164 CEMENTS, LIMES, AND PLASTERS. 



bonate of magnesia. A variable percentage of iron in the residue gives 

 the cement derived from this stone a reddish tint. 



"Second. For making stone, the burnt and ground magnesite 

 (oxide of magnesium) is mixed dry in the proper proportion with the 

 material to be united; that is, with powdered marble, quartz, emery, 

 silicious sand, soapstone, or with whatever substance forms the basis 

 of the stone to be imitated or reproduced. 



"The usual proportions are: for emery-wheels, 10 to 15 per cent of 

 oxide of magnesium by weight; for building-blocks, such as sills, lintels, 

 steps, etc., 6 to 10 per cent, and for common work for thick walls, less 

 than 5 per cent. 



"The dry ingredients are mixed together by hand or in a mill. A 

 hollow cylinder revolving slowly about its axis would answer the pur- 

 pose. 



"Third. After this mixing they are moistened with chloride of 

 magnesium, for which bittern water the usual refuse of seaside salt- 

 works is a cheap and suitable substitute. The moistened material 

 is then passed through a mill, which subjects it to a kind of trituration, 

 by which each grain of sand or other solid material becomes entirely 

 coated over with a thin film of the cement, formed by a combination of 

 the chloride with the oxide of magnesium. The bittern water is required 

 to be of the density of from 15 to 30 Baume. The mass on emerging 

 from the mill should be about as moist as molder's clay. The mixing- 

 machine used by the Union Stone Company is an improved pug-mill 

 invented by Mr. Josiah S. Elliott. It is represented as an excellent 

 mill, doing its work thoroughly. 



"Fourth. The mixture is formed into blocks by ramming or tamping 

 it in strong molds of the required form, made of iron, wood, or plaster, 

 precisely as described in paragraph 24, Report on Beton Agglomere. 

 The block may be taken out of the mold at once and nothing further 

 need be done to it. The setting is progressive and simultaneous through- 

 out the mass, as with other hydraulic cements, and requires from one 

 hour to one day, depending somewhat on the chemical properties of 

 the solid ingredients used, the carbonates as a rule requiring a longer 

 time than the silicates. 



/'Building-blocks will bear handling, and may be used when three 

 or four days old, although they do not attain their maximum strength 

 and hardness for several months. Emery-wheels are not allowed to be 

 used in less than four weeks. 



"This stone so closely resembles the natural stone, whether marble, 

 soapstone, sandstone, etc., from which the solid ingredients are ob- 



