Setting Expansion of Plaster of Paris: The Initial Contraction

¹ Department of Oral Rehabilitation, Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel

The early dimensional changes on setting (ambient temperature, 22°C., and relative humidity, 43 percent) of one gypsum product was studied under different conditions of restraint. Restraint was varied by allowing the material to set against tinfoil, plastic film, and mercury. Further variation was produced by altering the conditions of the normally free or exposed surface. This surface was (1) exposed to the air (open setting), (2) sealed with tinfoil or plastic film (closed setting), or (3) water was added (hygroscopic setting).

Specimens of uniform composition and manipulation, of the same length (200 mm.), were prepared in a horizontal hemicylindrical glass trough (lined with tinfoil or plastic film) and on a mercury bath.

Contraction and expansion (normal setting expansion, open and closed) or only expansion (hygroscopic expansion) was recorded soon after mixing; both methods (a piston sliding within the trough or a scribed marker in the plaster moving on the mercury) responded to the linear dimensional changes of the setting slurry.

External restraint on the setting material altered the magnitude of the initial contraction linearly but did not alter the time at which the contraction reached a maximum (or setting expansion began), provided that ambient temperature and humidity did not vary. This time of maximum contraction, with small variations, appeared independent of the method of measurement. The time of the externally observed loss of gloss, initial vicat needle setting time, and the onset of the initial contraction in the lined trough occurred at the same time.

During a period comparable to the initial contraction, only a linear expansion was observed when additional water was available to the setting mass against mercury, plastic film, and tinfoil, but the onset and rate of the early linear expansion varied. Mercury offered the least restraint, followed by plastic film and tinfoil, respectively.