Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/687

Rh M.xxUF.-c'rUR1»:.] annealing arch, where the bottles are placed in bins above one another. This arch is kept a. little below melting heat, till the whole quantity, which amounts to 10 or 12 gross in each arch, is deposited, when the fire is allowed to die out. Sl..(: GL.-ss.——Umler a patent obtained by lIr Bashley Britten, a manufacture of bottles has recently been established with every prospect of success, the leading peculiarity of the process being the use of blast-furnace slag, molten as it issues fron1 the furnace, as a principal ingredient. The use of slag in bottle-making is by no means new, but the catching of the intensely hot liquid mass and its innnediatc use for glass-making had not before been attempted ; aml therein results the great saving in fuel and consequent economy of the mzmufacturc. A company formed to work the process has erected glass-works in Northamptonshire in the innucdiatc neigh- bourhood of a set of blast furnaces, aml these worksaie now incon- stant aml successful operation. A regenerative gas furnace applied to a glass tank working on Dr Siemens’s continuous principle is used, aml in it the ingredients of the glass are fed at one end of the tank, where they are fused aml fined, and the fused “ lnctal " flows through a bridge to the other end of the tank, whence it is worked out, blown into bottles, and annealed in the usual way. The tank is from time to time fed with fused slag taken as itflows from tl1e blast furnaces, and with it is introduced the required proportion of the other in- gredients. The slag furnishes more than half the total material of the glass, and, as it is alrc.uly melted, its use effects a saving of about l1alf the heat or fuel and also half of the tin1e necessary for thc production of the “ metal." Thus the prime cost of the glass as it is worked out is considerably less than that of glass made in the ordinary way. The natural tint of the glass thus produced is greenish, l)ut it can be coloured to any required tint, and by careful fining and bleaching it ca11 be produced almost as colourless as com- mon window glass. The working qualities of the glass are excellent ; it comes fron1 the furnace in that beautifully plastic condition which renders it capable of being blown, cast, pressed, or otherwise moulded int’) any desired form, a11d the company expects to manufacture other articles besides bottles from amaterial so cheaply produced. Ol"l‘IC-AL GLASS is of two principal kinds—flint and c1'0'n—the combination of these two, with their different refractive powers, being necessary to produce perfect achromatism in the lenses of telescopes. For astronomical telescopes, formicroscopcs, and foralldelicate scien- tific instrmnents i11 which optical glass occupies a place, glass of the utmost purity, transparency, freedom from colour, streaks, and strite is of the highest importance ; aml to secure these qualities to the fullest extent much care, trouble, and expense are requisite. The first really successful maker of optical ﬂint glass was M. Guinaml of Solothurn in Switzerland who succeeded in making discs 9 inches in diameter free from strirc. Guinand died in 1823, and fromhis son, M. George Bontcmps learned his secret, and at (‘hoisy-le-Roi, ncar Paris, further improved the manufacture. In 1848 M. Bcntemps was induc d by Blessrs Chance of Birmingham to establish the art in their great works. In the hands of that eminent ﬁrm the preparation of optical glass has attained a perfection not approached by any other glass workers, and the chef ¢l'a:uz'rc of optical glass hitherto made consists of -.1 pair of flint and crown glass discs, :29 inches in diameter, exhibited by Chance Brothers at Paris in 1855. Ilcgarding these Sir David Brewster said, ‘‘I have entertained the hope that the linglish Government would purchase these discs and construct with them the grandest achromatic telescope that ever was contemplated by the most sanguinc astronomer.” They wc1'e, however, purchased b_v the French Government in 1867. Optical ﬂint glass contains more lead, and is consequentlyheavier and n1orc refractive, than the quality used for common purposes. It is made in a furnace having a single covered pot, and Guinand’s scerct consisted in constantly stirring the mass while it is in a mol- ten comlition so as to keep the heavier lead silicate from falling to the bottom. For the very highest qualities of optical glass, the contents of the pot are most scrupulously cleared, and the stirringis continued after the heat is lowered till the contents are cooled down to little more than a red heat. The furnace is then closed and the metal is allowed to cool and anneal gradually in the pot within the furnace. Vhen withdrawn the pot is broken, and the mass of glass is polished on two opposite sides so that any imperfections may be detected by examination. From the mass, cut horizontally, perfect discs of such size as can be formed are then obtained. Optical glass is also blown into thick (-ylimlers, aml cast in slabs from i inch to 1 inch in thickness. Chance Brothers make six kinds of optical glass, of which the vcrage densities and refractive imlices for the three hydrogen lines anal for the sodium line are given in the following table :— C. I) I-‘. G l_lard crou-n......... 1-.7141: 1 32x20 -‘ fr 01'-»“n 1-am 1-.'.-2:23 l.i;:lu ninr 1-571... 1-. »~_r ha-n-e tii:.t  1-u;17.'. 1-4;4..:; l-'.xIrn dense tl'nt .... .. 1-I;4.'.n 1-1:710’; Double (. xlra at-n.~e f‘lint......_ . 1-703:; GLASS I bility, aml cuts through ordinary (-mcry wheels with case. 665 In 1830 Faraday propo;ed the use of a cmn pound silicate and borate of lead glass, with adcnsity of 5'44, for optical use ; and Macz and Clemandot have successfully introduced a boro-silicate of zinc. An optical glass of higher refractive and dispersive power than any previously known has been made by Lamy from a mixture of silica, red lead, and carbonate of thallium. The glass is perfectly homo- genous, but of a yellow tint—-an evil, however, said to be avoidable by the use of the sulphate instead of the carbonate of thallium. The extraordinary refractive power of the glass rcndcrs it peculiarly adapted for the fabrication of imitation precious stones. S'rr.Ass.—l{emarkably faithful imitations of every kind of precious stone can be made from suitably prepared and, when necessary, coloured glass. The transparent basis from which arti- ficial precious stones is prepared is called strass or paste, a material which must of necessity be the purest, most transparent, aml most highly refractive glass that can be prepared. These qualities are combined in the highest degree in a flint glass of unusual density from the very large percentage of lead it contains. Among various mixtures given by Donault—Wicland as suitable for strass the following is a11 example:—powdered quartz 300 parts, red lead 470, potash (puriﬁed by alcohol) 163, borax 22, and white arsenic 1 part by weight. Special precautions are adopted in the melting of the materials, and the finished colourless glass is used for imitating diamonds. Vhcn employed to imitate coloured precious stones- the strass is melted up with various metallic oxides, to which refer- c11ce will be made nmler coloured glass. Artiﬁcial precious stones are, of course, easily distinguished from real stones by their infel ior hardnes-, and by chemical tests. They may also be generally detected by a comparatively soft warm sensation they communi- cate when applied to the tongue. Gmss TUBES, used for gauge glass for steam boilers and for many other purposes, are made by a very simple process; but as the manufacture is a separate department of industry it demands some notice. A gathering of glass is made 011 a blow-pipe, which is marvcred, aml slightlyblown, so as to form a thick-walled elongated globe. To the end of this globe opposite the blowing pipe :1 pontil- rod is attached by an assistant, and the two workmen move back- wards the one from the other, drawing out the tube as they recede. Une or two boys watch the process of elongation, and when the tube reaches the desired gauge they fan its surface so as to “ set " the glass, aml thus prevent further attenuation at that particular point. 'l‘he relation of the mass of metal to the original cavity determines the comparative stoutness and bore or internal diameter of the tube, and it requires much dexterity to make a tube at once straight and of uniform gauge aml diameter. In drawing out tubes of large gaug- the operatives recede from each other at a. slow rate ; and in proportion as the size of tube decreases the rate of drawing out increases. In the Venetian factories, where small tubes for beads are made, the workmen move at a smart walking pace. M.ssIvI5 GL.ss.——Under this term may be conveniently noticed the manufacture of various familiar solid glass objects which do not acquire their form either by blowing or pressing in the ordinary sense. Glass Rods forn1 the basis of many of these objects ; and the formation of a plain rod of glass is accomplished by a manipulation in all respects similar to that described under the head of glass tubes, the only difference being that the rod is drawn from a solid instead of fron1 a hollow or blown gathering as in the case with tube drawing. From solid rod glass, glass buttons of various forms are “pinched” by heating the rod till it softens, and immediately pinching it in heated moulds made and worked like ordinary pincers, but having moulds of suitable form in place of the gripping surface of the pincers. The small facets of glass lustre-s and girandolcs and glass marbles, are made by an analogous process. S1'L'.' GL.-tss.—Certain qualities of glass in the plastic condition are capable of being drawn out to threads of great tenuity, which, while possessing much brilliancy and beauty of colo11r, are perfect] y flexible and elastic, and feel to the touch soft aml smooth like ﬁne wool. A good deal of attention has been given to glass spinning in Vienna and in the Bohemian glass—works, the thread produced bcin g woven into many textiles for upholstery and wearing purposes. The material is specially useful in millinery ornaments owing to the fine colours in which it may be produced, and to the fact that it is unalterable in a11d unaffected by all kinds of weather. A remarkable and novel application of glass was patented in 1878 by Mr J. B. Hannay. It consists in making glass a cementing or binding substance i11 the manufacture of emery wheels, now so much 11sc(l instead of ﬁles. For preparing the wheels any broken fragments of glass are utilized. These are reduced to powder, mixed with proportions of powdered flints and emery, aml iii the form of a cake introduced on a layer of paper into a furnace where the material is submitted to a heat suﬂicicnt to fuse it into a compact ringing mass. The resulting cake is of intense hardness and dura- As the glassy emery wlzccls ean be made very much cheaper than those at present in use, there is little doubt that this material will come Into X.—S4