Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/685

Rh M.I'UFAC1‘URE.] size. In these troughs the emery powder is deposited in increasingly ﬁne division, according to the growing capacity of the trough and the consequent slow replacement of its contents. The last touches of the smoothing process can only be given by the hand, which at once detects any appearance of grittiness. Both sides of the glass are in succession submitted to these operations, after which it is again bedded in plaster and ﬁxed on the polishing table. The polishing is done with reciprocating rubbers, covered with ﬁne felt, and supplied with rouge (peroxide of iron) in a liquid state. While a reciprocating motion is connnunicated to the rubbers, the table itself moves backward and forvard in a transverse direction, so that all parts of the plate are equally brought under the polishing influence of the rubbers. About 40 per cent. of the Weight of the rough plate is removed in the three polishing operations. Ordinary ﬁnished plates vary in thickness from about ,§th to 3th inch, and the largest sizes measure about 17 feet by 9 feet 6 inches. The great St Gobain Company of France quotes regular prices 111) to 324e. (10 feet 8 inches) by 204C. (6 feet 8 inches), beyond which size the price becomes special. In the Paris Exhibition of 1878 that company showed a silvered plate 24 feet by 14 feet, the largest piece of plate glass which has hitherto been polished. 1.’olIr:a;l 1’latc.——.- form in which unpolished plate glass is exten- sively employed is the patent rolled plate, originally made by llartley 5; Co. of Sunderland. The surface of the casting table on which the rolled plate is spread, instead of being smooth and plain, is engraved or otherwise indented with ﬁne lines, grooves, or tlutcs, or it may be with small squares, lozenges, or even ornamental patterns, and the glass, of course, takes on its lovcr surface an accurate impression of any such pattern. Rolled plate is now very largely used for partitions, and in places where obscure lights are required. Such plates are always cast comparatively thin, and of moderate size, so that a large number may be piled in the annealing oven. They, as well as certain qualities of coloured glass, are cast by ladling the molten metal from huge pots which may contain about 2% tons of material. By this Iadling numerous “ air bells ” are enclosed in the glass, b11t the circumstance does not affect the durability and usefulness of the glass. FLll"l‘ GLAss on Cr.vsrAL.—'l‘he name flint glass originated in the circumstance that at first the silica used in the manufacture of this variety of glass was in the for111 of ground flints. The industry belongs characteristically to the United Kingdom, where it was first established on a large scale, and to the present (lay ﬂint glass is much more extensively manufactured in England than in any other country. ' Flint glass is a compound entirely ditferent from those above described, consisting as it does of a silicate of potassium and lead. As is the case with all kinds of glass, its composition and the pro- portion of ingredients used in its preparation vary widely. The average composition of a batch has been stated thus 2- Finc white sand parts. .linium (rcd lead) ..  66 ,, Reﬁned potash  -‘Z. ,, Nitre   10 ,, Cullct  .  GU to 100 ,, Small portions of bleaching ingredients, as white arsenic or man- ganese, are also sometimes added. Purity of materials is of the utmost consequence in the manufacture of ﬂint glass, which is prized in proportion to the transparency, uniformity, sparkle, and freedom from colour of the metal ; and, as ﬁnished objects are in many cases massive, defects of colourare very obvious. Flint glass is much more fusible than the kinds destitute of lead; it possesses great brilliancy, owing to its high refractive and dispersive power, but being comparatively soft its surface is easily scratched and dulled. It is also slowly corroded by alkalies, and contact with solutions of sulphides blackcn it. Its speeitic gravity varies according to the proportion of lead it contains ; and, as the silicate of lead tends to sink when the glass is in the molten state, Faraday found glass from one melting to vary from 3'28 to 3‘85, and in another instance from 3'81 to 4'75. The higher the speeitic gravity of the glass the greater is its refractive power and consequent brilliancy. Flint glass is in no case used for architectural or structural pur- poses, but its purity and lustrc peculiarly fit it for table glass, ornamental objects, glass globes and lustres, and for imitations of gems and precious stones. For the latter purpose a dense glass called strass, appropriately coloured when necessary, is employed, and a glass of still greater density and refractive power is used for optical puposes. The softness of Ilint glass adapts it for engraving, cutting, and polishing; and these methods of ornamenting the finished glass are very much employed. The special covered form of pot (fig. 2, szzpm) and the arrange- ment of the furnace have already been alluded to. Plate VI., ﬁg. 1, represents the various arrangements, tools, and processes connected with a tlint—glass house, the building in the centre being the large cone or chimney built over the furnace, which is seen through the arches a. At 5 and 6 men are seen at the working holes with- drawing metal from the pots on their long iron tubes; 7 is the marver on which the gathering is rolled till it acquires a circular shape ; at S a blower is seen in process of expanding a gathering GLASS 663 of glass by blowing ; and at 9 a servitor or second man is attaelnng a post or lump of metal he has gathered on a pontil or punty to the end of a blown globe of glass. The two masses of glass are thus united together, and that attached to the hollow tube is separated by touching it, mar to where the tube enters the globe, with a small piece of iron wetted with water. By this means the glass cracks, and a smart blow on the iron tube completes the disunion. The workman now takes the punty from his assistant, and laying it on his chair arm, rolls it backward and forward with his left arm, while with his right he moulds it into the various shapes required, by 111cans of a very few simple instru- ments. Iiy one of these, called a pueellas, the blades of which are attached by an elastic bow like a pair of sugar-tongs, the dimen- sions of the vessel can be enlarged or contracted at pleasure. Any superfluous matter is cut away by a pair of scissors. For smoothing and equalizing the sides of the vessel a piece of wood is used. After the article is ﬁnished it is detached from the punty and carried on a pronged stick to the annealing oven. The annealing oven or leer for flint glass is a low arched furnace, generally of considerable length, with several openings at each end between which narrow lines of rails run. On these rails, small waggons, or trays mounted on four wheels, are plaeed,and the articles to be annealed are filled into such waggons. They are slowly pushed to the hottest part of the chamber, and passing that point they vcry gradually approach the cold end of the oven, from which they are withdrawn fully annealed. As each waggon is withdrawn at one end, another is entered at the other so that the line from end to end is kept constantly full. The ordinary method of gradually decreas- ing the temperature around the articles stationary in the leer is also practised. Flint—Glass Cuttint, Engra-vilzg, (mrl Etchz'n_r/.—Tl1c sparkle and brilliancy of flint glass is developed by the process of grinding and polishing technically called glass-cutting. In ﬁg. 18 is seen a. representation of a glass-cutter’s mill, a. being the pulley and band communicating motion to the mill 1), which is made of wrought Elm it lzl 7-! N x ,, _ - _, _ 7 _ __, —; , __ Flt}. 18.-——(§lass-Cutter's Mill. or cast iron. Over it is suspended a wooden trough or cistern 0, containing a mixture of sand and water, which is fed on the wheel as required for the operation ofgrinding. Smoothing is done on a wheel of ﬁne sandstone to which water alone is applied, and for polishing, a wooden wheel supplied with emery, and tinally with putty powder (oxide of tin), is employed. The trough (l under the wheel receives the detritus of the grinding aml other operations. The articles are held in tlic hand, and applied to the mill while rotating. The punty marks are ground off tumblers, wine-glasses, and the like, b_v boys holding them on small stone mills. Ground or obscured glass is made by rubbing the surface with sand and water. Iron tools ﬁxed on a lathe aml moistened with sand and water are used to rough out the stoppers and necks of bottles, which are completed by hand with emery and water. ]-Engraving is the production of ornamental surfaces by a ﬁne kind of grinding mostly done with copper discs revolving in a lathe. Etching is variously done by