M, TECHNOLOGY. 521 



stone, which readily absorbs water to a certain depth, and 

 the treatment consists in the successive introduction of a 

 solution of an alkaline silicate and of alumina. The result 

 is the production of an aluminous silicate within the pores 

 of the stone, w T hich gives to the surface a considerable power 

 of resistance. The solutions employed are soluble glass and 

 sulphate of alumina. After the process is completed, the 

 stone may be polished like marble, which it greatly resem- 

 bles. Heated to a high temperature, the exterior layer vit- 

 rifies, and may therefore be colored or decorated according 

 to taste. The coloration may even be obtained by simply 

 mixing the desired pigment with one of the solutions em- 

 ployed. 



PRODUCTION OF PATINA, OF DIFFERENT COLORS, ON BRONZE. 



The attention of Christophle and Bouilhet has been special- 

 ly devoted for several years to the difference in the color of 

 bronzes, and the method of reproducing them. Their inves- 

 tigations were controlled by the conclusion, reached after 

 numerous experiments, that the patina of bronze can only 

 be permanent when it results from natural chemical reac- 

 tions, and is not developed by varnish or corroding agents. 

 As a result, they exhibited brown, red, orange-yellow, and 

 black colors. They state that they were obtained upon the 

 surface by reactions which cause the production of suboxide 

 of copper in two molecular modifications, and also of sul- 

 phide of copper. The objects allow of a coating of varnish 

 for protection, the chief condition of its successful applica- 

 tion being the slowness with which it is done. The process, 

 it is claimed, is practical and trustworthy, and it will repro- 

 duce the same three colors at any time. Addition of lead 

 to the alloy is not necessary to produce the black patina, 

 which under such circumstances has been found to be brit- 

 tle and wanting in permanence. 14 (7, CCXIIL, 1874, 447. 



PATE SUR PATE, A METHOD OF DECORATING PORCELAIN. 



The following is from an account of this method of decora- 

 tion given by Professor De Luynes, of the Conservatoire des 

 Arts et Metiers, Paris. A faint drawing is first made on 

 the article, completely formed in unbaked porcelain. All 

 the details are then represented by means of a colored 



