393 



GLASS MANUFACTURE. 



GLASS MANUFACTURE. 



391 



drical shape, and blows through at the other end of the tube. Another 

 workman then takes it ; and by a combined process of blowing, rotating, 



Blowing for Flint-glas;. 



and manipulation with small tools, the mass of glass gradually assumes 

 he form of a jug, without -being cast or pressed into any mould* A 



Flint-glass working. 



small quantity of glass is added and speedily fashioned into a foot for 

 the jug, and another similarly for the handle. All these operations 

 are effected with surprising quickness and precision. The glass is in 

 such a medium state between solid and liquid, that while, on the one 

 hand, it would drop from the tube if not kept rotating, it is, on the 

 other, susceptible of being pulled, stretched, cut, pressed, and worked 

 in various ways. The workman has nothing but his hand and eye to 

 guide him in giving accuracy of shape to the article manufactured. 

 The facility with which an additional piece attaches itself to and 

 becomes part of the larger mass, enables the workman to supply all 

 such appendages as the stems and feet of wine-glasses, the handles and 

 lips of jugs, &c. 



Flint-glass is used for a large variety of purposes, and many adjust- 

 ments of the manufacturing processes are found necessary. Thermo- 

 meter and barometer tubes are made by an operation which depends on 

 the remarkable ductility of glass. A workman collects a quantity of 

 glass on the end of a tube ; rolls it on an iron plate into a cylindrical 

 form ; blows into it to form a cavity within ; and attaches the other 

 end to a heated rod. Two men the one holding the tube and the 

 other the rod then walk backwards, stretching out the tube of glass 

 to a length of 40 or 50 feet. One of the most remarkable circum- 

 stances connected with this operation is, that the hollow within the 

 glass retains exactly the same shape, although the mass may be 

 extended from a few inches to 50 feet in length ; and the workman 

 can thus obtain either cylindrical or flattish tubes, as may be required. 

 The tubes are broken into smaller lengths for use. In making glass 

 beads, a very fine and narrow tube of glass is taken, and one end 

 is placed in the flame of a lamp while the workman blows in at 

 the other ; he expands the heated end into a small hollow sphere by 

 the action of his breath, and then breaks it off. So rapidly is this 

 done, that one workman can produce five or six thousand in a day. 

 Some of these beads, made of coloured glass, are used for necklaces ; 

 some, carefully treated afterwards, become dolts' eyes; while others, 

 made of a slightly bluish-white glass, become artifii ial pearlt, by being 

 coated on the inside with pearl-essence, or essence d'Orient, a liquid 

 prepared from the scales of the blay or bleak fish. The substance 

 called avantufine glass, a Venetian product, is applied to the manu- 

 facture of trinkets and ornaments, and is named, on account of its 

 resemblance to the natural crystal, avanturine. It is a yellowish-brown 

 glass, inclosing fine thin yellow lainime or scales of a brilliant metallic 



lustre. The manufacture is not well understood in England. Some 

 think that the yellow lamina; are produced by melting scales of rneta 

 or mica with the glass; but it is deemed more probable that a salt of 

 copper is mixed and melted with the glass, and that a powerful 

 reducing agent decomposes this salt during the melting, and separates 

 the copper in the state of thin metallic scales Venetian and Boliemian 

 glow are in general illustrations of the great diversities which can be 

 produced in glass, by adding metallic oxides to produce colour, and by 

 a peculiar treatment of the finishing processes ; but the rationale and 

 general character of the manufacture are sufficiently denoted by the 

 description given above. The fanciful productions known by the 

 names of Venetian ball, Venetian filagree, Miltefiorc glass, Mosaic glasf, 

 Smetz glass, Vitro di trino, &c., are all specimens of flint-glass which 

 have undergone peculiar treatment. Optical glass is mostly flint-glass, 

 although the different refrangibility of other kinds leads to the com- 

 bination of two or more in producing achromatic and aplanatic lenses. 

 But, in truth, opticians care little about the name given to their glass ; 

 they have long been trying, and still are trying, to produce pieces large 

 enough for telescopes of great diameter, and perfectly free from specks 

 or blemishes ; and if they succeed in this, it matters little whether the 

 substance more nearly resembles fliut, or plate, or crown glass. Mr. 

 Cooper, a glass-manufacturer at Aberdeen, sent to the Scottish Society 

 of Arts a few years ago two recipes for optical glass, which he had 

 found advantageous. The first consisted of well-washed and sifted 

 sand, 60 parts; oxide of lead, 60; purified carbonate of potash, 15 ; 

 saltpetre, 34 ; and broken flint-glass, 15 to 20. Another kind, heavier 

 and of greater refrangibility, was composed of sand, 60 parts ; oxide of 

 lead, 63 ; carbonate of potash, 14 ; saltpetre, 3.J ; and broken flint-glass, 

 20. In 1854, M. de Peyrony submitted to the Academic des Sciences 

 a proposal for a new mode of making optical glass for large lenses. In 

 the usual way, the mass of glass having been brought to a state of 

 fusion in the crucible, the material is simply stirred to make it homo- 

 geneous, and to drive out the included bubbles of air. But this double 

 object is never completely attained ; for the stirring itself occasions the 

 formation of stria;, or waving lines, which necessitate the rejection of 

 a large portion of the glass taken from the crucible, as being unfitted 

 for the formation of lenses. Hence the difficulty of obtaining lenses of 

 large diameter. M. de Peyrony proposed to give the crucible a motion 

 of rotation around a vertical axis ; the centrifugal force, he suggested, 

 would unite the air-bubbles towards the centre of the mass, while the 

 stria) made by the stirring would for the most part disappear ; those 

 that remain would probably assume a circular form, and would produce 

 little inconvenience, provided the optician took care to make the axis 

 of the mass coincide with that of the lefts. The finest optical glass yet 

 produced, perhaps, is a telescope-lens which was shown at the Paris 

 Exhibition in 1855. M. Lelebours obtained the piece of glass from the 

 glass-works at Choisy-le-Roi,and bestowed some months of labour upon 

 it ; it is 14 English inches m diameter. The French government bought 

 it for what was considered by scientific men an inadequate price, 

 25,000 francs, or 1000<. ; and it is now fitted into the finest telescope 

 at the Paris Observatory. At the same Exhibition, Messrs. Chance, 

 the glass-manufacturers of Birmingham, brought forward, not lenses, 

 but discs of glass intended for lenses, of a size never before equalled. 

 The discs were made, some of flint, and some of crown-glass; 

 two of them were twenty-nine inches in diameter each. Sir David 

 Brewster urged the British government to purchase these discs, and 

 therewith to make " the greatest achromatic telescope that was 

 ever contemplated by the most sanguine astronomer ; " but without 

 success. 



Flint glass, for domestic and ornamental purposes, undergoes many 

 processes after the moulding and shaping ; such as annealing, cutting, 

 engraving, gilding, &c. The process of annealing, intended to lessen 

 the brittleness of the glass, is noticed under ANNEALING. The cutting 

 consists in a grinding away of portions of the glass, to produce that 

 peculiar effect which is familiar to every one in cut-glass. This is 

 done by means of small wheels, made of cast-iron, wrought-iron, 

 Yorkshire stone, willow-wood, and other substances. Each wheel is 

 made to revolve very rapidly on a horizontal axis ; the edge or peri- 

 phery is that part by which the grinding is effected ; and different 

 shapes and thicknesses are prepared to suit different kinds of work. 

 The workman holds the glass decanter or other article against the 

 edge of a wheel, and modifies the position and pressure according to 

 the effect desired to be produced. The iron wheels, wetted with sand 

 and water, are used for grinding away the substance of the glass ; the 

 stone wheels, with clean water, for smoothing the scratched surfaces ; 

 and the wooden wheels, with rotten-stone and putty-powder, for 

 polishing. The engraving of flint-glass consists in the production of 

 devices on the surface more delicate than can be produced by the 

 cutting wheel. It is effected by the use of very small discs, generally 

 of copper, and moistened at the edges with emery and oil, and 

 requires great taste and judgment for its due performance. The 

 parti-crowed specimens of flint-glass are produced in a remark- 

 able way. The working-tube of the maker is first dipped into a 

 mass of colourless glass, and then into one of coloured ; the glass 

 is fashioned and annealed in the ordinary way, and then it is cut 

 by the wheels ; and according to the depth of the cutting or grind- 

 ing, so does the workman penetrate, nearly or quite, through the 

 coloured glass, giving rise to a beautiful play of tints. A different 



