Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/689

Rh M._2v'UFAC'.I‘UP.E.] “ That it consists in plunging glass heated to the melting point into a bath containing an oleaginous inixture, at a high teniperature, but considerably cooler than the glass itself ; and that this, accoi-dingto the speciﬂcation of the patcntee, is effected by re-heating already manufactured and annealed glass in a kiln, and passing it thence into the bath. After a rough trial of this process, which cer- tainly answers well for ﬂat or solid glass, we decided that it is defective, for hollow ﬂint glass, as hollow vessels, left to themselves in a kiln, are almost certain to collapse on reaching the required heat. To avoid this dit‘ﬂcult_v, and knowing that a vessel in course of manufacture, howcvcr hot, is always under control whilst it remains on the workiiiaii‘s rod, we placed a bath as near the mouth of the working-pot as possible, and directed the workman, instead of sending the ﬁnished vcssel to the annealing oven, to di'op it into the bath. The vessel is caught in a wire net, and is ready for removal as soon as it has acquired the temperature of the bath. For all vessels made in one piece, c.g., tumblers, ﬁnger b.isins, &c. , this process answers well; and it is obvious that if it proves to be the best way of treating hollow ﬂint glass, the use, for this description of glass, of the complicated machinery described in .I. do la liastic‘s speciﬁcation will be done away with, and the glass will be tempered in the course of niaiiiifactnrc, instead of being re—lieatcd and tempered after it has been already manufactured and annealed. We ascertained, with M. de la Bastie's aid, the right constituents and right tcinperature of a bath for ﬂint glass; for although the conditions for sheet, plate, and ﬂint glass are nearly the same, there is a difference, and it seems probable that every clicniicully diffei‘ent glass, and even cvery different thickness of glass, may require certain variations. In our experiments as to the hardness of the glass, we found that it could be iiinrkcd, but not cut, with the diamond, and, although it could be smoothed and en,-:i'aved in the ordi- nary way, that the disturbance caused by the wheel, when pcnetrating to any appreciable depth, tended to weaken, or even to cause the destruction of the entire mass. ’l‘lic value of the invention, as far as it concerns ﬂint glass, is at present somewhat modiﬁed by diﬂleultics in manipulation. 1. It seems to be iiii- possible to heat a. vessel made up of different pieces and of various tliickiiesscsto an absolutely equal temperature tliroughout, so that the whole may be equally tempered. 2. It seems also impossible to displace the air from a narrow-niouihed vessel quick enough for the inside and outside to be tempered siiiniltaiicously. llowever, setting aside these ditliculties. we come to a point which applies equallyto all sorts of hardened glass. II:irdeiied glass is not ‘unbreakable’; it is only harder than ordinary glass. and though it undoubtedly stands rough usage better, it has the disadvantage of being utterly disintegrated as soon as it receives the slightest fracture, and up to the present, until broken, of being undistinguisliable from ordinary glass. This class is known as ‘toughened’ glass, and we have seen the terms ‘ malleable ' and ‘ annealed‘ applied to it. Nothing can be more misleading than these unfoitunate epithets. The glass is hard, and not tough or malleable. and is the very opposite to annealed glass. Annealed glass is that the molecules of which have bcen allowed to settle them- selves; the iiiolcculcs of hardened glass have been tortured into their position, and until the glass is broken are subject to an extreme tension. It is the sudden change of temperature that ‘ liardens'; glass heated up together with the oil may be annealed, but decidedly is not hardened. A piece of hardened glass is only a modiﬁed l’.upert’s drop, i.c., it is case-hardened; the fracture of both is identical, both resist the diamond, and both can be amicalcd. .torcovcr, in the middle of imperfectly hardened glass a line is plainly visible, which seems to mark the extent of the casc-liardcniiig. This line resolves itself under the microscope into a mass of bubbles and St1‘l{l‘.; it seems to be the nucleus of breakage, and consequently as soon as the cutting wheel app)'(ia(-I105 it, utter destruetion ensues." The great anticipations which at first were fornied as to the extended use of hardened glass have not been realized. M. de la iastie has improved several of his processes, but the (lciiiaiid for his productions, at no time great, is understood to decrease rather than to increase. For a short time the process was worked lay both lIcssrs Powell and Messrs Pellatt in London, but both these eminent ﬁrius have given it entirely up. Sheet glass hardened by the process does not appear ever to have come gencrallv into the market, the niost serious obstacle to its introduction bciiig the im- possibility of cutting it with the diamond, after which the utter destruction resulting from fiactu re is a serious defect. For laboratory piirposes—as ﬂasks and bcakcrs, &c.—it has been suggested that the glass has great advantages, but cxpcriincnts have proved that its great resistance is not absolutely reliable, and that hardened vessels sub- mitted to a high heat lose their distinguisliiiinr peculiarities and become as common glass. Thus a glass, 1)ai'tialTy ﬁlled with water and heated considerably above the boiling point at the parts un- covered with water, broke, the bottom of the glass showinrr the fracture peculiar to liardened glass, while the upper 111l00V0l‘0(T part was bro_l~:en into large sharp-cdgcd fragments like common glass. A modiﬁed process of hardening, patented by Herr F. Sicnicns, consists 111 pressing and suddenly cooling the glass in moulds specially constructed to conduct away the heat with the various degrees of rapidity found to produce the best results. 1. .. . . ,., v. br._iisT1cs or 1111:_GL.tss '1 r..)E.—.iccoi-ding to a factory report of 1811, there were 111 that year 240 glass-works in the United lxingdoin, ciiiployiiig, in addition to stcani-power, ‘.21, 434 operatives, of whom 2116 were icinales. Of these works 213 were in Enrrland 19 in Scotland, and 8 in Ireland. Further, there were at tlu? saine date 37 glass-cutting factories, employing 500 people, principally situated 111 the county of Warwick. The quantities and value of glass niaiiufacturcs exported were as under in the year 1878 :— Plate glass  sq. ft. £106,906 Flint glass    93,112 C“-t3_ 239,936 Common bottles   __ ;',7.'._]m) 3,Q_4R1 Other glass manufactures. 9:) 170 £754,523 In 18/ 8 the imports of manufactured glass were thus given :— Window glass   652.325 cwts _£ -2 gng Flint glass  _ 14_r;_(6_-, ,, Plate glass  :u1.19s I 2 2 (‘-lass manufactures ' L :,- .. 3:35.751 ,, ___ GLASS 667 The following table shows the comparative imports and exports of glass in the ten years ended 1877 :— ‘ Exports of British Glass. 1' Imports, Foreign. . N Plate Glass. l Other Kinds Cwts. Square Feet. Cwts. 1868 609.806 91 1.330 868. 9.70 1509 601.070 1,076,130 933,475 1870 6I)2,.‘l76 1,3-57,50-‘5 799,2-52 . 1871 ($29,472 I .641},-57.5 847.988 187:! 688,156 ‘2.131,9‘.2-I 1,00'2.~l'.l8 I 1873 807.410 2,133,106 1,1-l.‘~',6~t0 187-! 946.903 l,~i11,:?I;8 1,] 14.253 I 187-3 983,677 I,609.1.’~'0 880,983 1:474; i,m;o.:;u1 l,779,(3'.’S 74:,-,ao5 In the Bulletin (la la Sociélé d’Encom'agc7nrmt pour l’2'92dush'z'c nationals for 1877 there is an elaborate statistical computation of the extent and value of the glass 1ll8.ll1lfaf‘t1ll‘e throughout the world, based chiefly on returns applicable to 1874. The writer, M. Ilenry de Fontaiiic, arrives at the conclusion that the annual production of glass has almost doubled in the past twenty years, and estiniates the total yearly production throughout the world at a value of six hundred millions of francs. .B1'l:l2'0g'ra])hg/.—Tlic literature of glass-making of English orig.'n is scanty and imperfect. In France and Germany the subject has received much fuller attention. The following list embraces the principal works :—Antonio Keri, Ars I'2't7'ar2'a, cum 1llcr7'z'tt2' obscrimionibus, Amst., 1668 (Neri’s work was translated into Eng- lish by C. Merritt in 1662, and the tianslation, The Art Qf 'n1ak£7zg Glass, was privately reprinted by Sir T. Phillipps, Bart, in 1826):; J oliann Kunkel, I’olIsta.ml2'_.r/c Glasmaclzcr-Ifmzst, Nuremberg, 178.) ; Apsley Pellatt, C'm'ios2'l£cs of Glass-making, London, 1849; A. Sauzay, J[a.9'L-cls Qf Glass-makiazg (from the French), London, 1869; G. Bontcmps, Guide (lu I/'c2"rz'c7, Paris, 1868; E. Peligot, La I'crrc, son Iu'stoz'rc, sa fabrication, Paris, 1878 ; WV. Stein, Die Glas-_fabrz'— Ication (in Bollcy's Tcclwzologic, vol. iii.), llrunswick, 1862; H. E. Benratli, Die G'Ias_faI29'2'I.-at2'o71, Brunswick, 1875; J. Falck and L. Lobiiicyr, Die Glasinclustﬁc, Vienna, 1875. (J. PA.) GLASS PAINTING. The manufacture of coloured glass, which is the basis of the beautiful and interesting art of glass painting, originated at a period of remote antiquity, and the use of enamels, to vary or ornament its surface, was known to the ancient Egyptians; but the formation of windows of mosaics of coloured glass upon which the shapes of ﬁgures and orna- iiieiits are painted with an enamel ﬁxed by ﬁre is niediacval, and emphatically a Christian art. In all probability it was suggested by the mosaic pictures with which churches were adorned from an early period for the instruction of the illiterate, as was shown by the inscription which they bore, “ sanctzc plebi Dci.” The step from mosaic pictures to glass mosaic windows was merely a question of time; it is not known when the step was taken, but coloured windows existed in St Sophia at Constantinople in the 6th century, whilst the basilicas of St John Lateran and of St Peter at Rome were adorned about the same time in the same manner. In the year 709 Vilfrid, bishop of York, invited workers in glass from France (“artiﬁces lapidearum et vitrearuni fenestraruni prinius in Angliain aseivit ”). The French claim the honour of having invented the process of painting upon the mosaic windows of coloured glass, and of thus transforniing them into works of art, and also of teaching this to the English, who in their turn instructed the Germans; but Muratori, in the second volume of his ..-inn‘.-lam Italiclze of the Middle Ages, printed a treatise on mosaic and painted glass written by an anonymous Italian in the 8th century, and probably not later than the 11th was written the interesting essay 1)z'versarmn A7'tz'2zm Sclmlula T /mop/cili P2'esI»_2/tcri ct Jfonac/12', which details with minute accuracy the process of painted glass as it has been practised with some additions and modiﬁcations, throughout the best periods of the art; it may reasonably be assuuicd that Theophilus describes methods invented before his time. Probably the oldest specimen of glass
 * 1.477 1,140,694 1,157,063 792,424