper part of the jaws, by screws passing through
elongated holes which render them capable of adjustment.
[Illustration: 521 522 523 524]
The patentee states, that in addition to the index described in his
former patent, which is applicable to what is called round-seam sewing
only, and which permits the leather to expand but in one direction, when
the needle is passed through it, namely, upwards; he now makes two
indexes of different construction, one of which he calls the receding
index, and the other the longitudinally grooved index. _Fig._ 521.
represents an end view, and _fig._ 522. a top view of the receding
index, which is particularly adapted for what are called “drawn sewing,
and prick-seam sewing;” this index, instead of biting to the top, is so
rounded off in the inside from the bottom of the cross grooves, as to
permit the needles, by being passed backwards and forwards, to carry the
silk thread on each side of the leather without passing over it. _Fig._
523. represents an end view of the longitudinally grooved index, partly
open, to show the section of the grooves more distinctly; and _fig._
524. represents an inside view of one side of the same index, in which
the longitudinal groove is shown passing from _k_ to _l_. This index is
more particularly adapted to round-seam sewing, and permits the leather
to expand in every direction when the needle is passed through it, by
which the leather is less strained, and the sewing consequently rendered
much stronger.
It is obvious that the parallel horizontal movement may be effected by
other mechanical means besides those adopted here, and the chief novelty
claimed with respect to that movement, is its application to the purpose
of carrying the index used in sewing and pointing leather gloves.
Importation of leather gloves for home consumption; and amount of duty
in
1836. 1837. | 1836. 1837.
1,461,769 | 1,221,350 | _£_27,558 | _£_22,923
[Illustration: 525 526 527]
GLOVE-SEWING. The following simple and ingenious apparatus, invented by
an Englishman, has been employed extensively in Paris, and has enabled
its proprietors to realize a handsome fortune. The French complain that
“it has inundated the world with gloves, made of excellent quality, at
30 per cent. under their former wholesale prices.” The instrument is
shown in profile ready for action in _fig._ 525. It resembles an iron
vice, having the upper portion of each jaw made of brass, and tipped
with a kind of comb of the same metal. The teeth of this comb, only one
twelfth of an inch long, are perfectly regular and equal. Change combs
are provided for different styles of work. The vice A A is made fast to
the edge of the bench or table B, of the proper height, by a thumb-screw
C, armed with a cramp which lays hold of the wood. Of the two jaws
composing the machine, the one D is made fast to the foot A A, but the
other E is movable upon the solid base of the machine, by means of a
hinge at the point F. At I I is shown how the upper brass portion is
adjusted to the lower part made of iron; the two being secured to each
other by two stout screws. The comb, seen separately in _fig._ 527., is
made fast to the upper end of each jaw, by the three screws _n n n_.
_Fig._ 526. is a front view of the jaw mounted with its comb, to
illustrate its construction.
The lever K corresponds by the stout iron wire L, with a pedal pressed
by the needlewoman’s foot, whenever she wishes to separate the two jaws,
in order to insert between them the parallel edges of leather to be
sewed. The instant she lifts her foot, the two jaws join by the force of
the spring G, which pushes the movable jaw E against the stationary one
D. The spring is made fast to the frame of the vice by the screw H.
After putting the double edge to be sewed in its place, the woman passes
her needle successively through all the teeth of the comb, and is sure
of making a regular seam in every direction, provided she is careful to
make the needle graze along the bottom of the notches. As soon as this
piece is sewed, she presses down the pedal with her toes, whereby the
jaws start asunder, allowing her to introduce a new seam; and so in
quick succession.
The comb may have any desired shape, straight or curved; and the teeth
may be larger or smaller, according to the kind of work to be done. With
this view, the combs might be changed as occasion requires; but it is
more economical to have sets of vices ready mounted with combs of every
requisite size and form.