Delayed setting and hygroscopic linear expansion of three gypsum products used for cast articulation.

Statement Of Problem: Wetting of the set gypsum product used for cast articulation may result in additional expansion.

Purpose: The purpose of this study was to evaluate the linear expansion of 2 type II and 1 type III gypsum products with and without addition of water after setting, over a time period of 120 hours.

Material And Methods: The gypsum products tested in this study were Model Plaster, Lab Plaster, and Mounting Stone. All materials were hand mixed for 5 seconds and mechanically mixed under vacuum for another 15 seconds. The mixes were poured into a linear expansion-measuring instrument (n=20). Half of the specimens were wet with 25 ml of water immediately after the setting time recommended by the manufacturer, and at all time intervals, immediately after data collection. No treatment was performed on the remainder of the specimens. The expansion values were recorded with an expansion-measuring device over a period of 120 hours. Collected data were subjected to a 2-way repeated-measures analysis of variance (ANOVA) (alpha=.05).

Results: The highest expansion values for all gypsum products were recorded at 96 and 120 hours. No significant difference in setting expansion values was recorded between these 2 time intervals for any of the materials and treatment conditions (dry vs. wet). Mean expansion values ranged between 0.08% +/-0.06% for dry Mounting Stone specimens at 15 minutes, to 0.57% +/- 0.02% for wet Lab Plaster specimens at 96 and 120 hours. Significant differences (P<.001) between different mounting gypsum products and time were identified. A significant difference between wet and dry specimens was not observed.

Conclusions: The expansion of all gypsum products used for articulation purposes was complete at 96 hours. The type III gypsum product demonstrated lower mean expansion values than type II products. Time and material were more important factors than the dry/wet condition when measuring total expansion values. Type II gypsum products demonstrated 80% or more of the total expansion values in the first 45 minutes, while type III stone demonstrated the same expansion after 24 hours.