334 



ON HYDEAULIC LIMES, ARTIFICIAL STONES, &c. 



[1855. 



atiou of works of sculpture, and ornaments of tlie most delicate 

 workmansliip ; for when the silicifying process — " silicatisa- 

 tlon" — has been effected on well-dried chalk, without which 

 n-Qod results are not possible, the surface remains unalterable. 



" Some attempts made to render this stone applicable for li- 

 thography give promise of great success. 



" This method of converting soft limestone into siliceous 

 limestone is likely to become a great acquisition in the art of 

 building. * Ornaments, unaffected by damp, and of great hard- 

 ness, may thus be obtained at little cost ; and, in many cases, 

 a plaster made with a solution of silicate of potash will preserve 

 from subsequent decay ancient monuments formed of soft lime- 

 stone. This same plaster may become of general application 

 in those countries where, as in Champagne, chalk forms almost 

 the only building material." 



I have shown experimentally that one part of the silica from 

 the silicate becomes separated by the action of the carbonic acid 

 of the air, but that those parts of the silicate which have come 

 into contact with a sufficient quantity of carbonate of lime, 

 pass into the state of silicate of lime. My work, presented to 

 the Academy in 1841, pointed out numerous industrial pur- 

 poses to which the impregnating of porous bodies of mineral 

 substances might be applied, whether the objects operated upon 

 were organic or inorganic. Considering these applications of 

 the art as of the first importance in building, I have attempted 

 to extend them, and I have just laid before the Academy a new 

 series of observations. 



IIAR3I0XIZIXG THE SUADES OV THE SILICIFIED STOXE. 



I have given the name of " silicatisation" to this remarkable 

 conversion of soft and porous limestone into siliceous and com- 

 pact limestone. As the operation of this process to articles of 

 sculpture and building materials gives rise to a colouring very 

 frequently so marked as to render the joinings more apparent 

 and the veins more distinct, I have been compelled to seek a 

 remedy for this objection. 



There are two essential and general points to be met. Chalk 

 walls are too white, while some kinds of ferruginous limestones 

 are too sombre in their shades. To obviate this inconvenience, 

 I perform the silicatisation of limestones which are too white 

 with a double silicate of potash and magnesia. This is a vi- 

 treous substance, which forms a brown solution, and which 

 when used in th% process causes a little oxide of manganese to 

 be deposited in the artificial siliceous paste. Oxide of cobalt, 

 too, will combine, though in very small quantities, with silicate 

 of potash. Silica precipitated by a current of carbonic acid is 

 of a brilliant azure blue. This silicate may be used in the treat- 

 ment of white marbles. 



When the shades of the stone ai-e too decided, and that is 

 the most common defect, I obtain good results by mixing in 

 the silicate solution a small c^uantity of artificial sulphate of 

 barytes, which in penetrating the porous stone, whilst it forms 

 a layer of silica, remains fixed, entering, as we shall see below, 

 into a state of chemical combination. As regards the joints, 

 they may be made with common cements, the shades of which 

 may be rendered lighter by means of some white substances, 

 but they may be still more entirely concealed with broken 

 pieces of the stone itself mixed with silicate of potash, the 

 whole being well pulverised previous to its use, and applied in 

 a state of liquid paste. 



COLOUEIXe THE STONE. 



In the course of niy researches for giving to these silicified 

 stones shades which would cause thoseportionsof our buildings 



which had been submitted to this pi'ocess to harmonise with 

 those which had not, I was led to submit the stones to an ac- 

 tual dyeing process by impregnating them in the first instance 

 with certain metallic salts, which by precipitation would pro- 

 duce the required colour. 



Thus, impregnating the stone with salts of lead or copper, 

 and afterwards bringing it into contact with sulphuretted hy- 

 drogen gas, or a solution of hydrosulphuret of ammonia, I ob- 

 tain at will grey, black, or brown shades ; with salt of copper 

 and ferrocyanide of potassium I get shades of copper colour, &c. 



In the present case I have made an observation which in a 

 chemical point of view is not devoid of interest. 



I have stated that the porous limestones, when submitted at 

 a boiling heat to solutions of metallic sulphates whose bases are 

 insoluble in water, give rise during the whole reaction, to a 

 disengagement of carbonic acid, and to the fixing in sufficient 

 depths metallic oxides in intimate combination with sulphate 

 of lime. When the metallic sulphates have a coloured base, 

 very beautiful tints of different and perfect shades are obtained. 

 Thus, with sulphate of iron we get the production of a tint in 

 red rust, more or less deep according as we operate with solu- 

 tions of green vitriol more or less cencentrated; with sulphate 

 of copper the stone takes a magnificent green tint ; with sul- 

 phate of manganese brown shades are obtained ; with a mixture 

 of sulphate of iron and sulphate of copper we get a chocolate 

 colour. I have also experimented with sulphates of nickel, 

 chromium, cobalt, &c., and with mixtures of these sulphates. 



The affinities which determine the reactions in question are 

 sufficiently powerful to cause the metallic oxides to be com- 

 pletely absorbed by the stone, so much so, that certain oxides, 

 such as that of copper, for instance, entirely disappear from the 

 solutions after boiling with an excess of chalk. 



It is remarkable, that when in operating with mixtures of 

 salts of copper and salts of iron or of manganese, the oxides of 

 iron and manganese are the first to be thrown down. 



When we operate with sulphates having a colourless base, 

 such as sulphate of zinc, magnesia, or alumina, we equally ob- 

 tain the precipitation from the oxide, and their penetration to 

 a certain depth in the stone, with a disengagement of carbonic 

 acid. 



The bi-sulphate of lime gives analagous results. 



In general, when we intend to use coloured stones in build- 

 ings, &c., or to form mosaics, it will be found useful to increase 

 their hardness by the silicifying process. 



We may proceed in the same way with articles in shell, white 

 coral, &c., in which the colour may be produced by the same 

 process, acting at different pressures. 



I will conclude this head with an important observation, 

 which is, that the double sulphates which are formed in pene- 

 trating the stone, make a body with it, and increase its hardness 

 to such an extent, that when certain sulphates are employed, 

 such as that of zinc, the silicifying process becomes unnecessary. 



The editor of " Cosmos" adds, the process which has just 

 been described, is likely to tend to the production of a great 

 and new industry, splendid specimens of which are to be found 

 in the Exposition Universelle, placed in the central gallery of 

 the " Annexe," on the banks of the river, opposite the produce 

 from the mines of Anzin. We shall examine these specimens 

 with care, and give a detailed account of them when we treat 

 of the section of the chemical arts. The display made by the 

 celebrated chemist of Lille is one which deserves great at- 

 tention. 



