Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/684

Rh G62 ﬁres being arranged and the inside partitioned so that the split cylinder gradually approaches the greatest heat, where the flatteninnr takes place, and then passes round in a decreasing temperature_tilI at the opening into the annealing arch it has attained sutﬁeient. consistency to be moved into the. _vct cooler annealing arch, round which the sheet is slowly carried till it arrives fully annealed at the point where it is withdrawn. _ Although the average size of ﬁnished sheet glass as now made is about 50 inches by 36 inches, very much larger sizes may _be and occasionally are produced, the extreme trade limit being 85 inches long by -19 broad; but both these extremes caimot _be obtained in the same sheet, and few workmen attain the dexterity necessary for properly manipulating the larger sizes. In Belgium sheets from blown cylinders measuring 10 feet by 4 feet have been made. The thickness of ﬁnished sheet glass is estimated by the amount it weighs per square foot, and the coiniiiereial range of wei ht is from 15 oz. to 42 oz. per foot, the thicker and heavier kinds, of course, selling at the highest price. Polz'shz'n.g of Sheet Glas-s.—Polislied sheet glass is known in com- merce as patent plate glass, to distinguish it from ordinary polished cast-plate. The practice of polishing sheet glass was ﬁrst intro- duced and patented by Chance of Birmingham, and polished sheet is now in considerable demand for photographic purposes, for fram- ing cnrrravings, and generally where a ﬁne true surface combined with lightness or thinness is requisite. The polishing process in- volves t.wo operations——smootliiiig and ﬁnishing or polishing. The smoothing is done on a thick slab of slate, which must possess a surface as smooth and level as possible. This is covered with a piece of wet cotton cloth, and, the sheet of glass to be smoothed being laid thereon, by gentle pressure all air is expelled from between the surfaces, and the atmospheric pressure then keeps the glass ﬁrmly in position. The process of smoothing and the subsequent ﬁnishing are from this )0l1lt the same as in the case of ordinary plate glass, under which head these operations will be detailed. It is only selected plates of ﬁne clear metal that are used for polishing. PLATE GLASS is manufactured by a. process entirely differeiit from any of those yet described, consisting as it does of glass cast and spread in sheets. As plate glass is invariably thicker than blown sheet, it is of the utmost consequence that the “metal” should be prepared from the purest possible materials, and that the found- ing or melting should be done with great care to ensure the expul- sion of air bubbles. It is only thus that the ﬁne clear homogeneous and ﬂawless plates aimed at in the manufacture can be produced. Formerly the French excelled all other nations in the manufacture of plate glass, and the manufactures of the great St Gobaiii Com- pany yet command the highest prices in the market; but the increased care which has now for about a quarter of a century been given in England to the purity of materials, and the ingenious machinery which has been devised for polishing plate in England, have resulted in placing British-niade plate glass on a level with the best productions of France. The following is given as an example of the materials employed for the preparation of ﬁne French plate :— White quartzose sand. Sodic carbonate....... Lime (slaked). Manganese pero. dc Cullet .1000 parts. 333 . 14-3 ,',' . 0-15 ,, ....10o-0 ,, The materials are melted in furnaces and pots of the ordinary construction, but in some cases the melting pots are of greater capacity, and contain charges of from 2 to 21; tons. In French establishments, and in many others, two forms of pot are placed within the furnace———(]) the ordinary melting pot, and (2) a. pouring pot or cutcttn, quadrangular in form, made of the same material as the melting pot, and capable of holding suflicient metal for casting a single sheet of the superﬁcial extent and thickness desired. Melting pots and cuvcttcs are placed side by side in the furnace, and the molten mixture is ladled with copper or malleable iron ladies from the pot into the cuvette, in which it is allowed to clear before casting. In some works, however, the molten glass is poured direct from the melting pots, and in other cases it is ladled to the casting table from the huge melting pots with larrre malle- able iron ladies. D The casting table is a heavy thick fl-at table of cast-iron, of a length and breadth exceeding the size of the largest plate of glass which may be cast on it. At one end is placcdba heavy cast-iron roller, the full breadth of the table, and ﬁtted to roll the whole length of the table by means of spur wheels working into gearing along its sides. The cast-iron roller determines the thickness of this glass by the height at which it is caused to roll above the table, and that height is regulated by plar-iiig nairow strips or ribs of GLASS metal of the required thickness of the glass along the edges of ' the table on which the two extremities of the roller bear as it revolves. The breadth of the plate again is determined within the limits of the table by the two sides of the “gun,” an apparatus consisting of two plates of cast-metal, p_lae,ed in front of the roller, and bolted together by cross l-ars at a IllFlﬂl2('C apart 'lllL'll can be [MANUFACTURE easily altered and adjusted according to the breadth of plate the apparatus is intended to control. The edge of the plates abutting against the roller are accurately ﬁtted to the roller curvature, mid thus they and the roller form tln'ee enclosing sides for the molten mass poured on the table, and as they travel forward they carry in front of them all the semi-fluid niass except the uniform layer which represents the distance between the moving surface of the roller and the surface of the casting table on which the glass is spread. As the glass does not instantly solidify the moment the roller has past over and spread it into a sheet, the edges generally assume a rounded and somewhat wavy cast-like appcaraner-. Immediately the plate has suﬁicicntly solidiﬁed to bear moving, it is taken to the annealing furnace, the heat of which has been care- fully raised to the proper pitch. The sole of the annealing oven on which the plate is to rest (for with large plates any piling on edge is impracticable) must be as smooth and level as possible, since the still senii-plastic mass moulds itself to the surface on which it is laid. As the oven only accommodates a single plate of the largest dimen- sions, aiid since the annealing process occupics several days, a large number of ovens, occupying a great space and involving the cxpcii- diture of iinicli labour and fuel, are needed in works where iiiaiiy plates are cast. In dealing with plates of small size they may be laid horizontally on the furnace bed for a little till they come to the fur- nace temperature, after which they are piled up on edge in l.‘L'lltl(S or thirties, leaning against a range of iron bars running across tl.e furnace. 'hcn the plates have been deposited in the annealing oven, the openings are all carefully stopped up, and the furnace with its contents is cooled by slow degrees down to a temperature at which the glass may safely be withdrawn and exposed to ordi- nary atmospheric inﬂuences. The cooling of such furnaces is now hastened as far as the safety of the plates and the completion of the annealing will permit by allowing cold currents of air to pass under their sole. The plates, as withdrawn from the annealing oven, have a very irregular, rough, undulating surface, and although the glass is per- fcctly pure, they have, owing to their uneven surface, no trans- p-areiicy. In this condition they constitute the “ coninion rough plate ” of commerce, and as such they are extensively used for the glazing of roofs, for floor and cellar lights, and generally in positions where light without transparency is requisite. Polz‘shing.—Whcii the annealed plates are witlidrawii from the oven they are carefully examined for any defects, such as spots, air-bells, &c., which they frequently exhibit. If sciious defects are found, the plates are cut into the largest __ pieces which can be obtained free from ‘ flaws, and the select- ed pieces are then submitted to the operations of polisli- , ing, consisting of— ,' _ ____ __ .' (1) grinding, (2) ' smoothing, and (3) -" ,3, - I. polishing. Various -. v‘ forms of machine ‘-._ have been devised and introduced for effecting these opera- tions. The grind- ing aiid sinoothing - F table invented by Mr Daglish of Raven- hend works consists FIG. 17.—Platc-Glass Grinding Table. of a great revolving ﬁat table A (ﬁg. 17), with a strong ﬁxed bar ('13, stretched across it about 10 inches above its surface, to which two runner- frames B, 1,’-, shod with iron, are pivoted at Q. The glass to be ground is cemented with plaster of Paris on the surface of the table, and the runner-frames rest on that surface. 'hen the table A is set in motion, each oblong runner-franic 13 has communicated to it a revolving motion also, owing to the excess of friction brought. into action by the more rapid revolution of the outer edges of the revolving table. The ctfcct of these com- pound revoliitions is that every point of the surface_of the glass is equally and uniformly abraded, when the apparatus in r_evolution is fed with sharp sand and water. _ hen alevcl surface is thus pre- pared, the smoothing process is next bcguii on the same table, the only difference being that emery powder of increasing degrees of ﬁneness is substituted for the sand, and as_the operation approaches completion the utmost care must be exercised as to the purity and uniformity of the emery, seeing a single particle of grit would eil'ec- tually destroy the smoothness of a whole plate. _ The quality of the emery is thus of essential consequence, and an ingenious apparatus is brought into use for arranging the sizes by means of a current of water of diminishing velocity, obtained by passing a uniform stream _ with agitation successively through cylinders or troughs of iiiercasing