GLASS CUTTING AND GRINDING 675 



spreads from the heated point over the whole glass, and increases in intensity until it 

 becomes nearly a black-rod. This coloured glass can be again rendered colourless 

 by fusion and slow cooling ; its colour is again produced by a repetition of the heating 

 process. If, however, it is suddenly cooled it cannot again be made to resume its 

 ruby colour. This is also an example confirmatory in the highest degree of the view, 

 that no chemical change takes place ; but that all the phenomena are due to altera- 

 tions in molecular structure. The practice of flashing colourless glass with the ruby 

 glass from gold is commonly adopted. The beautiful examples of the Bohemian glass 

 manufacture, in which we have a mixture of rich ruby and the purest crystal, are 

 produced in this way. A globe of hot colourless glass is taken from the pot, and a 

 cake of ruby glass, prepared with a composition called Schmelze, is warmed and 

 brought into contact with the melted globe ; this ruby glass rapidly diffuses itself over 

 the surface, and the required article is blown or moulded with a coating of glass, 

 coloured ruby by gold, of any required thickness. 



Schmelze is prepared with 500 parts of silica, 800 of minium, 100 of nitre, and the 

 same quantity of potash. A very small portion of a solution of gold in aqua regia is 

 intimately mixed with 500 parts of Schmelze, 43 parts of prismatic borax, 3 or 4 of 

 oxide of tin, and a similar quantity of oxide of antimony. This mixture is heated for 

 twelve hours in an open crucible placed in a flat furnace, and then cooled slowly in 

 an annealing oven. A Bohemian ruby, especially so called, is prepared by melting 

 together fulminating gold rubbed in with oil of turpentine, quartz powdered, and 

 fritted minium, sulphide of antimony, peroxide of manganese, and potash. Bohme 

 has given an analysis of a Venetian ruby glass, in which ^th of a grain of gold is 

 combined with about 150 of the ordinary ingredients of glass, with some tin and 

 iron. 



Manganese is sometimes employed to give a fine amethystine colour to glass ; 

 care is however required to prevent the reduction of the peroxide of manganese in 

 the process. 



GREEN. Green colours may be obtained by a variety of metallic oxides. Protoxide 

 of iron imparts a dull green ; an emerald-green colour is given by oxide of copper. 

 Either copper-scales or verdigris dried and powdered are employed, the colour being 

 much finer with a lead glass, than with one containing no lead. Translucent or dull 

 glass is converted into a deep blue or turquoise colour by oxide of copper and not 

 into a green. An emerald green is also produced by the oxide of chromium. Two 

 kinds of Bohemian green glass, known respectively as the ancient and modern emerald 

 greens, are prepared from mixtures of the oxides of nickel and of uranium. 



BLUE. The only fine blue is produced by cobalt. The manufacture of smalt or 

 zaffre is so important that it will be treated of in a separate article. See SMALT and 

 COBALT. 



BROWN. Peroxide of manganese with zaffre yields a fine garnet-like brown. 



PINK or FLESH-COLOUR. Oxide of iron and alumina, obtained, by heating a mixture 

 of alum and green vitriol. 



ORANGE. Peroxide of iron with chloride of silver. 



JASPER. A Bohemian glass, generally black, but of fine lustre, prepared by adding 

 forge scales, charcoal, and bone ashes to the ordinary materials for glass. 



Amongst the different varieties of glass, artificial gems may be enumerated. For a 

 description of their manufacture, see GEMS, ARTIFICIAL. 



GLASS, STAINED. See GLASS, COLOURED. 



GLASS CUTTING AND GRINDING, for common and optical purposes. By 

 this mechanical process the surface of glass may be modified into almost any orna- 

 mental or useful form. 



1. The grinding of crystal ware. This kind of glass is best adapted to receive 

 polished facets, both on account of its relative softness, and its higher refractive 

 power, which gives lustre to its surface. The cutting-shop should be a spacious long 

 apartment, furnished with numerous skylights, having the grinding and polishing 

 lathes arranged right under them, which are set in motion by a steam-engine or water- 

 wheel at one end of the building. A shaft is fixed as usual in gallewses along the 

 ceiling ;_ and from the pulleys of the shaft, bands descend to turn the different lathes, 

 by passing round the driving pulleys near their ends. 



_The turning lathe is of the simplest construction. Fig. 1103, D, is an iron spindle 

 with two well-turned prolongations, running in the iron puppets a a, between two 

 concave boshes of tin or type-metal, which may be pressed more or less together by 

 the thumb-screws shown in the figure. These two puppets are made fast to the 

 wooden support B, which is attached by a strong screw and bolt to the longitudinal 

 beam of the workshop A. E is the fast and loose pulley for putting the lathe into 

 and out of gear with the driving shaft. The projecting end of the spindle is furnished 

 with a hollow head-piece, into which the rod c is pushed tight. This rod carries the 



