LIME AND CEMEl^T INDTJSTEIES 663 



allowed to burn itself out, after which the burned stone is drawn 

 off at the bottom of the kiln. The principle of such a kiln is 

 bad, because it necessitates a mixture of the stone and the fuel, 

 whose ashes may dirty the lime (pi. 3). 



In the continuous kilns, the fuel does not come in contact with 

 the stone; for fireplaces are built in the sides of the kiln several 

 feet above the bottom. These fireplaces are arched openings, 

 which extend from the exterior to the interior of the kiln. Into 

 these the fuel is put, and burned, thus permitting the flames to 

 pass upAvard through the stone, but preventing the ash of the fuel 

 from coming in contact with it. 



The older forms of kilns were massive stone structures, with 

 thick walls, and having the chamber lined with one or two layers 

 of fire brick. The more modern ones are circular in form, with 

 much thinner walls, and boimd with sheet iron plates. 



In the accompanying plates are shown several forms of lime- 

 kilns (pL 4, 5, 6). 



The lime obtained by the burning of limestone is a white, 

 amorphous, more or less dense mass, with a specific gravity of 

 3.09. It is infusible. Lime weighs from 1400 to 1800 pounds 

 to the cubic meter, the variation in weight depending on the 

 density of the original rock and the degree to which it has been, 

 burned. Denser stone gives a denser lime. 



Impurities. " Limestone containing silica and alumina should 

 not be burned at too high a temperature, because of the sintering^ 

 that takes place on the outside of the lum^os and thereby inter- 

 feres with the escape of the carbonic acid, yielding dead burnt 

 lime, which does not slake completely. 



It is said that dead burnt lime is more apt to be formed if 

 the impurities are evenly diffused in the stone. The ashes of the 

 fuel and also the alkalis in the stone may cause dead burning. 



The best limestone, if heated too quickly but not long enough, 

 may give dead burnt lime, in which case a basic calcium carbonate 

 is formed, which mth water forms a mixture of calcium carbonate 

 and calcium hydrate and hardens.^ 

 1 Frasch. Min. ind. 7 : 483. 



