Retardation of Gypsum Plaster by Calcium Tartrate

Mixtures of laboratory plaster and artificial stone normally give properties intermediate between those of the two components and approximately in relation to the proportion of each. This was found not to be the case with one particular batch of a commercial dental plaster (Sample B). When this batch was added to a particular artificial stone, the mixture took an inordinately long time to set, several hours longer than did either component alone. Preliminary tests showed that the batch of plaster in question was unusual only insofar as a 10 per cent aqueous slurry gave a pH value of 11.4 compared with the normal 8.6 (Sample A). This alkalinity was found to be due to the presence of 0.56 per cent lime. Further, the anomalous setting occurred only when the batch was mixed with artificial stone containing tartrate as a modifier. As a further example, an experimental commercial mixture of equal quantities of an artificial stone containing tartrate and a dental plaster submitted for test (Sample C) gave a setting rate even slower than the previous mixture. It was found to contain 2.7 per cent lime, giving a pH value of 11.7. An investigation into the explanation for this phenomenon was carried out by examining the rates of setting, evolution of heat, and the change in the crystal habit.