298 ENGRAVING ON GLASS 



and it can be set up and printed with the typo, a mode which no other style of art 

 can rival in simplicity and cheapness. The taste for elaborately-executed wood- 

 engravings may again decrease, as we find it did for nearly two centuries ; but it was 

 never a lost art, and never will be, owing to the practical advantages wo speak of, 

 unless it be superseded by some simpler mode of doing the same thing hitherto undis- 

 covered. The number of persons who practise wood-engraving in London .-ilum- is 

 very large, and when we consider the quantity done in the great cities of the Con- 

 tinent, and the largo amount of book illustration in constant demand, the m-ativo 

 power of one single genius Thomas Bewick shines forth in greater vigour than 

 ev er. F.W.F. 



When a large number of impressions are to be taken from a wood-block, a cast in 

 metal is taken from it, in the manner described under STEREOTYPE. In somo cases 

 electrotype copies are taken in copper, and the comparatively thin sheet of copper 

 produced is mounted upon a wood-block of the proper thickness. 



Photography has been employed most serviceably by the wood engraver. Drawings 

 are reduced by the aid of the camera to any required size, and the impression re- 

 ceived directly upon the wood which is to be cut. Most of the wood-engravings 

 which illustrate this volume have been produced by the use of the photographic pro- 

 cess. See PHOTOGRAPHY. 



Engraving on brass plates has been used as a cheap mode of superseding wood- 

 engraving for works of a cheap character. 



ENGRAVING OW GXiASS. This work does not deal with mere matters of 

 manipulation, therefore the process of engraving on glass, by means of the glass-cutter's 

 wheel, and with tools, finds some notice under GLA.SS only. One process of engraving 

 on glass is carried out by covering the glass with an etching wax, carefully cutting out 

 the pattern, then covering the glass with powdered fluor-spar, and pouring sulphuric 

 acid upon it. Or, by preparing fluoric acid in a metal bottle, and projecting tlio 

 vapour as formed directly upon the exposed part of the glass. Attention has of late 

 been drawn to a process of engraving glass by the sand-jet, an American invention. 



Mr. Tilghman's attention seems first to have boon directed towards cutting stone or 

 hard metal by a jet of sand impelled by escaping steam under high pressure. His 

 early experiments were with very high pressure ; but, as he progressed in the knowledge 

 of results obtainable with various velocities, a great use for this process seemed to 

 develop itself in sand driven by moderate air-blasts, and applied to grinding or de- 

 polishing glass for ornamental purposes. 



For grinding glass a common rotatory fan is used, 30 inches in diameter, making 

 about 1,500 revolutions per minute, which gives a blast of air of the pressure of about 

 4 inches of water, through a vertical tube, 2 feet high by 60 inches long, and 1 inch 

 wide. 



Into the top of this tube the sand is fed, and falling into the air-current and ac- 

 quiring velocity from it, is dashed down against the sheets of glass, which are slowly 

 moved across, about 1 inch below the end of the tube. About 10 or 15 seconds' 

 exposure to the sand-blast is sufficient to completely grind or depolish the surface of 

 ordinary glass, so that sheets of it carried on endless belts may be passed under this 

 1 inch wide sand-shower at the rate of 5 inches forward movement per minute. In 

 the machine in use for this purpose the spent sand is reconveyed to the upper hopper 

 by elevators, and the dust made by the sand-blast (which might otherwise be a 

 source of annoyance to the workman) is drawn back into the fan, and thence passes 

 with the wind into the blast ton, and again mingles with the shower of sand upon the 

 glass. 



By covering parts of the glass surface by a stencil or pattern of any tough or 

 elastic material, such as paper, lace, caoutchouc, or oil paint, designs of any kind may 

 bo engraved. 



There is a kind of coloured glass made by having a thin stratum of coloured glass 

 melted or ' flashed ' on one side of an ordinary sheet of clear glass. If a stencil of 

 sufficient toughness is placed on the coloured side, and exposed to the sand-blast, the 

 pattern can bo cut through the coloured stratum in from about 4 to 20 minutes, 

 according to its thickness. 



The theoretical velocity of a current of air of the pressure of 4 inches of water, 

 Mr. Tilghmau calculates, is (neglecting friction), about 135 foot per second; the actual 

 velocity of the sand is doubtless much loss. 



If a current of air of less velocity is used, say about 1 inch of water, very delicate 

 materials, such as the green leaves of the fern, will resist a stream of fine sand long 

 enough to allow their outlines to bo engraved on glass. By graduating the time of 

 exposure with sufficient nicety, so as to allow the thin parts of the loaves to bo p.irtly 

 cut through by the sand, while the thicker contra! ribs and their branches will resist, 

 the effect of a shaded engraving may be produced. 



