Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/688

G66 extensive use, in the rapidly increasing applications of emcry to grinding, smoothing, and cutting. CoLOL'r.En GL.ss.—When to the ordinary materials in the melting pot small quantities of various metallic oxides and other i:1ii)eraT substances are added, coloured but still transparent glass is produced. The colours yielded vary in intensity according to the proportion of oxides used; arnl the temperature at which the fusion is effected, the length of time the molten glass remains in the melting pot, not onl_v modify, but actually change altogether, the resulting colour. Indeed, it has been asserted by M. Bontcmps that all the colours of the spectrum may be obtained by the use of one oxide alone, if employed in varying proportions and at different temperatures. The materials, temperatures, and other conditions employed by manufacturers for producing certain of their colour effects in glass are kept as trade secrets; although, in a general way, the substances vhich produce particular colours are perfectly well known. Blue is obtained by the use of cobalt,—tl1e ordinary blue pigment smalt being a powdered cobalt glass. Yellow glass can be prepared from several sources : uranium yields a beautiful opalesccnt yellow; salts of silver are the source of fine shades of yellow; a different tone of the same colour may also be obtained from oxide of antimony; and a dull yellow is produced from powdcrcd charcoal. Green was at one time prepared chiefly by the use of cuprir; oxide and of ferrous oxide; now oxide of chroinium—which produces a beautiful emerald green—is much employed, that substance being mixed with other oxides when modified colours are desired. For red glass, cuprous oxide is employed to produce an intense ruby tint; and the purple of Cassius—a c0n1pound of gold with tin oxide —-yields magnificent shades of ruby, carmine, and pink, while oxide of iron also is the source of a brown'ish-red colour. For the p1‘0(luc— tion of violet tint_s the black oxide of manganese is depended on, and a mixture of the oxides of manganese and cobalt is employed for black. The deep-black glass prepared at Venice for making glass beads contains a large percentage———about 11'40—of manganese. Avanturine. an imitation in glass of the mineral bearing that name, is a warm, brown, opaque glassy body, studded with innumerable minute spangles having a. metallic lustre. It was originally made in the Venetian glass-houses, but can now be manufactured gene- rally throughout the Continent. According to Von Pcttenkofer, the metallic spangles consist of euprous oxide, and one means of preparing the glass consists of melting equal parts of euprous oxide and ferrous oxide with the glassy mass. The other opaque varieties of coloured glass are obtained by using the various metallic oxides with the compounds that produce milky or white glass. The uses of coloured glass are various, the most obvious and usual being for ornamental windows, for signal lights, for imitation precious stones and ornamental table glass, &c. For the imitation of precious stones the strass already alluded to forms the basis, and both flint glass and Bohemian (potash) glass are much used for coloured domestic glass. Window and signal coloured glass are made both as rough plate and as sheet glass. In the case of plate glass the metal is, of course, uniformly coloured throughout, but coloured sheet glass may either be composed of “ pot metal " or it may be “ flashed colours.” Pot metal consists of glass uniformly coloured throughout; but in flashed colours the body of the glass is transparent sheet metal covered on one surface only with coloured glass. It is very simply made: the workman, taking up on the end of his blowing tube a. gathering of clear metal from one pot, dips this into a. pot of coloured metal, thus gathering over the transparent mass a uniform stratum of coloured glass. The whole is then blown in the ordinary manner, and according to the original relative proportions of clear and colonrc-l metal will be the thickness of each in the finished sheet. lt is obvious that this process admits of many variations, such as gathering the coloured metal first, or making alternate gatherings of coloured and clear metal so as to have clear within coloured, coloured within clear, and so on. Inrm-zscrzxr GLAss.—-Ancient glass, which has for ages been sub- mitted to the slow disintegrating influence of the da.mp of the earth and other gently 0 rating agencies, in many instances dis- plays an iridescent play o colours of a most magnificent description. 'l‘he iridescenee thus shown, it has been long known, is due to a pro- cess of decomposition resulting in the fonnation of excessively thin scales of glass. Numerous attempts have been made to imitate by artificial means the gorgeous display of colours thus produced by the slowly acting influences of manv centuries, and a certain amount of success has attended some of these efforts. The Venetian glass workers possess the means of giving the surface of their glass a kind of metallic iridescenee; and in certain Hungarian glass houses iridescent glass has been made for at least about 20 years. But in 1873, at the Vienna Exhibition, iridescent glass formed a prominent feature, and since that time it has become veryeommon. The iridescent glass now generally seen is a plain flint glass having a slightly metallic tinge and a play of colours like a soap bubble. It is probable that several methods of producing iridescenee in glass are practised, as the nacreous lustre in different examples varies considerably. GLASS The subje -t was investigated by M.l. I"rem_v _ [.LNL'FACTUl‘.E. and Clemandot; and under a patent obtained by the latter gentle- man, one method, commonly practised, has been made public. It consists in submitting the object to be iriulizcd to the influence of a weak acid solution—-such as water with 15 per cent. of hydrochloric acid—under the combined influence of heat and pressure. The cfl'cct certainly falls innnenscly short of the iridcsccnce of ancient glass, but the glass assumes permanently a pearly iridescenee, and, though the effect is tiresome, the process will doubtless continue to occupy a place among the methods of ornamenting table glass, 8:4-. Ul’.Ql'l-‘. (il...‘:~'.—_-l)Sen('c of transparency in glass may be due to any of three causcs—(l) to the grinding, or roughening by othcr means, of the surface of ordinary clear glass; ('2) to dcvitrilica- tion or crystallization of the substance; and (3) to the mechanical iutcrmixturc of an opaque substance in the glassy mass. ()lI§Clll'L'tl glass was formerly principally prepared by a process of grinding the surfacc,—thc means cmployed in the case of sheet and plate glass being simply the smoothing process, which forms an intu.-rmcdiate stage in the operation of polishing plate glass. Now the greater part of ordinary obscured glass is prepared by the agency of Tilghman's sand blast, an apparatus by which a fine stream of sand is blown with great violence against the glassy surface. 'l‘l.e impinging grains of sand abrade the surface with extraordinary rapidity, and by protecting certain portions with suitable stencils, elaborate patterns in clear glass are produced in a very simple manner. Alabaster glass, so called on account of its resemblance to that substance, is an opaque variety of glass which has been long known and used. Its opacity is due toaproccss of devitrification it readily undergoes, favoured by the excess of uncombincd silica used in its preparation. The material is prepared from a combination such as 100 parts of sand, 40 of potash, 5 of borax, and 5 of tale (silicate of magnesia). Into the composition of this glass it will be observed no lime enters, although sometimes bone-earth is added to the materials. For enamel glass a mixture of varying proportions of lead and tin oxides is prepared by calcining the two metals together, and using the compound in the preparation of a mixture for fusing, of which the following is an example :-—sand 100 parts, pure potash 80, and mixed oxides 200 parts. The proportion of tin varies within wide limits, and oxide of antimony may be substituted for the tin. Bone glass, milk glass, and opal glass, differing in degree of opacity, are made by adding to the materials of clear glass large proportions of bone-ash, or of oxide of tin, or both together, and with these substances white arsenic may also be combined. The following is an example of a batclr for opal—flint glass :—sand 100 parts, bone-ash 30, potash 30, borax 5, and red lead 5 parts. Such a glass was formerly in extensive use for the opal shades of gas lights and moderator "lamps, &e.; but the ruddy glow possessed by the rays passing through the imperfectly opaque glass was an objectionable feature in the resulting material. lluring recent years an opal or milk glass free from such a defect has been introduced, and it is now in extensive use for globes. This preparation, which diffuses light from its surface in a clear pure white glow, owes its milky opacity to the use of cryolite—a mineral substance consisting of a double fluoride of ahnninium and sodium (Al:F6, 6l'al") obtained from Greenland. The cryolite glass was first brought promi11cntly into public notice by the Ilot-Cast Porcclain Company of Philadelphia, by whom it was made on a large scale, although the material had been in use in Bohemian and Silesian glass-works for some years previous to the commencement of the manufacture in America. For milk white glass the materials used arc—sand 100 parts, cryolitc 40. and zinc oxide 10 parts. The lllllsln-(l glass, which is remarkably strong, hard, and indifferent to acids contains about 15 per cent. of nmlccomposcd cryolitc, to which its opacity is due. The copious evolution of fumes of hydrofluoric acid during the melting, which continues even in the working, is the source of serious difficulty in the manufacture of cryolitc glass. T()L’GllI-‘..'ED on ll.r.nr..'1-:n GI..ss.—ln the year 1875 the announcement that a French gentleman, M. dc la liastie, had dis- covered a means of rendering glass practically unbrcakal_)le attractc-l a great amount of at.tcntion ; and his statements vcre nnmediately made the subject of practical investigation thro_ughout the glass- making connnunity. All the experiments made 1n_conncx1on with M. dc la Bastit-’s process tended to confirm his claim to h-rve dis- covercd a. method of rendering glass capable of bearing a shock or strain variously estimated at from 30 to 100 times greater than the same material annealed in thc connnon way. ])e la llastie was led to undertake the prolonged series of experiments, which ultimately resulted in his discovery, by the consideration that_thc brittleness of glass arises from weak cohesion of its molecules; and his efforts were first directed to improving its molecular arrangement, by sub- mitting glass, in a molten state, to forcible compression. _ This series of experiments led to no practical result; and the hue of investigation he ultimately pursued, as well as the merits and_ defects of his process, are thus succinctly stated by Mr II. J. Powell of Whitefriars Glass Works. with whom M. de la llastie carried out his first practical experiments with hollow glass. Mr Powell,

writing in August 1573, says of the process:—