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clear glass, and for this purpose peroxide of manganese, arsenious acid, and nitrate of potash are the materials generally used. These bodies oxidize carbon compounds which may be present, and neutralize to a large extent the colour yielded by iron by converting its protoxide into peroxide. Too much manganese, however, gives the glass a reddish tinge, and excess of arsenic produces a milky cloudiness. The various substances employed to produce coloured and opaque varieties of glass will be enumerated when these special kinds are described. The requisite proportions of the raw materials ground and prepared are intimately mixed with the aid of a mixing apparatus, and in this form constitute the "batch." Formerly it was the habit to frit or partially decompose and fuse the ingredients in a form of reverberatory furnace called a calcar arch, but since the use of kelp was abandoned that operation is no longer essential, and generally the well-mixed batch is placed at once in the melting pots, or the tank in the case of tank furnaces.

Melting Pots. These pots or crucibles are made of the finest fire-clay, that from Stourbridge in Worcestershire being exclusively used for glass pots in Great Britain. Great care is requisite in the selection, and in cleansing the clay from extraneous particles, the presence of which, even in the smallest degree, will injure the pot. A fine powder procured by grinding old crucibles is generally mixed, in a proportion seldom larger than a fourth, with what is termed the virgin clay. This mixture dries more rapidly, contracts les's while drying, and presents a firmer resistance to the action of the fire and alkali used in the composition of glass than the simple unmixed clay. These ingredients, having been mixed, are wrought into a paste in a large trough, and carried to the pot loft, covered in such a way as to exclude dust and other minute particles. Here a workman kneads the paste by trampling it with his naked feet, turning it from time to time until it becomes as tough as putty. It is then made into rolls, and wrought, layer upon layer, into a solid and compact body, every care being taken to keep it free of air cavities, which would, by their expansion in the furnace, cause an immediate rupture of the pots. After pots are made, very great care is necessary to bring them to the proper state of dryness before taking them to the annealing or pot arch. In drying they commonly shrink about 2 inches in the circumference. When pots are made during summer, the natural temperature is sufficient for drying them; but in winter they are kept in a temperature of from 60° to 70° Fahr. They remain in the room where they are made for a period varying from nine to twelve months. Being afterwards removed to another apartment, where the heat is from 80° to 90° Fahr., they are kept there for about four weeks. They are then removed for four or five days, more or less, according to their previous state of dryness, to the annealing arch, which is gradually and cautiously heated up till it reaches the temperature of the working furnace, whither, after being sufficiently annealed, they are carried as quickly as possible. Pots last upon an average from eight to ten weeks, and they form a costly item in the manufacturing operations, as each pot is worth on an average about £10; and many of them, notwithstanding all care, crack and give way as soon as they are placed in the melting furnace. For all varieties of glass, excepting lead glass, open pots in the form of a truncated cone, as represented in fig. 1, are employed; but for flint glass a covered pot with an opening at the side, as shown in fig. 2, is essential. Dr Siemens proposed a form of melting pot divided into three compartments, the materials being melted in the first, and passing into the second by an opening at the lower part of the partition, where the metal was to be fined and freed from included air-bubbles, and afterwards to pass by a like

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and the best melted portions pass into the second compartment, in which, under the influence of the direct furnace heat, it would be cleared. There, similarly, the perfectly fined glass falling to the bottom would pass into the cooler working compartment, which is protected by a covering cap. Dr Siemens's idea has been practically developed in his continuous tank referred to below.

Furnaces.-A glass-melting furnace or oven is a modified form of reverberatory furnace, which assumes many different shapes and arrangements according to the kind of glass to the manufacture of which it is devoted, and the nature of the fuel used. As regards the latter cause of difference it may be noted that, while coal is the principal fuel employed in Great Britain, dried wood and peat are extensively consumed in Germany, and in modern times gas furnaces on the Siemens and other principles are being freely introduced. In the construction of a furnace the principal objects to be kept in view are not only the production and maintenance of an intense heat, but its uniform distribution throughout the furnace, and the bringing of the charges of glass materials directly under its fusing influence. The form assumed by melting furnaces is, in general, square or oblong for sheet and plate-glass making, and circular in English flint-glass making. The fire-space or grate occupies the centre of the furnace, and the fire, when fuel is used for direct heating, is either fed or stoked from both ends, or raised from under the bars by a patented method. The fire-grate is usually on a level with the floor of the house in which it is erected, but under it is an arched subterranean passage forming the "cave or ash-pit, both ends of which extend to the open air outside the glass-house. The fire-grate bars are placed in the top of this arched passage, which thus serves as a canal for the atmospheric air required to maintain combustion within the furnace; and for regulating the admission of air, and so controlling the heat, there are doors at both ends of the archway. In some cases two such arched passages at right angles to each other, and intersecting at the fire-bars, are constructed, so that either can be used according to the prevailing direction of the wind, &c. In general no flue or chimney is directly connected with the furnace, the only exit for the products of combustion being the working holes, and thus the heat is directed around and over each pot placed opposite a working hole in the furnace. Within the furnace, around the grate space in the case of circular furnaces, or on both sides of it in quadrangular furnaces, is a raised bank or narrow platform termed the "siege," on which the melting pots are placed. The number of pots arranged in a furnace vary from four to ten, and each is reached, either for charging or for working off the prepared metal, by means of "working holes " in the side of the furnace situated directly over the pots. The general form and construction of a six-pot crown-glass furnace, which also may be taken as the type of sheet and X. - 83

Plate V. plate-glass furnaces, is shown in Plate V., where fig. 3 is a ground plan at the level of the siege of a common form of furnace, while in fig. 1 is seen a front elevation of the same furnace, 1, 2, and 3 being the working holes, 4, 5, 6, and 7 pipe-holes for heating the blowing pipes, and 8, 9, and 10 foot-holes for mending the pots and sieges. The furnace

FIG. 3.-Section of Flint-Glass Furnace. is covered with a low-roofed crown or dome, and the whole structure is bound together with a system of iron bars. The materials used in the construction and lining of all furnaces must be selected with the utmost care, and built with special regard to the enormous temperature to which they are subjected. Formerly a fine-grained purely siliceous sandstone was much used, but now the principal materials

are large moulded bricks or blocks of fire-clay of the most infusible and refractory description. For the crown of the furnaces used in plate-glass melting Dinas silica blocks are employed. In laying the blocks and throwing the arches no mixture containing lime can be used, but only fire-clay or Dinas sand, in as small quantity as possible. Should any of the materials of the crown of the furnace gradually fuse under the influence of the heat, the dropping of the molten matter into the glass-pots is the cause of most serious annoyance and loss to the manufacturer.

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An English flint-glass furnace furnishes the type of circular furnaces. Usually a large number of pots, sometimes ten, are provided for in such a furnace, because, the objects made in flint glass being in general of small size, the metal is worked off only slowly, and a large number of glassblowers can be accommodated at the separate work-holes. The arrangements of the cave and fire-grate are the same as in the case of square or oblong furnaces, but flint-glass furnaces differ from the prevailing rule in others by being provided with a system of flues and chimneys, one flue being placed between each pair of pots. The general appearance presented within a flint-glass house is illustrated in Plateate VI. fig. 1; and the accompanying woodcut (fig. 3) is a sectional illustration showing the construction and internal appearance of a seven-pot furnace. The furnace is com. posed of a double arch or vault springing from strong pillars or abutments. bb. The space c, between the outer arch and the vault proper of the furnace d, is a common receptacle for the flues ff led from within the furnace, and the products of combustion escape by the chimney i The work-holes are at h, and at that place the furnace wall is taken down when a pot requires to be removed and renewed. The "cave" or air canal is seen at k; n is the fire-grate, stoked in this case from one side only; 7 shows openings at which the blowing tubes are heated; m is an opening for cleaning the flues; and a is the bank or siege with the position of the pots indicated. Fre quently instead of being arched the outer portion of the furnace is carried up in the form of a wide truncated cone or open chimney stalk, and in other cases short separate chimney stalks are built for each flue terminating within the glass-house, which itself forms such an open-topped

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