Brery.| Manufacture of Plasters. 167 
Retarders and Accelerators. 
Although much has been written in regard to the action of 
retarders, which are frequently used, especially in the purer 
grades of plaster, to defer the setting, the theory of their use 
seems to be asimple one. If, as has been assumed, the plaster 
sets because there are really a few small crystals that have not 
been dehydrated, and which ‘‘set the example,’’ so to speak, 
to the other crystals to form, and if the strength of the set ma- 
terial is due to the mass of interlacing crystals, then any sub- 
stance added to the water with which the calcined plaster is 
mixed, or to the dry plaster, which will keep the molecules 
apart, or from too close contact, will retard the setting. Such 
substances are dirt, or organic matter that is not of a crystalline 
character. Animal refuse, glue water, starch or bodies of the 
class known as ‘‘colloids’’ are thus used for this purpose, and 
wisely. Too much insoluble material or carbonates of the alka- 
line earths will of course dilute the plaster so that it will not 
set as quickly as a purer sample, and when it does set it will 
not be as strong, as there is not so much of it in a cubic foot; 
that is, there is not so much of a mass of interlacing crystals of 
hydrated calcium sulphate in a cubic foot asif it be pure. Itis 
probable also that the use of organic retarders beyond a cer- 
tain low limit will seriously reduce the strength of the set plas- 
wees 
On the other hand, it is a well known fact that a supersaturated 
solution of a salt, as for instance sodium sulphate, can be made, 
and immediately when a crystal of the salt is dropped into this 
solution the whole mass will crystallize. Crystals, or even 
solid particles of foreign material, will hasten the crystalliza- 
tion of the material. Apply the same reasoning to the setting 
of plaster, and in the light of known facts it is easy to see what 
will accelerate the setting. To make a good dental plaster, for in- 
stance, some other crystallizing salt, as alum or borax, is added, 
in small quantity to the plaster. The result of this is that after 
solution the added salt has a tendency to crystallize, and in so 
doing starts that process in the dehydrated gypsum. It is even 
