The linear shrinkage during 24 h after setting of a number of proprietary condensation silicone dental impression materials was studied. Weight loss and change in volume were measured over the same time period by weighing similar specimens in air and water. The weight-loss data conformed to a linear t 1/2 plot, indicating that the loss of volatile material was diffusion controlled. From the diffusion coefficient so determined, linear shrinkage values predicted from the weight loss data were compared with the experimental results. For all except the highly-filled putty materials, the correspondence between theoretical and experimental data was reasonable; in the case of the putty materials, the shrinkage was very much greater than that predicted theoretically. This is possibly due to elastic memory effects consequent on moulding the putty materials.
