SLAG BRICKS AND SLAG BLOCKS. 681 



a top pressure by means of a wedge-shaped plunger, forcing the mate- 

 rial well into the sides and corners of the mold, and a final pressure 

 from below, which completely presses out the indentation made by 

 the wedge-shaped plunger and gives a good finish to the sides and cor- 

 ners of the brick. The necessity of this arrangement will be apparent 

 when it is understood that ground slag does not become plastic under 

 pressure as does ordinary clay, and that a material when filled into a 

 mold by gravity naturally piles in the center, and if directly pressed 

 would produce bricks of greater density in the center than at the 

 sides. 



"The hardening-chamber s like a boiler without flues, 45 ft. long 

 by 6 ft. in diameter. In contains 6000 bricks, and must be capable of 

 withstanding the pressure of steam, which is used for their indurating. 

 One end of the chamber is removable and held in place by hinged bolts 

 threaded on to a back ring, the joint being made by a projection on 

 the cover fitting into a recess in the shell, the bottom of the recess 

 being filled with ordinary red rope packing. The chamber will per- 

 mit of two steamings per day one during the daytime and the other 

 at night. Hence each chamber with high-pressure steam serves for 

 12,000 bricks per day. 



" The brick wagons must be strongly constructed, as any deflection 

 of the platform might tend to crack and spoil the bricks, which in the 

 green state require some care in handling. It is necessary that roller 

 or ball bearings be used for the axles, as under the action of the steam 

 any oil or grease would be burnt out of ordinary bearings. 



" It will be noted that no binding material whatever is mixed with 

 the slag. The process is really the production of a concrete. In grind- 

 ing the slag fine enough to pass a 10 per inch mesh a very large pro- 

 portion of it is reduced to a fine dust, which acts as a hydraulic cement, 

 the coarser particles forming the aggregate. Where the slag is very 

 hard, and consequently only a small proportion of dust is produced, it 

 is necessary to reduce a portion in a ball mill or other suitable fine-grind- 

 ing machine. The precise action which takes place during the harden- 

 ing is difficult to determine; but evidently the result is due to a com- 

 bination being effected between the free lime found in all limy slags 

 and the silica and alumina. 



" It may be assumed that the silicious compounds in the slag become 

 soluble in the presence of heat and moisture, in which condition it is 

 readily attacked by the free lime present in the slag. 



" With some slags high-pressure steam gives better results than low- 

 pressure, besides requiring less time to effect the hardening. In speak- 



