Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/675

Rh M,NUF.-lCI‘URB.] clear glass, and for t';_is parpisg peroxide of manganese, arsenious acid, and nitrate of potash are the materials gene- rally used. These bodies oxidize carbon compounds which may be present, aml 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 con- stitute the “batch.” Formerly it was the habit to frit or partially decompose and fuse the ingredients in a for1n 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 i11 the melting pots, or the tank iu the ease of tank furnaces. .l[+=/tin_r/ I’ot.-.—These pots or crucibles are made of the finest fire-clay, that from Stourbridge in Vorcestershire being exclusively used for glass pots in Great Britain. Great care is requisite iii 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 less while (lrying, 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 C11‘I‘l(3Ll to the pot loft, covered in such a way as to ex- clude dust and other n1i11ute 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 a11d 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 pet arch. I11 drying they com- monly shrink about 2 inches iii 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 fron1 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 d-Lys, more or less, according to their previous state of drynt ss, to the annealing arch, which is gradually and cautiously heated up till it reaches the temperature of the working furnace, whither, after being suﬂieiently 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 em- ployed ; 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 compart- ments, the materials being melted in the ﬁrst, a,nd 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 GLASS G57 opening to the third compartment, whence it was to be drawn for working. The specific gravity of the charge in the first compartment would rise in proportion as the materials melted and became homogeneous in structure. Therefore the metal would sink in proportim as it melted; FIG. 1.—Crown-Glass Pot. and the best melted portions pass into the second com- partment, in which, 11nder the inﬂuence of the direct fur- nace heat, it would be cleared. There, similarly, the per- fectly ﬁned glass falling to the bottom would pass into the cooler working compartment, which is protected bya cover- ing cap. Dr Siemcns’s idea has been practically developed in his continuous tank referred to below. Furnaces.——A glass-melting furnace or oven is a modiﬁed forn1 of reverberatory furnace, which assumes many diﬂ'c-rent 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 em- ployed 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 i11tro- duced. I11 the construction of a furnace the principal objects to be kept in view are not only the production a11d maintenance of an intense heat, but its uniform distribution throughout the furnace, a11d 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 u11der 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 main- tain 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 intersect- ing at the ﬁre-bars, are constructed, so that either can lie used according to the prevailing direction of the wind, &c. In general no ﬂue or cl1inn1ey is directly connected with the furnace, the only exit for the products of combustion being the working holes, a11d 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 pets arranged in a furnace vary from four to ten, and each is reached, either for charging or for work- ing 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 '. — 8 3