STONE, ARTIFICIAL 915 



stone. To get rid of the sulphate, the caustic solution of soda has added to it, in the 

 tank D, a quantity of caustic baryta, obtained by burning the commercial carbonate 

 of baryta with wood-charcoal. The caustic baryta seizes upon the sulphuric acid 

 contained in tho sulphate of soda, and forms with it an insoluble sulphate of baryta, 

 which is precipitated on tho bottom of the tank. The depurated lye is then drawn 

 off by the pipe d, into the lower closed tank E, and the sulphate-of-baryta sediment 

 passes off by the cock at the bottom. From E, the prepared solution of the caustic 

 soda is pumped into the vertical boiler or digester F. This digester, in which the 

 process of dissolving the flints is effected, is a cylindrical vessel, having a steam- 

 jacket,/, into which steam from the boiler A is supplied by the pipes 1, 2, y. Tho 

 inner cylinder F, is provided with a wire-basket G, reaching the whole length of the 

 vessel, and serving to hold a collection of nodules of common flint. When F has 

 been filled with the caustic lye, and the basket with flints, the manhole at the top is 

 closed and well screwed down, so as to be able to resist a pressure of at least 60 Ibs. on 

 the square inch. The cock at y is then opened, and the full pressure of steam from 

 the boiler passes into the jacket f, and causes the lye in F to rise to the same tem- 

 perature. The condensed steam in the jacket /returns to the boiler by the pipe 12, 

 which it enters below the water-line. The pressure maintained in the digester is 

 generally about 60 Ibs., and this is continued about 36 hours ; at the end of which time 

 the strength of the solution is tested. The workmen employed to superintend this 

 part of the process generally use the tongue as the most delicate test. If the solution 

 has a decidedly caustic alkaline taste, they conclude that there is still too much free 

 soda in the cement, and the boiling is allowed to continue until the cement has a 

 slightly sweetish taste, which occurs when the alkali has been nearly neutralised by 

 combination with the silicic acid of the flints. A more scientific mode of testing tho 

 strength of the solution is to take a wine-glassful and drop a little hydrochloric- acid 

 into it ; by this means the whole of the silica in the solution is thrown down by the 

 acid combining with the soda, so as to form chloride of sodium. The precipitated 

 silica presents an appearance resembling half-dissolved snow, and its comparative 

 volume gives a good idea of the strength of the solution of the alkaline silicate. 



When it is judged that the alkali has taken up as much of the silica as it is capable 

 of doing, at the temperature to which it is subjected in the digester, the stop-cock y, 

 in the steam-pipe communicating with the jacket, is shut, and a cock in the pipe 8 is 

 opened. The pressure of the steam in F then forces the fluid silicate, through the 

 pipe 8, into the vessel H, where it is allowed $o stand for a short time to deposit any 

 sediment which it may contain. From H it is then conveyed by the pipe 9 to the 

 evaporating pan, K, which has a steam-jacket, k, supplied with steam by the pipe 10. 

 The cement is then boiled in the evaporating pan until it becomes of the consistency 

 of treacle, when it is taken out. The specific gravity of the cement when ready for 

 use is about T600. The general proportions of the materials used in making up the 

 artificial stone are about the following : 



10 pints of sand, 1 pint of powdered flint, 1 pint of clay, and 1 pint of the alkaline 

 solution of flint. 



These ingredients are first well mixed in a pug-mill, and kneaded until they are 

 thoroughly incorporated, and the whole mass becomes of a perfectly uniform consis- 

 tency. When worked up with clean raw materials, the compound possesses a putty- 

 like consistency which can be moulded into any required form, and is capable of re- 

 ceiving very sharp and delicate impressions. 



The peculiarity which distinguishes this from other artificial stones consists in 

 the employment of silica both as the base and tho combining material. Most of the 

 varieties of artificial stone hitherto produced are compounds, of which lime, or its 

 carbonate, or sulphate, forms the base ; and in some instances they consist in part 

 of organic matters as the cement, and having inorganic matters as the base. 



To produce different kinds of artificial stone, adapted to tho various purposes to 

 which natural stones are usually applied, both the proportions and the character of 

 the ingredients are varied as circumstances require. By using the coarser description 

 of grits, grinding stones of all kinds can be formed, and that with an uniformity of 

 texture never met with in the best natural stones. Any degree of hardness or porosity 

 may also be given, by varying the quantity of silicate employed and subjecting it to a 

 greater or less degree of heat. 



For some descriptions of goods a portion of clay is mixed with the sand and other 

 ingredients, for the double purpose of enabling the material to stand up during the 

 process of firing in the kiln and to prevent its getting too much glazed on tho surface. 



The plastic nature of the compound allows of the most complex and undercut pat- 

 terns being moulded with greater ease than by almost any other material we are 

 acquainted with, if we except gutta-percha, which, however, has the drawback of being 

 affected bv common temperatures. 



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