ENGRAVING ON STONE 299 



The grinding of such a hard substance as glass by an agent which is resisted by 

 such a fragile material as a green leaf, seems at first rather singular. The probable 

 explanation is, that each grain of sand which strikes with its sharp angle on the glass 

 pulverises an infinitesimal portion, which is blown away as dust, while the grains 

 which strike the leaf rebound from its soft elastic surface. 



The film of bichromatised gelatin, used as a photographic negative, may be suffi- 

 ciently thick to allow a picture to be engraved on glass by fine sand, driven by a 

 gentle blast of air. 



For cutting stone, the inventor uses steam as the impelling jet; the higher the 

 pressure the greater is the velocity imparted to the sand, and the more rapid its cutting 

 effect. 



In using steam of about 100 Ibs. pressure, the sand is introduced by a central iron 

 tube, about -^ths-inch bore, while the steam is made to issue from an annular passage 

 surrounding the sand tube. 



A certain amount of suction of air is thus produced, which draws the sand through 

 the sand-tube into the steam-jet, and both are then driven together through a tube, 

 about 6 inches long, in which the steam imparts its velocity to the sand, and 

 finally strike on the stone, which is held about an inch distant from the end of the 

 tube. 



At the part struck a red light is visible, as if the stone was red-hot, though really 

 it is below 212 Fahr. The light is probably caused by the breaking-up of the 

 crystals of the sand and stone. 



The cutting effect is greatest when free escape is allowed for the spent sand and 

 steam. In making a hole of diameter but slightly greater than that of the steam jet, 

 the rebounding steam and sand greatly interfere with, and lessen the efficiency of the 

 jet. 



Under favourable conditions, using steam which he estimated as equal to about 1 

 horse-power, at a pressure of about 125 Ibs., the cutting effect per minute was about 

 1 cubic inch of granite, or 3 cubic inches of marble, or 10 cubic inches of soft brown 

 sandstone. 



By means of flexible or jointed connecting tubes, the blast-pipe is made moveable in 

 any direction ; grooves and mouldings of almost any shape can thus bo made, or by 

 means of stencil plates, letters or ornaments can be cut either in relief or in intaglio, 

 with great rapidity in the hardest stone. 



At a high velocity, quartz sand will cut substances much harder than itself, as before 

 stated. With a steam jet of 300 Ibs. pressure, a hole 1^ inch in diameter was cut 

 through a piece of corundum 1 inch thick in 25 minutes. 



A hole 1 inch long and % inch wide was cut through a hard steel file ^ inch thick, 

 in 10 minutes, with a jet of 100 Ibs. steam. 



A stream of small lead shot driven by 50 Ibs. steam, wore a small hole in a piece of 

 hard quartz ; the shot were found to be only very slightly flattened by the blow, show- 

 ing their velocity to have been moderate. 



Among the curious examples of glass cut by this sand-blast was shown a piece 

 of ordinary window-glass, which, having been partially protected by a covering of 

 wire gauze, had been cut entirely through, thtis producing a glass sieve, with openings 

 about- -th of an inch, the intervening glass meshes being only ^th of an inch wide. 

 This seems to have been produced more as a curiosity than for any practical purpose. 

 Should such a sheet of perforated glass be required, it is questionable if it could be 

 produced from a solid sheet by any other method. 



A microscopic examination of the sheet glass depolished by this process shows a 

 succession of pits formed by the blows of the impinging grains of sand, and looks more 

 uniform than do surfaces ground by the rubbing process. 



One curious fact connected with the use of this process is that when a surface to be 

 cut in intaglio or otherwise is partially protected by templates of metal, these tem- 

 plates curl up under the blows of the sand, so that paper patterns are really more 

 durable than patterns cut from brass. Sheet steel, cut into shapes and then hardened, 

 will also curl up under the blows of the fine particles of sand, unless protected by 

 sheets of yielding material. Fine lace will protect glass during the depolishing process, 

 and leave its designs in polished lines on a ground surface. This account is derived 

 from the ' Journal of the Franklin Institute.' See GXASS. 



ENGRAVING ON STONE!; S A 1* OR GEIK ENGRAVING. Some 

 very delicate and beautiful results are obtained by engraving upon lithographic stones. 

 The art, is not, however, of sufficient importance to occupy our already crowded space. 

 Seal or gem engraving is an art of great antiquity. We may here simply allude to the 

 apparatus in general use by the workman, and which consists of a lathe turned by a 

 treddle, and moving a small horizontal steel cylinder, into which the tools are inserted, 

 allowing them to revolve against the gem, which is held by the hand of the workman 



