MANUFACTURE OF NATURAL CEMENTS. 235 



The following seems to explain the difference in practice satisfac- 

 torily. It has been noted above that the lower the Cementation 

 Index the higher the temperature necessary to secure perfect combina- 

 tion of the lime and clay. The Rosendale rock, where the Cementa- 

 tion Index averages around 1.50, will therefore be properly burned 

 at a much lower temerature than the Akron rock, where the index falls 

 below 1.10. But a low temperature means low fuel costs, and so there 

 is not the temptation to economize on fuel in burning a rock of high 

 index that there is in burning one of low index. When rocks of high 

 index are burned, therefore, the temperature and the fuel supply will 

 often be allowed to exceed their proper amount, resulting in the pro- 

 duction of clinkered material which has been burned higher than was 

 necessary and is therefore inferior. But in burning a rock of low index, 

 it will be necessary to almost clinker the material before perfect com- 

 bination is secured. Because of the natural tendency to economize 

 on fuel, the temperature will usually be carried a little lower than is 

 necessary. The result is that in burning rocks of low index the 

 clinkered material is correctly burned and gives the best cement, while 

 the softer masses are really underburned and therefore inferior. 



The respective values of the hard and soft parts of the product 

 will therefore depend on the Cementation Index. The following seems 

 a fairly satisfactory statement of the case. 



(a) When burning a product having an index lower than 1.10, the 

 clinkered portions will furnish the best cement. If separated and 

 ground to proper fineness, they will give a cement approximating to 

 Portland in its physical properties. 



(U) When burning a product where the index is between 1.10 and 

 1.25, the clinkered portions and the softer masses will probably be of 

 almost equal value. 



(c) When burning a product where the index is above 1.25, the 

 clinkered portions may be rejected as worthless; and when the index 

 rises above 1.60, even some of the softer masses may be overburned, 

 unless the temperature be kept sufficiently low. 



Seasoning and slaking. It is obvious that in burning a natural 

 rock in a kiln whose temperature cannot be controlled closely, there 

 are opportunities for great differences between the degree of burning 

 which the rock should have received and the degree which it actually 

 does receive. When the Cementation Index of the product is very 

 high over 1.60 for example even a light burning will suffice to decar- 

 bonate all of the limestone and to secure combination of the lime with 

 the clayey matter. This is especially true if the silica is comparatively 



