200 K L. Troocell— Plaster of Paris. 



Petempering. 



After plaster or cement has started to set and before the set- 

 ting is final it may be mixed again, a process called " retem- 

 pering." To a certain extent this can be done without lessening 

 the strength of the material. In plaster of paris there is really 

 no true initial set, since the process is a continuous one vary- 

 ing only in the degree to which the chemical action has taken 

 place. Even though the crystals are well started in the pro- 

 cess of formation, they may be broken apart and disturbed 

 without harm provided there is enough liquid plaster left to 

 cement the fragments together. Plaster once started to set 

 hardens very rapidly after being disturbed, because each crys- 

 tal already formed furnishes a nucleus around which the mass 

 rapidly accumulates. A quick-setting plaster may therefore 

 be made by mixing in the dust ground from a piece of set 

 plaster, thus suppling the nucleii. 



Experimentally retempering gave the following results : 

 Plaster, remixed after standing forty minutes, showed a higher 

 tensile strength than that which had stood a period of ten 

 minutes. Neither was quite as strong as that which was retem- 

 pered after twenty minutes. Forty minutes was about the 

 limit, for at fifty minutes the mixture set so rapidly, after 

 being disturbed, that it became hard and brittle before- it could 

 be put into the molds. It apparently matters little how long 

 the plaster has stood provided it is soft enough to permit of 

 remixing. 



Retempering finds its practical application in the use of 

 plaster as a cement where it is to be used little by little. A 

 mixture of gum arabic and plaster may be useful, roughly, for 

 a half hour, but if glue be added in a large proportion it may 

 stand for days and still be available for use. In this case it is 

 probable that the plaster does not function as an adhesive, but 

 merely as a filler forming a body for the glue. 



Colored Plaster. 

 Effect of Coloring. 



Briquets made up partly of coloring material are necessarily 

 not so strong as those of pure plaster, for the adhesive and 

 cohesive properties of the pigment are not so good, and further 

 the small foreign particles interfere with the growth of the in- 

 terlocking crystals of the plaster. 



The experiments to determine the variation of the strength 

 with the amount of coloring matter gave two curves (fig. 1), 

 one for raw sienna and the other for bone black. These curves, 

 though irregular, show a certain similarity of trend and in 



