CALCULATION AND CONTROL OF THE MIX. 395 



Methods of control. The chemist having determined the standard 

 of composition which he wishes to maintain in the mix, several different 

 methods of maintaining this standard are possible. Theoretically, of 

 course, the best of these methods is: 



(1) Both raw materials are analyzed as they arrive at the mill; 

 the mix is made according to these analyses; after grinding the mix 

 is analyzed as a check, and if seriously incorrect is corrected by the 

 addition of the necessary ingredients. This method is actually prac- 

 ticed at some plants, but in general one or the other of its two elements 

 is gradually dropped out, so that most plants approach one of the two 

 following extremes in practice. 



(2) The raw materials are analyzed, either by borings in the quarry 

 or by an arrival at the mill, and the mix made in accordance with these 

 analyses. The mix may be analyzed occasionally as a check, but no 

 serious attempt is made to correct it. In this method the entire reli- 

 ance is placed on the analyses of the raw materials. With hard, dry, 

 raw materials varying little in composition the plan works well. In 

 dealing with marls, etc., the third plan is most used. 



(3) The raw materials are mixed without analysis in approximately 

 correct proportions, according to previous experience, and the ground 

 mix is analyzed and brought up to proper composition (standardized) by 

 the addition of whichever raw material proves to be deficient. In this 

 method the correction of the mix is a regular part of the procedure. For 

 convenience the mix is usually made always a little low in the same con- 

 stituent, so that only one tank or bin of raw material needs to be kept on 

 hand for standardizing. The following blank order shows how this is 

 arranged in actual practice under the chemist's direction: 



CLAY ORDER. 



Date 



Tank No requires one hopper 



of clay for each 



inches of marl. 



Slurry tank No - ... 



Changes in Composition During Manufacture. 



In theory the cement produced should correspond in composition 

 to the mixture from which it is made. In practice it is found that, in 



