LIME AND CEMENT INDUSTRIES 661 



be the percentage of quarry moisture in it; while the looser or 

 more spongy it is, the more moisture will it absorb. Marl and 

 chalk may be looked on as the loosest forms of limestone, and 

 in them the moisture may reach 36$ or 40$. In marls and bi- 

 tuminous limestones the loss in burning will of course be much 

 greater than 44$, owing to the contained water and bitumen. 

 A dense limestone is much harder to burn than an open-textured 

 one, and requires more fuel, but this increased consumption is 

 more than made up for by the quality of the lime obtained. In 

 a clean, dense limestone the percentage of quicklime may be 54$ 

 while in an impure one it may amount to only 30$ or 35$. 



In addition to the decrease in weight in burning, the limestone 

 also decreases somewhat in volume, as much as 12$ to 21$, but 

 usually 16$ to 18$. x 



In burning it is important to observe that the temperature re- 

 mains as constant as possible and varies only between certain 

 limits; for, when limestone is overburned, the lime made from 

 it slakes slowly and incompletely. In lime rock with clayey im- 

 purities a sintering is very apt to occur and this should in all 

 cases be strictly avoided; but it is true that the higher the tem- 

 perature within the permissible limits the denser will be the lime. 

 On the other hand, the temperature must not get too low, as in 

 this case any large pieces of limestone that may be in the kiln 

 will not become thoroughly burned. The unburned core result- 

 ing from underburning makes the lime lean, and, to avoid such 

 an occurrence as far as possible, it is advisable not to put too 

 large pieces into the kiln. 



The quicker such lime is burnt at the highest temperature pos- 

 sible the more readily it slakes, and therefore a slow burning 

 process is disadvantageous. 



Many different types of lime rocks are available for the manu- 

 facture of lime, those only being excluded which are contami- 

 nated with clay; for this latter substance often affects their most 



important properties, and it is only since the beginning of this 



f- ■ ■ ■ 



1 Schoch, C. Die moderne aufbereilung und wertung der iLortel-mate- 

 rialien, p. 57. 



