CHAMBERS'S INFORMATION FOR THE PEOPLE. 



of sugar-tongs, the glass is moulded to the form 

 required. The shapes into which flint-glass is 

 manufactured are so numerous, that it would be 

 almost impossible to describe them all. The 

 operations are extremely simple and beautiful, 

 and are performed with a rapidity which is truly 

 astonishing. The workman is furnished with a 

 pair of compasses and a graduated scale, to 

 measure the articles which he is making, by which 

 they are kept of a uniform size. When finished, 

 the articles are all weighed, to see that the right 

 quantity of glass has been used in their manufac- 

 ture, and after this they are put into the annealing- 

 furnace. 



Optical glasses are made from fine flint-glass. 

 The utmost care is necessary to keep the metal 

 entirely free from waves, otherwise the glasses will 

 be useless. An achromatic object-glass for a tele- 

 scope or microscope that is, an object-glass 

 which does not produce coloured fringes around 

 the edge of the image known as chromatic 

 aberration must consist of three lenses made of 

 different kinds of glass, differing in the proportion 

 which their refractive bears to their dispersive 

 power. Flint-glass and crown-glass are well 

 adapted for being formed into such a compound 

 lens, the dispersive power of the former being 

 nearly double that of the latter, while the mean 

 refractive powers of the two kinds are nearly the 

 same. 



Bottle-glass. 



Bottle-glass is composed of the coarsest mate- 

 rials, generally soap-boilers' waste and sand. The 

 furnaces and pots for preparing bottle-glass are 

 similar to those used for crown-glass ; and the 

 raw materials are treated much in the same way. 

 As the mixture always contains a very small rela- 

 tive proportion of the alkaline ingredient, its vitri- 

 faction requires a high temperature ; but it is 

 usually complete in eighteen or twenty hours. 

 After the undissolved matter has subsided, the 

 sandiver, or ' scum,' which rises being skimmed 

 off, and the glass cooled down to blowing con- 

 sistency, the mass may be worked up into bottles. 

 For this purpose, the workman introduces his 

 tube, and when sufficient is gathered upon the 

 end, he rolls the glass upon a stone, blowing into 

 it at the same time. When sufficiently distended, 

 he gives it the bottle form by turning it rapidly 

 round, and shaping it with a wooden or iron tool. 

 But bottles are now usually made by the mould- 

 ing process, which is begun in the same way by 

 the workman, who gathers the metal on his blow- 

 pipe, and mavers it on the stone, as before men- 

 tioned. He then puts the metal into a brass or 

 iron mould of the shape of the bottle to be made, 

 and blows through the tube until it comes to the 

 desired form. This mould is so contrived as to j 

 open down the middle by means of a spring which 

 the blower works with his foot. The mould is 

 open when he puts in the metal at first ; it is 

 then immediately closed, and opened again when 

 the bottle is formed, which is handed over to the 

 finisher. The finisher detaches the tube from the 

 mouth of the bottle, and fixes the punty-rod to 

 the bottom. He then warms the bottle at the 

 furnace, and takes out a small quantity of metal, j 

 which is turned round the upper part of the neck, 

 and forms the rim usually seen on bottles. The 

 finisher next employs a pair of shears to give the 



361 



right shape to the neck : on one of the blades of 

 the shears is a piece of brass resembling a cork, 

 by which the inside of the neck is formed. The 

 bottles thus finished are sent to the annealing- 

 arch, which is kept a little below melting-heat 

 until full, when the fire is allowed to die out. 



Cutting Grinding Etching. 



The instrument universally employed in cutting 

 window-glass is the diamond, which is set in 'a 

 metal socket, attached to a wooden handle for 

 this purpose. The cutting point of the diamond 

 must be a natural one ; artificial points, as well 

 as those produced by breaking the diamond, only 

 scratch the glass, without producing the deep cut 

 which is necessary % 



What is called glass-cutting, or grinding, is a 

 separate trade from blowing in all glass-manufac- 

 tories. The cutting-wheel is driven by means of 

 a belt proceeding from a large drum attached to 

 an engine or other moving power. Above the 

 cutting-wheel is a conical box, from which wet 

 sand drops upon it, while another is placed below, 

 to receive the sand as it falls from the wheel. 

 The wheels used are three in number : the first 

 is made of cast-iron, by which the rough glass is 

 ground ; the second, of Yorkshire stone, by which 

 the vessel is smoothed ; and the third, of willow- 

 wood, by which the final polish is communicated. 

 For this last purpose, the wooden wheel is dressed 

 with rotten-stone or pumice-stone ; and for im- 

 parting the highest degree of polish, putty-powder 

 is used. These wheels are of various forms, 

 according to the shape of the vessel to be cut. 

 They may be broad or narrow, flat-edged, two- 

 edged, concave, convex, &c. The cutter holds 

 the glass to the wheel while it is revolving, and 

 the most beautiful and regular figures are engraved 

 in this manner with astonishing rapidity. Imita- 

 tions of cut-glass vessels are made by blowing the 

 soft glass into a polished metallic mould, the form 

 of which it acquires with as much faithfulness as 

 wax. 



As stated under CHEMISTRY, hydrofluoric acid 

 acts energetically on glass, and is employed for 

 the purpose of etching on this material. ' The 

 art,' says Parnell, ' may be practised on all kinds 

 of glass, but the most proper description is good 

 crown-glass. The facts on which the art is 

 founded are, that glass becomes powerfully cor- 

 roded by exposure to the acid in question, and 

 that certain parts of the glass may be easily pro- 

 tected by wax, or other kinds of varnish, on which 

 the acid exerts no action, except at a high tem- 

 perature.' A variety of processes are now in use 

 for decorating the surfaces of glass-ware ; space 

 prevents us, however, from noticing any of these ; 

 we refer the reader, therefore, to other and larger 

 works, for a description of them. 



Staining Colouring Enamelling. 



The art of staining or colouring glass is believed 

 to be coeval with the discovery of the article itself. 

 It is certain that it was known in Egypt several 

 thousand years since, and tradition gives the 

 honour of the discovery to an Egyptian king. 

 The art of combining colours so as to produce 

 pictures is of more recent date. The early speci- 

 mens of stained glass exhibit a number of different 

 pieces of various colours, joined together like 



