1 854.] 



ON SILICA AND SOME OF ITS APPLICATIONS TO THE ARTS. 



107 



this silicated alkali by disolving broken flints in a solution of 

 caustic alkali at a temperature of 300° Fab.* And, more 

 recently, Mr. Way has observed that the sand which he has 

 described will combine with caustic alkali at boiling heat, also 

 producing a water-glass. 



This water-glass has been applied to several important pur- 

 poses, three of which were specially noticed. 



1. To protEct huilding-stones from decay. The stone .sur- 

 faces of buildings, by being exposed to the action of the 

 atmosphere, become liable to disintegration from various 

 causes. Moisture is absorbed into their pores. The tendency 

 of their particles to separate, in consequence of expansion 

 and contraction, produced by alternation of temperature, is 

 thus increased. Sulphurous acid is always present in the 

 atmosphere of smoke-burning cities, and cannot but corrode 

 the calcareous and magnesian ingredients of oolites and dolo- 

 miies. li is true that good stone resists these sources of injury 

 for an indefinite time, but such a material is rarely obtained. 

 As a preventive of destruction, whether arising from physical 

 or chemical causes, it has been proposed to saturate the surfaces 

 of the stone with a solution of water-glass. 



It is well known that the affinity of silica for alkali is so 

 feeble that it may be separated from this base by the weakest 

 acids, even by carbonic acid. According to the expectation 

 of those who recommend the silication of stone, the carbonic 

 acid of the atmosphere will set the silica free from the water- 

 glass, and the silica, thus separated, will be deposited within 

 the pores and around the particles of the stone. The points of 

 contact of these particles will thus be enlarged, and a sort of 

 glazing of insoluble silica will be formed sufficient to protect 

 the stone against the effiact of moisture, &c. This cause of 

 protection applies chiefly to sandstone. But wherever carbo- 

 nate of lime or carbonate of magnesia enters notably into the 

 composition of the building-stone, then an additional chemical 

 action, also protective of the stone, is expected to take place 

 between these carbonates and the water-glass. Kuhlmann f 

 remarks " Toutes les fois que Ton met en contact un sel inso- 

 luble avec la dissolution d'un sel dont I'acide put former avec 

 la base du sel insoluble un sel plus insoluble encore, il y a 

 echange ; mais le plus soutent cet echange n'est que partiel." 

 Inconsequence of this "partial exchange" an insoluble salt 

 of lime may be looked for whenever a solution of water-glass is 

 made to act on the carbonate of lime or carbonate of magnesia 

 existing in oolitic or dolomitic building-stone. 



This expectation, however, has not been altogether sanc- 

 tioned by experiment. A gentleman, eminently conversant 

 with building materials, J immersed a piece of Caen stone in a 

 solution of a silicate of potass in the month of January, 1849. 

 This fVagraent, together with a portion of the block from which 

 it had been separated, was placed on the roof of a building in 

 order that it might be fully exposed to the action of atmosphere 

 and climate. After four j-ears the silicated and the unsilicated 

 specimens were f jund to be both in the same condition, both to 

 be equally corroded. These specimens were exhibited in the 

 theatre of the Institution. But whatever ultimate results may 



* Report of a communicntion mnde to the Royal Institution by Trof . 

 Faraday, May 20, 1848. Vide Athemcmn, June 17, 1818. 



f Experiences Chimiiiues et Agronomiques, p. 120. 



X Charles II. Smith, Esq., one of the Authors of the " Report on tlio 

 Selection of Stone for the building of the New Houses of Parliament." 



ensue from this process, the immediate effects on the stone are 

 remarkable. Two portions of Caen stone were exhibited, one 

 of which had been soaked in a solution of water-glass two 

 months before. The surfiioe of the unsilicated specimen was 

 soft, readily abraded when brushed with water, and its calcare- 

 ous ingredients dissolved in a weak solution of sulphurous acid. 

 The silicated surface on the other hand, was perfectly hard, and 

 resisted the action of water and of dilute acid when similarly 

 applied. § 



II. Another proposed use of the water-glass is that of 

 hardening cements mortars, &c., so as to render them imper- 

 meable by water. 



Fourteen years since, Anthan|| of Prague proposed several 

 applications of the water-glass. Among others he suggested 

 the rendering mortars waterproof He also suggests that this 

 substance might be beneficially employed as a substitute for 

 size in whitewashing and staining walls. It was demonstrated 

 by several experiments that carbonate of lime mixed up with 

 a weak solution of water-glass and applied as a whitewash to 

 surfaces, was not washed oif by sponging with water, and that 

 common whitewash laid on in the usual manner with size, was 

 rendered equally adhesive when washed over with water-glass. 



ni. The Sfereochrome of Fuchs. 



The formation of an insoluble cement by means of the water- 

 glass, whenever the carbonic acid of the atmosphere acts on 

 this substance, or whenever it is brought in contact with a lime- 

 salt, has been applied by FucLs to a most important purpose. 

 The stereochrome is essentially the process of fresco secco^I in- 

 vested with the capability of receiving and perpetuating works 

 of the highest artistic character, and which may be executed 

 on a vast scale. Fuch's method is as follows**: — 



" Clean and washed quartz-sand is mixed with the smallest 

 quantity of lime which will enable the plasterer to place it on 

 the wall. The surface is then taken oif with an iron scraper in 

 order to remove the layer formed in contact with the atmos- 

 phere, the wall being still moist during this operation. The wall 

 is then allowed to dry ; after dijing it is just in the state in 

 which it could be rubbed off by the finger. The wall has now 

 to be fixed i. e., moistened with water-glass.ft (-^n important 

 point is not to use too much water-glass in the moistening the 

 wall.) This operation is usually performed with a brush. 

 The wall must be left in such a condition as to be capable of 

 receiving colours when afterwards painted on. If, as fre- 

 quently happens, the wall has been too strongly &s.cd, the 

 surfoce has to be removed with pumice and to be fixed again. 

 Being fixed in this manner, the wall is suffered to diy. Before 



§ Siliman'g jVmerican Journal, January, 1854, contains a notice of 

 the application of the -vvatcr-glass ^to the decaying surfaces in the 

 Cathedral of Noh-e Dame in Paris. 



II Neuore Mittheilungcn uber die Nutzanwcmlung dcs Wasscr-glasos 

 1840. This subject has been fully treated by Kuhlmann in his 

 " Memoiro de I'lntcrvcntion de la potasse ou de la soude dans la 

 formation drs chaus hydrauliqucs," &c. 1841. Experiences Chimiques 

 et Agronomiqucs. 



T[ Vide Eastlake's Materials for a History of Oil Painting, p. 142. 



** These particulars were obtained by Dr. Ilofmann from Mr. Echter. 

 A stcreochromic picture by Editor, and a sample of the water-glass as 

 • prepared by Fuchs, were also exhibited by Dr. Hofmann. 



f f The composition of the specimen produced was — Silica, 2:3-21 per 

 cent.; Soda, 8-90 per cent.; Potass, 2-C2 per cent.; and the specific 

 gravity of tho solution was 3-81. 



