MAKING MONEY OUT OF WASTE 13 



in the air has been made prisoner, so to speak, and converted 

 into a most valuable factor. The mixture left after the 

 iron has been extracted from the ore sometimes called slag 

 which represents the debris of the iron ore, is now one of 

 the most valuable compounds coming from the blast fur- 

 nace, although but a few years ago it was thrown away. In 

 fact, blast furnaces have been built on the edge of swamps 

 and bodies of water, so that the slag could be thrown into 

 these places and used for filling them up. Very good glass 

 is now made from this slag, as well as paving blocks and 

 bricks, artificial porphyry, and a cement which is equal to 

 the best. Ground with six per cent, of slaked lime, build- 

 ing mortar is also made from slag ; and ornamental copings 

 and moldings, window sills, and chimney pieces are fash- 

 ioned of it. 



Slag brick is stated to be quite as strong as ordinary 

 brick, and much less permeable to moisture. To make 1,000 

 brick, 6,000 or 7,000 pounds of granulated slag, and from 

 500 to 700 pounds of burned lime, are consumed. Good 

 bricks also can be made from granulated slag mixed with 

 dust from slag, though the hardening process is rather slow. 

 Slag is also used for steampipe and boiler wrappings, in 

 which form it is called "silicate of cotton." Coal slag is 

 a good structural material when mixed with slaked lime. 

 Basic slag is used in large quantities by manufacturers of 

 fertilizers, instead of phosphate rock. 



The greatest metal industry in the world, which is now 

 being built in Indiana, forming an entire city in itself, is 

 provided with iron smelters from which the gas as it rises 

 will be returned to the fires beneath the ore and used for 

 heat. By this system the cost of coal to smelt the ore will 

 be about one half the expense if the gas were not secured 

 as stated. Waste gas has been utilized by inventors for the 

 direct operation of engines so large that they have a force 

 equal to the power of a thousand horses. As it issues from 

 the smelter, the gas enters a large cover, as it might be 

 termed, placed above the furnace. In the center of the 



