GLASS 



245 



flashed i.o. a thin veneer of colour coat* a 

 greater thickness of clear glass. In thin nine the 

 i\v layers are 'gathered ' from different pots on the 

 Mowing iron, ami llo\vn out together us one sheet. 

 Sometimes a very thin coloured coating is put on 

 dear glass ly spreading, nay, a red glassy powder 

 on the nurface of the latter, and then carefully 

 fusing it. When the chloride or other salt of sil\rr 

 is used to give a yellow, orange, or red, the glass 

 is merely xtuini-il on the surface. In painted glass 

 the decoration is usually produced hy the use of 

 cnam-'l i -"lours painted on with a brush, and after- 

 wards fired at a moderate heat. Single sheetH of 

 glass, each with several shades of the same colour, 

 are now made for glass-stainers, by which much 

 shading l>y hand is dispensed with. A pane or 

 vessel of flashed glass may l>e ornamented by 

 partially removing the coloured layer, either by 

 cutting or etching ; and in the case of many designs 

 additional enamel colours are added. Hydrofluoric 

 acid, which corrodes glass, is commonly used to 

 produce etched patterns upon it, by protecting 

 certain portions with a varnish, and allowing the 

 acid to act upon the unprotected parts. For painted 

 windows, see GLASS (PAINTED) ; and for artificial 

 gems of glass, see STONES ( PRECIOUS ). 



The beautiful iridescence of much very ancient 

 glass is known to l>e due to the partial decomposi- 

 tion of its surface and the formation of innumerable 

 thin scales. Many attempts have been made to 

 produce a like result artificially, and several 

 methods have l>een successful. One is to submit 

 the object to the influence of acid solutions, with 

 the help of heat and pressure. 



Venice, which prior to 1859 produced almost 

 nothing in glass but beads, now, thanks to Salviati 

 (q.v. ), is once more making on a large scale glass 

 objects, whose quaint forms and rich colours are but 

 little, if at all, inferior to the best products of her 

 ancient glass-houses. The well-known Bohemian 

 glass, much of which is coloured and gilt, but 

 which in past days was often more showy than 

 tasteful, has recently shown quite remarkable 

 advances in the character of its decoration. Some 

 very tastefully ornamented coloured glass is also 

 made in France. 



Glass-cutting and Engraving. It is usually flint- 

 glass that is so treated ; ana vessels intended to 

 have cut patterns are blown with thick walls. The 

 first operation in glass-cutting is usually done on 

 an iron grinding- wheel 10 or 12 inches in diameter, 

 and about three-fourths of an inch thick, which is 

 made to revolve vertically by means of a belt and 

 pulley. Immediately above a hopper-shaped cistern 

 is placed, which supplies the wheel with the neces- 

 sary mixture of sand and water. If a faceted 

 pattern is to l>e given to a decanter or other object, 

 it is first roughed out on this wheel by the grinder 

 holding the vessel against it. The facets now 

 formed on the glass are next made smoother by a 

 fine sandstone wheel, fed with water only, and simi- 

 larly driven. For many purposes this wheel is of 

 an angular section on the edge. The ground parts 

 are finally polished upon a wooden wheel, supplied 

 with moist putty-powder (oxide of tin) or other 

 fine polishing material. The obscuring of glass by 

 the ordinary process is done with sand and water 

 alone ; but much of this kind of work is now done 

 by Tilghman's sand-blast process, to be presently 

 noticed. Engraved patterns are produced by means 

 of small copper discs, revolving in a lathe, emery 

 powder, mixed with oil, being applied to the edges 

 of their circumference. We have already referred 

 to the use of hydrofluoric acid for etching glass. 



Tilghman's Sand-blast. This is a very striking 

 invention. The well-known fact that windows 

 exposed to the action of wind-blown sand by the 

 seashore eventually l>ecome completely obscured 



appear* to have suggested the process to Mr Tilgh- 

 maii. Tin- Matlhe\\son's patent sand blast appar- 

 atus, manufactured bv the Tilghinan' Patent nand- 

 blast Company, is altogether inde]M-ndent of any 

 blower or engine, and occupies a very small space, 

 being alxuit 2 feet square and 3 feet high. The 

 piece of glass to be operated on is held on the top 

 of the machine. The sand is set in motion by a 

 steam ejector (part of the machine), being drawn 

 by the vacuum caused by the flow of steam into an 

 annular space where it mingles with the steam, 

 and is ejected through a small pipe against the 

 glass. The machine does exceedingly tine and 



auick work as regards obscuring lx>th plain and 

 ashed glass. It cuts away the flashed surface 

 almost as soon as the glass is held in position. The 

 parts which are to remain bright are protected 

 either by a composition or by blotting-paper, 

 which, having been soaked in glycerine and glue, 

 has been stuck on to the glass, and from which the 

 pattern is cut out. The blast will drill holes in a 

 glass plate J-inch thick of a diameter from .j inch 

 to 1 inch in less than two minutes. This machine 

 is not adapted for obscuring large sheets of glass, 

 which is accomplished by a machine in which the 

 sand is set in motion by a Baker's blower driven by 

 a non-condensing engine. The latter apparatus is 

 not capable of perforating glass or of removing the 

 flashed surface. 



Pressed Glass. By this name is known a certain 

 cheap class of objects, such as tumblers, small 

 dishes, &c. , with patterns in imitation of cut glass. 

 It is an American invention, and the process 

 consists in pressing or shaping glass into form 

 by means or a metal mould and reverse, called 

 a plunger, or, for larger work, by a weighted lever, 

 or a screw and fly-wheel. The chief seat of this 

 branch of the trade in Great Britain is at New- 

 castle-on-Tyne, where a glass in which baryta is 

 largely or wholly substituted for lead is used. By 

 a somewhat similar but much older process, 

 ' pinched glass ' objects such as buttons are largely 

 made at Birmingham. 



Soluble Glass. When silica (flint or sand) is 

 fused with an excess of akali, a glass is formed 

 which is slowly soluble in cold, but readily soluble 

 in hot water if powdered. The soluble silicate of 

 soda or of potash formed by this or by other 

 methods is known as soluble glass or water-glass. 

 When pure and solid it has the appearance of com- 

 mon glass, and it is the more soluble the larger the 

 quantity of alkali that it contains. This substance 

 has a number of applications in the arts. When a 

 solution of it is mixed with sand, ground chalk, 

 dolomite, or other minerals, it gradually binds them 

 into a stony mass. See STONE ( ARTIFICIAL ). It 

 is also employed as a Cement (q.v.). Soluble glass 

 is useful as a material for rendering calico and even 

 wood non-inflammable, for improving the cleansing 

 power of cheap soaps, and as a dung substitute in 

 dyeing. A small quantity of silicate of soda mixed 

 with hard water improves it for washing purposes. 



As far back as 1825 Fuchs of Munich suggested 

 the application of soluble glass to the surface of 

 fresco-painting, in order to fix the colours, the 

 climate of northern Europe not being suitable for 

 the preservation of this Kind of decoration, when 

 simply executed in the old way, with colours applied 

 with water on a plaster ground. It has frequently 

 been stated that Fuchs's plan of applying solutions 

 of silicate of soda or silicate of ]>otash to fresco- 

 painting has effectually preserved it. But in most 

 cases it has not done so. The action of the car- 

 bonic acid of the atmosphere upon either of these 

 compounds has usually, in the course of time, 

 brought out an efflorescence like mildew on the 

 surface of the picture. Professor Barff, who ha* 

 paid a good deal of attention to the behaviour of 



