676 



GLASS 



1103 



1104 



cutting or grinding disc plate. For heavy work, this rod is fixed into the head by 

 a screw. When a conical fit is preferred, the cone is covered with lead to increase 

 the friction. 



Upon projecting rods or spindles of that kind 

 the different discs for cutting the glass are made 

 fast. Some of these are made of fine sandstone or 

 polishing slate, from 8 to 10 inches in diameter, 

 and from to inch thick. They must be care- 

 fully turned and polished at the lathe, not only 

 upon their rounded but upon their flat face, in 

 order to grind and polish in their turn the flat and 

 carved surfaces of glass vessels. Other discs of 

 the same diameter, but only | of an inch thick, 

 are made of cast tin truly turned, and serve for 

 polishing the vessels previously ground ; a third 

 set consists of sheet iron from to an inch 



" *^3 thick, and 12 inches in diameter, and are destined 



to cut grooves in glass by the aid of sand and water. Small discs of well-hammered 

 copper from to 3 inches in diameter, whose circumference is sometimes flat, and 

 sometimes concave or convex, serve to make all sorts of delineations upon glass by 

 means of emery and oil. Lastly, there are rods of copper or brass furnished with 

 small hemispheres from ~ to \ of an inch in diameter, to excavate round hollows 

 in glass. Wooden discs are also employed for polishing, made of white wood cut 

 across the grain, as also of cork. 



The cutting of deep indentations, and of grooves, is usually per- 

 formed by the iron disc, with sand and water, which are allowed 

 constantly to trickle down from a wooden hopper placed right over 

 it, and furnished with a wooden stopple or plug at the apex, to regu- 

 late by its greater or less looseness the flow of the grinding 

 materials. The same effect may be produced by using buckets, as 

 shown in fig. 1104. The sand which is contained in the bucket F, 

 above the lathe, has a spigot and faucet inserted near its bottom, 

 and is supplied with a stream of water from the stopcock in the 

 vessel G, which, together, running down the inclined board, are con- 

 ducted to the periphery of the disc as shown in the figure, to whose 

 lowest point the glass vessel is applied with pressure by the hand. 

 The sand and water are afterwards collected in the tub H. Finer 

 markings which are to remain without lustre, are made with the 

 small copper discs, emery, and oil. The polishing is effected by the 

 edge of the tin disc, which is from time to time moistened with 

 putty (white oxide of tin) and water. The wooden disc is also em- 

 ployed for this purpose with putty, colcothar, or washed tripoli. 

 For fine delineations, the glass is first traced over with some 

 coloured varnish, to guide the hand of the cutter. 



In grinding and facetting crystal glass, the deep grooves are first cut, for example, 

 the cross lines, with the iron disc and rounded edge, by means of sand and water. 

 That disc is one-sixth of an inch thick and 12 inches in diameter. With another iron 

 disc about half an inch thick, and more or less in diameter, according to the curva- 

 ture of the surface, the grooves may be widened. These roughly cut parts must be 

 next smoothed down with the sandstone disc and water, and then polished with the 

 wooden disc about half an inch thick, to whose edge the workman applies, from time 

 to time, a bag of fine linen containing some ground pumice moistened with water. 

 When the cork or wooden disc edged with hat-felt is used for polishing, putty or 

 colcothar is applied to it. The above several processes in a large manufactory are 

 usually committed to several workmen on the principle of the division of labour, 

 so that each may become expert in his department. 



2. The grinding of optical glasses. The glasses intended for optical purposes being 

 spherically ground, are called lenses ; and are used either as simple magnifiers and 

 spectacles, or for telescopes and microscopes. The curvature is always a portion of a 

 sphere, and either convex or concave. This form insures the convergence or diver- 

 gence of the rays of light that pass through them, as the polishing does the brightness 

 of the image. 



The grinding of the lenses is performed in brass moulds, cither concave or convex, 

 formed to the same curvature as that desired in the lenses ; and may be worked i-ithor 

 by hand or by machinery. A gauge is first cut out of brass or copper plate to suit 

 the curvature of the lens, the circular arc being traced by a pair of compass, s. In 

 this way both a conyex and concave circular gauge aro obtained. To these gauges 



