REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1903 135 



would start the process of crystallization. Their growth would 

 cause the turbidity of the solution noted by Marignac, and would 

 result in a precipitation of small gypsum crystals, thus forming 

 a crystal net-work which constitutes the set of plaster. 



If the plaster is underburned the gypsum is not reduced to its 

 proper fineness and uniformity, and so would not permit the 

 crystallization to go on in the way it would in the properly 

 burned plaster. But of more importance the hydrate represented 

 by plaster of paris w^ould not ibe formed. 



If the plaster is overburned, the plaster will be so completely 

 comminuted that no minute crystals will be left to start the 

 crystallization. Where the plaster is slightly overburned, the 

 crystals are extremely fine and crystallization goes on very 

 slowly and imperfectly. 



Retarders. The question of retarding the set of plaster of paris 

 is one that has received a great deal of attention in the past few 

 years, and the substances which are used are almost too numer- 

 ous to mention. The principal materials used are different forms 

 of organic matter, as glue, starch and animal refuse. These when 

 moistened form a gelatinous liquid, which hinders the formation 

 of the crystals. 



The carbonates of the alkaline earths are sometimes used to 

 dilute the plaster. Slaked lime or calcium hydrate is used to 

 make a lime plaster putty, which sets quite slowly and is used 

 largely in the manufacture of relief forms for ornamentation. 



As a usual thing, the addition of any material w^hich is not a 

 stronger cementing material than the plaster of paris will have 

 a tendency to render the hardened plaster weaker than it would 

 be in the pure state. It is possible, however, that even the addi- 

 tion of material that may be considered a diluent may give hard- 

 ened plaster that will be fully as strong as the pure material 

 would be, if not stronger. Experiments have not been made with 

 a view of ascertaining the truth of this, but the ultimate strength 

 of the hardened plaster seems to be due to the filling of all 

 crevices in the material and the addition of a material which 

 helps to fill the crevices should have this effect. 



It is known that in the manufacture of Portland cement, equal 

 portions of cement and sand can be ground together, and the 



