The original method for molding the briquettes was worked out in 

 the laboratory of the Massachusetts Highway (V)niinission and differs 

 somewhat from the method as described above. In the earlier prac- 

 tice the requisite amount of rock dust to make a briquette was weighed 

 out while dry, mixed with 3 to 4 cubic centimeters of water, and the 

 briquette immediately molded from the wet dust. It is well known 

 to practical road builders that the binding power of many -rocks 

 increases under the combined influence of water and traffic as time 

 goes on. This question has received a great deal of attention and 

 investigation in this laboratory. Experiments have shown that the 

 cementing value is increased if the dough made from a rock dust is 

 allowed to stand for some time before being molded, and it is still more 

 increased if the dough is kneaded. This is plainly seen in the results 

 obtained on a sample of dolomite. This dolomite is a celebrated road- 

 building material in the locality in which it is found, and its most 

 marked characteristic is the way that its binding power increa-< - 

 after it has been on the road for some time. A series of experiments 

 was conducted, bearing on this point, in which the rock dust was made 

 to a stiff dough and briquettes molded at stated intervals. A new lot 

 of dust was made up and well kneaded by hand for one hour. The 

 results were as follows: 



Cementing 

 value. 



Dough tested at once 16 



4 hours old 50 



LM hours old 81 



7'2 hours old 7!) 



'.'(I hours old 77 



120 hours old 79 



144 hours old 83 



8 days old 81 



Kneaded 1 hour 190 



An inspection of this table clearly shows the importance of conduct- 

 ing the operations of this test under uniform conditions. In mixing 

 the dust with water to form the dough, the operator should be trained 

 to knead each specimen a> nearly as possible in the >ame manner for 

 the same length of time. After twenty-four hours standing in a 

 < ln-ed jar a cementing value is developed which remains fairly con- 

 -tant. When carried out in a uniform manner by a skilled operator 

 this te-t has no higher percentage of variation on homogeneous sam- 

 ple* than i> found in testing the tensile strength of cement briquettes. 

 Road materials vary widely in this important property, but the test is 

 undoubtedly of the highest comparative value and practicability, for 

 it distinguishes between excellent, good, fair, and poor binding mate- 

 rials. It should be remembered that from the very nature of the 

 materials to be tested it is impossible to expect a degree of accuracy 

 such as is obtained in the testing of perfectly homogeneous substances. 



