MAGNESIA BRICKS AND OXYCHLORIDE CEMENTS. 



161 



part light magnesia. The dense, chemically stable " heavy magnesia " 

 is thus the base of the brick; the light magnesia is added to give plas- 

 ticity to the mixture, enabling it to be molded, and also to harden on 

 exposure to the atmosphere. 



From 10 to 15 per cent of water is added to this mixture, and the 

 resulting stiff paste is pressed into form in iron molds. The brick will 

 gradually harden on simple exposure to the air, after which it is usually 

 made still more resistant by reburning at a low red heat. Bricks or 

 other objects made in this manner may, if not sufficiently solid for 

 the use for which they are intended, be hardened by dipping into a 

 cold dilute solution of boracic acid in water. After this they should 

 be dried and reburned. 



Composition of magnesia bricks. 



TABLE 66. 

 ANALYSES OF MAGNESIA BRICKS. 



1. Brick made from Hungarian magnesite. Mineral Industry, vol. 10, p. 439. 



Grecian 



Trans. Am. Inst. Min. Engrs., vol. 26, p. 268. 

 Mineral Industry, vol. 10, p. 439. 



Trans. Am. Inst. Min. Engrs., vol. 26, p. 268. 



Physical properties of magnesia bricks. The brick * whose analysis 

 is given in column 3 of Table 66 was made in Pittsburg from Styrian 

 magnesite. Its specific gravity was 3.44, equivalent to a weight of 

 160.9 Ibs. per cubic foot. The brick whose analysis appears in column 6 

 of the same table was made from Grecian magnesite. Its specific gravity 

 was 3.54, corresponding to a weight of 170.2 Ibs. per cubic foot. 



Le Chatelier tested two kinds of magnesite bricks (Austrain and 

 Grecian) for expansion with increase of temperature, obtaining the 

 results quoted in Table 67. The expansions given are in millimeters 

 for a bar 100 mm. in length, and are therefore equivalent to percent- 

 ages. 



* Trans. Am. Inst. Min. Engrs., vol. 26, p. 268. 



