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GLASS.

Spina. Within the present year (1862), glass-reflectors for telescopes, of great size and accuracy, have been made in France. See TELESCOPE.

As regards processes of making, that called the cylindrical was used by the ancients, and is mentioned by Theophilus at the end of the 12th century. The rotatory process was first introduced in Bohemia, subsequently into France in 1730, but not into England till 1832. Pressed glass was invented in America. In England, the tendency has been to throw the trade into fewer hands, there having been 24 window-glass factories in 1847, and only 8 in 1858; but the value of the export increased from £26,694 in 1848 to about £500,000 in 1855.-Franks, A. W., Vitreous Art in the Art Treasures of the United Kingdom (Manchester) Exhibition (4to, 1858); Pellat, A., Curiosities of Glass-making (4to, Lond. 1849); Exhibition of Works of Industry of all Nations (1851); Reports of Juries (1852), Ch. xxiv. p. 521.

Manufacture. The manufacture of glass, as at present carried on, may be classed under the following heads Bottle-glass, Crown Window-glass, Sheet Window-glass, Plate-glass, Flint-glass, Colouredglass. The first is the coarsest kind in common use. In this country, it is made generally of soap-makers' waste (which contains a quantity of soda-salts), fresh-water river-sand, brick-dust, calcined-lime, and marl; to these a quantity of cullet, or the broken glass of the works, is always added at a certain stage of the manufacture. This is the mixture employed in making what are called black bottles, used for wine, beer, &c. Of late years, light-green coloured glass has been preferred for many purposes, such as medicine bottles, sodawater bottles, &c. This colour is commonly produced by adding a large proportion of the cullet of crown-glass, which, by its light colour, dilutes the darker material; if, however, it is wanted of a finer quality, it is made of sand of a light colour, containing only about two-tenths per cent. of the oxide of iron. To 50 parts of this sand are added 20 parts of heavy spar (Sulphate of Baryta), 30 parts of soap-makers' waste, and about two-tenths per cent. of oxide of manganese.

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made by hand, and require great skill and care. The bottom is first moulded on a board. When the bottom is finished, the workman begins to build up the side of the pot by first forming a ring of the same height all round, taking care to round off the upper edge to a semicircular curve of great regularity; upon this he begins bending over other lumps of the paste until another equal layer is formed, and these are continued until the pot is complete; but the workmen do not work continuously at each pot until it is finished, they leave off from time to time, spreading wet cloths over the edge when they discontinue working. This is necessary, to admit of a certain amount of drying, otherwise the large weight of clay used would prevent the form from being kept, and the pot would fall to pieces, or lose shape seriously, the building of the pot is consequently extended over several days. Those made in the form of fig. 6 are from three to four inches thick, but the flint-glass pots are only from two to three inches. After the potter has finished his work, the pots are removed into the first drying-floor, where they are only protected from draughts, so that the drying may be conducted with the greatest possible uni formity. When they have progressed sufficiently, they are removed to the second drying-floor, which is heated with a stove, and the drying is here completed. They are then placed in the store, where usually a good stock is kept on hand, as time In France, kelp and wood-ashes are used to fur- improves them, and they are seldom kept less than nish the alkaline portion of the mixture; in other six or nine months. When required for use, they respects, the material is essentially the same. In are placed for four or five days in the annealing Germany, where a rich brown tint is in fashion for furnace, which is on the reverberatory principle, and bottles for the light-coloured Rhine wines, the they are there kept at a red heat. This furnace is materials consist of a light-coloured clay, 16 parts; so situated, that the pots, when ready, can be most a light yellow-coloured sand, 20 parts; kelp, 8 quickly transferred to the main furnace an operparts; wood-ashes, 38 parts; cullet, 15 parts; and ation of exceeding difficulty, and requiring great oxide of manganese, 3 parts. skill and dexterity, as they have to be removed whilst red-hot, and it must be done so quickly that no sudden cooling shall injure the pot, a difficulty which can only be understood by remembering that the ordinary pots are nearly four feet in depth, are the same in width at the mouth by about thirty inches at the bottom, and they weigh several hundredweights. The enormous amount of labour bestowed upon these pots makes them very expen sive, their value being from £6 to £10 each. Their removal from the annealing oven to the main furnace is effected by an immense pair of forceps several feet in length, which are placed horizontally upon an upright iron pillar about three feet in height, which rises from a small iron truck on four wheels, so that the whole apparatus can be easily moved from place to place. By means of this instrument the pot is lifted and dexterously withdrawn from the oven, and as quickly transferred to its position in the main furnace, in which usually four or six are placed on a platform of firebrick or stone, each pot being opposite to a small arched opening, through which it can be filled and emptied.

One of the first essentials to a successful manufacture of glass, is the preparation of the meltingpots. These pots are composed of clay, which is required to be as free as possible from lime and iron. A clay obtained from the carboniferous shales of Worcestershire, in the neighbourhood of Stourbridge, is the most esteemed for this purpose; it consists of pretty nearly equal proportions of silica and alumina. The clay is carefully dried and sifted, after which it is mixed with hot water, and worked into a paste; it is then transferred to the kneading-floor, and when sufficiently kneaded-which is done by men treading it with naked feet-it is laid in large masses in a damp store-cellar to ripen, a process the theory of which is not well understood. When required for forming the pots, a sufficient quantity is taken and again kneaded with onefourth of its quantity of the material of old pots, which are ground to fine powder and carefully sifted; this material gives firmness and consistency to the paste, and renders it less liable to be affected by heat. The pots are of two kinds, the open (fig. 6)

GLASS.

The entrance to the main furnace, through which the pots have been introduced, is then closed with a movable door of firebrick, and covered over with fireclay, to prevent the escape of heat; the pots in the furnace are filled with the prepared materials for glass, now called frit, mixed with about a sixth or eighth part of cullet or broken glass; the openings are closed temporarily for two or three hours, by which time the first charge of material has melted down, leaving room for a further supply, which is then thrown into the pot, and this is repeated two or three times until the pot is completely full. The openings are then closed, and the heat increased to the utmost for ten or twelve hours: this part of the operation is called founding, and the result of it is to perfectly melt and vitrify the materials. The heat of the furnace is now somewhat reduced, and the scum is removed from the surface of the melted material, now technically called metal, by a workman called the skimmer, whose labour requires great care and much experience, as the metal is at a glowing white heat, and is only with difficulty distinguishable in the fierce white glare of the furnace. The metal is now ready for the commencement of the journey, as the operation of working it up is called. This term, like most others in the glass trade, is derived from the French.

The arrangements so far apply equally to all kinds of glass. We now, however, return to the manufacture of glass bottles, in order the more fully to understand which, we give the following groundplan of one of the houses in which this is carried on (fig. 8). a is the main furnace, which in this case

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s square, and made to hold only four pots; at each corner is an opening, which allows the fire to enter four small reverberatory furnaces, b, b, b, b, called arches; two are called the coarse arches, and the others the fine arches. In the two former, the soapmakers' waste is calcined at a red heat for at least four hours, or whilst a set of pots is being worked out that is to say, one journey. Then the calcined material is ground and sifted in the grinding and sifting house, h, after which it is mingled with the sand, &c., and transferred to the fine arches, where for the term of another journey it is again calcined. At the end of that time, the pots being empty, are refilled with this material.

When the furnaces are opened for a journey, the skimmer first removes the scum, and makes the

way clear for the blower and moulder, who takes his blow-pipe of iron, six feet in length, the part held in the hand being guarded by a covering of wood and other non-conducting materials. After heating the end of the blow-pipe in the furnace mouth, he dips it into the pot, and turning it round, gathers as much metal on the end as is sufficient to form a bottle of the size required. Usually, in bottle making, one gathering suffices, but in larger operations, such as blowing window. glass, more gatherings have to be made. The operator then blows gently down the pipe, and having thus slightly distended the bulb of red hot plastic glass (fig. 11, a), he takes it to a plate of polished iron, forming a low bench called the maver, or mavering table. On this he turns it round, moulding the round lump of glass into a conical form, the change being represented in fig. 11, b. This operation, called mavering, is per formed in all cases where glass is blown; and as it is necessary that the glass should be pretty firm before mavering, it is often cooled by sprink ling with water, and even, as in the case of window-glass and other large blowings, turning it in a cavity containing water, which is made by hollowing out a block of wood, usually, if attainable, that of the pear-tree, which is said to be best for the purpose.

After being mavered, the glass is held to the mouth of the furnace, and the operator blows down his blow-pipe, and further distends his glass. Formerly, he commenced moulding it into the form of a bottle with his shears, one arm of which was of charred wood, and the concave bottom was made by pushing a little piece of glass, called a punty, at the end of an iron rod called the pointel; the blow. pipe was then detached by a slight blow of the shears, and the partly formed bottle was left at the end of the pointel attached by the punty in the hands of a boy who attends upon the man, and brought and applied the punty. The man then took the pointel in one hand, and after softening the bottle in the mouth of the furnace, moulded the neck by means of his shears, regulating the size of the opening by means of a small brass mould, the size and shape of a cork, attached to the middle of the shears; heating the neck again, he formed with a small portion of metal from the pot the ring round the mouth of the bottle. Now, however, after mavering, and the first slight blowing, the operator inserts the glass into an iron or brass mould, which is formed in two pieces, opening or closing by the pressure of the foot on a lever. When the mould is closed, he blows down the pipe, and the bottle is completed all but the neck, the ring of which has to be formed by the addition of a fresh piece of metal, as before des cribed. By this process, bottles are made with wonderful rapidity and exactness. At this stage of the manufacture, by either process, the bottles are taken from the workman by a little boy, who inserts the prongs of a fork into the necks, and carries them to one of the annealing arches, d, d, d, d, d, d, where they are carefully arranged in proper bins until the arch, which usually holds 144 dozen, is full; it is then closed, and the heat is raised nearly to melting point, and then allowed gradually to subside until it becomes cold, when the bottles are removed to make room for a fresh charge. In the plan, fig. 8, e and ƒ are the sand and alkali stores; 9, 9, are stores for the prepared frit; and i, j, are sifting-cribs in the sifting-house.

Window-glass, whether crown or sheet, is made of much more carefully selected materials. They are slightly varied by different manufacturers, but the following are the ingredients used in one of the

GLASS.

largest glass-houses in Great Britain: Sand (well and after turning it about for a minute or two dried), from the neighbourhood of Leighton Buz- in the air until sufficiently cooled, he then dips zard, in Bedfordshire; sulphate of soda, ground; it in again, and over the first he makes a second subcarbonate of soda, white oxide of arsenic, man- gathering, which increases the weight to about ganese, Welsh anthracite, chalk; limestone from Hopton Wood, Derbyshire; nitrate of soda; cullet, about as much as is equal to an eighth part of the other ingredients. The exact proportions are only known to the manufacturers. Each ingredient is carefully powdered before mixing, and they are afterwards calcined or fritted, except the anthracite, which is added in the pot for the purpose of decomposing the sulphate of soda, and dissipating its acid; and the manganese and arsenic, which are only added in very small quantities, to improve the colour; too much, however, of each is sure to injure the glass, and therefore these materials can only be safely used by experienced manipulators. The bulk of the glass, however, consists of the sand, and carbonate and sulphate of soda.

The arrangement of the window-glass houses is different, and on a much larger scale than in the houses for bottle-glass, and excepting in gathering and mavering, all the operations subsequent to the founding are different. Fig. 9 will give a general

a

three pounds weight; the same cooling process is repeated, and a third gathering is made, which brings up the weight to about nine pounds; he then holds his blow-pipe perpendicularly with the glass downward, so that it may by its own weight pull downward from the pipe in the form of a symmetrical pear-shaped bulb; he next takes it to the hollowed block before mentioned, and turns it round in the water placed in the cavity by which it is made ready for the mavering table. The workman, by skilful management, mavers the bulb of glass into the form b, fig. 11, and then forms a little knob at its apex, by turning it on a fixed bar of iron called the bullion bar; he then commences blowing, and soon the bulb of nearly' solid glass is expanded into a large hollow sphere (c, fig. 11), still, however, with the little nipple made by the bullion bar. A little boy now comes forward with an iron rod, the pointel, upon the end of which has been gathered a small lump of metal, called the punty, about the size of a hen's egg, this he applies to the nipple, to which it firmly adheres, the workman meanwhile resting his blow. pipe on a fixed rest called the casher-box, placed for the purpose; by the pressure of the pointil the globe of glass is flattened as in d, fig. 11. The application

dd d

dd d

Fig. 9.

plan of the house for crown window-glass, and fig. 10 gives an elevation of one side of the main furnace, with the three openings through which the glass is gathered from the pots. In fig. 9, a is the main furnace; b, b, two flashing furnaces; the projecting piece of brick-work, b', being the screen which protects the workman from the fire; and c, c are two annealing furnaces or ovens.

When the founding or melting and the skimming are completed, the workman takes his blow-pipe, which is about seven feet in length, heats it at the end, and dipping it into the pot of melted glass

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Fig. 11.

of a piece of iron, cooled for the purpose by keeping it in water, to the junction of the glass with the blowpipe, detaches it instantly, and the globe of glass is now held with the pointil. The operator carries it next to the nose-hole (b, fig. 10), and presents the opening formed by the detachment of the blow-pipe, to the action of the furnace; this again softens the glass, which is kept continually revolving by turning the pointil on an iron rest or hook fixed to the masonry of the furnace. The revolutions are at first slow, but are gradually accelerated as the softening of the glass goes on, and the centrifugal force so produced throws the edges of the orifice outwards, as in e, fig. 11. As the glass flattens, it is revolved with greater rapidity, and advanced so near to the mouth of the nose-hole as to draw the flames outward, by contracting the draught. This completes the softening of the glass, which is done suddenly, with a rushing noise like the unfurling of a flag in the wind, caused by the rapid flying outward of the softened glass and the rush of the flames outwards. It becomes perfectly flat, and of equal thickness, except at the bullion or centre, formed, as before described, by the bullion-bar and the punty. The flashing is now complete; and the workman removes it from the nose-hole, and still continuing to turn it in his hands, in order to cool and harden it, as he walks along, carries it to the annealing oven, where another one receives it on a large flattened fork-like implement at the moment the flasher, who has hold of the pointil, suddenly detaches it by a touch of his shears. It is then passed through the long horizontal slit which forms the opening into the annealing oven, and when fairly in, it is dexterously turned on

GLASS.

its odge; here it remains at a temperature somewhat below that required to soften glass, until the oven is filled with these so-called tables of glass, when the heat is suffered to decline, until the whole is cold, when they are removed to the packing-room, to be packed in crates for sale.

Until lately, crown-glass was almost universally employed for windows, but now that which is called German sheet has become quite as common, besides which British sheet, which is the same glass polished, and plate-glass are much used. The operation of making the sheet-glass is very different from that employed in making crown-glass, inasmuch as a long and perfect cylinder is sought to be produced by the blower instead of a sphere of glass. This necessitates also a different arrangement of the glass-house, as is seen by the ground-plan shewn in fig. 12: aa is the furnace, is the annealing oven,

a

Fig. 12.

heated by the flue b', which opens into the main furnace; the leer, or annealing oven, is often, however, an independent structure; c, c, c, c, c, c, c, c, are the eight pots, which is the number usually employed in these works. These, of course, are opposite to the openings for working them, and in front of each opening is a long opening in the ground, about eight feet deep and three feet in width; d, d, d, d, d, d, d, d. The workman stands on the edge of this pit, and having made his gathering, as in the crown-glass manufacture (a, fig. 13), he next mavers it, without, however, using the bullion-rod (b, fig. 13). He next proceeds to blow his glass, holding it downward whilst doing so, that its weight may widen and elongate the bulb, and from time to time dexterously swings it round, which greatly increases its length (c, d, fig. 13). As it cools rapidly in this operation, he from time to time places his pipe in the rest which is fixed before the furnacemouth, and gently turning it round, he brings it again nearly to the melting-point, then he repeats the blowing and swinging, standing over the pit, to enable him to swing it completely round as it lengthens out. These operations are continued until the cylinder has reached its maximum size, that is, until it is of equal thickness throughout, and sufficiently long and broad to admit of sheets of the required size being made from it (e, fig. 13). Sometimes these cylinders are made 60 inches in length, allowing sheets of glass 49 inches in length to be made from them. The next operation is to place the pipe in the rest, and apply the thumb so as to close

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turns the cylinder, still with its end to the fire, and the softened edges of the opening, which at first are curved inwards, are flashed out until they are in a straight line with the sides of the cylinder (g, fig. 13). It is then removed, and placed on a rest or casher-box, when a small punty of melted glass at the end of a pointil is brought by a boy; this the workman applies to one side of the cylinder, just below the shoulder formed at the blow-pipe end (fig. 13, g), and drawing it out to a thin string, wraps it quickly so as to draw a line round the cylinder; after a second or two, he withdraws this line of red-hot glass, and touching it quickly with his cold shears, the shoulder and neck drop off as neatly as if cut with a diamond.

The cylinder (fig. 13, h) is now placed for a short time in the annealing oven (b, fig. 12), where it is prepared for cutting; it is next placed in a groove lined with green baize, and a diamond fixed to a sliding rule makes a perfectly straight cut from end to end. The split cylinder is then taken to the flattening arch or furnace, where it is laid on the bottom, with the diamond-cut upwards. The bottom is a perfectly smooth stone, kept constantly free from dust by the workman; here the heat is sufficient to soften without melting the glass, and the flattener, as it softens, opens the two edges of the crack until they fall outward flat on the stone; he then takes an implement in the form of a rake, made by placing a piece of charred wood transversely at the end of a long handle, and this is gently rubbed over the glass, producing a very smooth surface. At the back of the flattening arch is an annealing oven, communicating with the arch by a narrow horizontal slit, through which the sheet of glass is now pushed on to a plate of iron, which receives it; and as this plate is one of a series linked together so as to form an endless band, which can be turned round, the sheets move forward into the annealing oven, where the workman gently lifts them on edge until the oven is filled, when, as in the case of crownglass, the heat is allowed to decline until perfectly cool, the sheets are then ready for use. Very much larger sheets are obtained by this process than by the former one, hence it is becoming of great importance; but it is not easy to obtain workmen sufficiently powerful and dexterous to blow and twirl the largest-sized cylinders; at present, we obtain almost all the operatives so employed from Belgium.

Glass-shades are made in the same manner as above described; indeed, they are nothing more than the rounded ends of the cylinders before being burst. When wanted oval or square, these forms are produced by boxes of wood charred inside, of the size the shades are required, through which the

GLASS.

cylinder is passed when being blown, until the soft glass touches, and receives shapes from the inside of the box or mould: they are afterwards annealed, and cut to the lengths required. If of large diameter, they require immense strength and great skill in the operator, who sometimes aids the power of his breath by taking into his mouth a little spirit, which he blows down the pipe; this, of course, is instantly converted into vapour, when it reaches the red-hot cylinder, and by its expansion aids in distending the glass.

Plate-glass is made in a totally different manner; and as its value depends chiefly on its purity, the greatest possible care is taken to procure materials

of the best quality, and almost every manufacturer has his own private formula for the mixture. It may, however, be said to consist chiefly of sand and alkaline salts, as in other kinds of glass, and the following is one receipt known to be in use: Fine white sand well washed, to free it from impurities, 720 lbs.; sulphate of soda, 450 lbs.; slaked lime, 80 lbs.; nitrate of potash, 25 lbs.; and cullet of plateglass, 425 lbs. These ingredients, when melted and skimmed, should yield about 1200 lbs. of perfectly clear metal, which is the quantity usually required for a casting. When melted and ready for use, the pot is lifted out of the furnace (aa, fig. 14) by means of the forceps, and wheeled up to the casting-table

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(cc, fig. 14); here it is seized by a crane and tackle, | by the narrow openings, ff; and, after they have by which it is lifted, and so nicely poised over sufficiently cooled, are removed through the openthe table, that it can be easily tilted so as to pour ings at each end, g, g. out its contents. All this requires so much care and steadiness, that the men, impressed with the great danger of carelessness, usually preserve perfect silence during their work. The table is of large size -20 feet or more in length, by 8 or 10 feet in width. When the red-hot liquid glass is poured on, it immediately begins to spread; two bars of iron, a little thicker than the plate is intended to be, are quickly laid on each side of the table, and a steel roller is laid across, resting on these bars: this roller is worked by hand, and rapidly spreads the glass all over the table, the bars preventing it from running over the sides, and regulating its thickness. In a very short time, it begins to cool; the men then seize the end of it with pincers, and pull it forward with great dexterity on to an endless band of wire-gauze, which, being made to revolve, moves the immense plate forward to a slit-like opening to the annealing oven (fig. 14, ff), where it is worked on to another table on wheels, which is pushed forward to make room for another. The annealing oven is usually of immense length, as, in the case of plate-glass, the sheets cannot be set on edge. At the works at St Helen's, in Lancashire, where glass of all kinds is extensively made, there are usually two annealing ovens to each shed, the furnaces being placed between them; each oven runs to the end of the shed, and these sheds are usually over 300 feet in length. The ground-plan shewn in fig. 14 will give a general idea of the arrangement of one of these vast work-shops. The main building is a shed, with the doors at each end, and both doors and windows are made so as to exclude drafts of air, which, if admitted during the operation of casting, are highly injurious to the quality of the manufacture. a, a, are the two melting-furnaces; b, b, b, b, b, b, the pots; c, c, the casting-tables; d, d, the endless bands of wire-gauze for moving the plates to the annealing ovens ; e, e, where they enter

The plates are next removed to the first polishing. shed, where each is imbedded in a matrix of stucco, leaving one surface exposed; the whole is enclosed in a frame, which holds both glass and stucco securely. Two of these frames are placed one over the other, with the two exposed surfaces of glass in contact. The lower frame is fixed, and the upper is made to move by machinery with great rapidity backward and forward with a swinging motion, so as to describe an opposite curve with each backward and forward motion. Sand and water are continually thrown on the surface of the fixed plate, and thus the first stage of polishing is performed. The plates are then readjusted in the frames, and the other surfaces are brought upwards, and receive a similar rubbing down with sand and water. The plates are next removed to the second polishingroom, where women are usually employed; here they are again fixed on low tables, and each woman rubs the surface for a long time with a piece of plate-glass, covering from time to time the whole face of the plate with emery-powder and water. After both sides have received this hand-polishing, the plates are removed to a third room, where they are again imbedded on tables which movable by machinery, so that the whole surface of the plate may be brought under the action of the polishers. These are large movable blocks, covered with woollen cloth and leather, and loaded so as to press on the glass; the polishing material used is colcothar, the red oxide of iron; this completes the polish which gives so much beauty to plate-glass. It is a long and laborious process, and is the chief cause of the high price of plate as com pared with other sheet-glass. British plate is only the cylinder glass polished by the processes just described; its comparative cheapness is due to the rapidity with which the cylinder can be blown. Of this rapidity, the best estimate may be formed from

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