Investigation on the compressive strength of several gypsum products dried by microwave oven with different programs.

Different programs in a microwave oven were selected to dry several gypsum products. A type III dental stone (Moldano), a type IV high-strength dental stone (Glastone) and a type III partial denture casting investment (Multi-vest) were selected. Specimens of the products were dried by microwave radiation set at the lowest and highest power levels for 5 and 15 minutes. The dried specimens were tested at intervals of 2, 4, 24, and 48 hours for compressive strength. Specimens dried in ambient room air were used as the control group. A statistical analysis of the test results was performed with Student's t-test at p < or = 0.05 level. Microwave radiation at the highest power level resulted in a decrease in the compressive strength of type IV dental stone (Glastone), whereas the values of the other gypsum products did not differ at any time interval from values attained for the air-dried specimens. The 2-hour low power test of the investment (Multi-vest) had a higher compressive strength than the air-dried specimens after 24 hours.