THE BEHAVIOR AND MEASUREMENT OF HYGROSCOPIC EXPANSION OF DENTAL CASTING INVESTMENT

¹ School of Dentistry, University of Michigan, Ann Arbor, Mich.

The results of the controlled water addition experiment can be summarized as follows:

1. The hygroscopic expansion is directly related to the specific amount of water picked up by the investment.

2. Hygroscopically expanding investments have limiting capacities to pick up water and expand. This limit has been called the critical point and is affected by water:powder ratio, manipulative procedures, and the condition of the investment. Other variables which may affect the critical point, but which were not specifically investigated at this time, include setting time and the nature of the investment.

3. Although variations in spatulation, water:powder ratio, batch number, and age do not significantly affect the value of the expansion below the critical points established for these variables, a slight deviation exists for thick mixes.

The three technics of wet asbestos liner, complete immersion, and controlled water addition were investigated with respect to the reproducibility and isometric nature of their hygroscopic expansions. The results of this analysis indicate that accurate compensation by the established technics is an elusive entity and that a method of controlled water addition may serve as a basis for resolving this difficulty.