M. L. Troxell — Plaster of Paris. 199 



a briquet withstood a force of 619 pounds before breaking. 

 The average for the pure plaster briquets, 72 in number, was 

 a fraction over 400 lbs. 



An important factor contributing to the strength of these 

 briquets is the ease with which pure plaster mixes with pure 

 water, allowing a minimum of water and thus producing a more 

 dense substance. Foreign ingredients in the plaster such as 

 coloring matter, or in the water as glue or gum arabic, hinder 

 very greatly the mixing, and demand a greater amount of 

 water. Briquets, in which a minimum of water was used, 

 which were made of plaster so stiff that it could support its 

 own weight and which had to be packed into the molds, aver- 

 aged at least a hundred pounds greater strength than those 

 in which an excess of water was used. This is an unfortunate 

 condition since it is essential in casting that the mixture be 

 liquid enough to pour. 



An analogy has been pointed out between anhydrite and 

 plaster mixed with a minimum of water and between gypsum 

 and that mixed with an excess of water. In the one case they 

 are more nearly anhydrous, are stronger, and perhaps more lia- 

 ble to alteration, passing over toward the other extreme of cal- 

 cium sulphate compounds represented by gypsum, which in 

 turn is not so strong but is known to be more stable. This 

 analogy may be only fanciful. The chief reason for the weak- 

 ness of the excess water plaster is due to its additional poros- 

 ity. The space occupied by the additional water is left vacant 

 when the plaster dries out. 



Plaster of paris is most soluble at a temperature of 80° C. 

 It is also found to mix more readily with water at about this 

 temperature, therefore it is possible to use a smaller quantity 

 of hot water, thus producing a more dense mixture. This was 

 shown experimentally by tests which gave an average of over 

 sixty pounds per square inch more for the plaster mixed with 

 hot water. 



Only a temporary weakening is noticeable as a result of soak- 

 ing briquets in water. The strength is decreased by at least 

 two hundred pounds if they be soaked in water and pulled 

 while wet. On the other hand, briquets when dried thoroughly 

 after this treatment seemed to give as high or a higher test 

 than others made under the same conditions but not again 

 soaked in water. In one experiment on twelve briquets, those 

 of even number, put in water five and one half hours and then 

 dried, averaged nine pounds per square inch stronger than the 

 odd ones kept dry. In another case four briquets soaked and 

 redried averaged each thirty pounds greater strength than four 

 dry ones. However, in still another case four soaked briquets 

 gave tests averaging nineteen pounds less than those which were 

 not treated. 



