Freeze-drying and scanning electron microscopy of setting dental gypsum

The initial and final forms of reactive gypsum products have been photomicrographed previously. However, the purpose of this project was to document the microscopic morphology of setting dental stone at various stages during the reaction.

Two dental products, a conventional (Type IV) die stone and a fast-setting (Type III) stone, were investigated. At selected times ranging from 1 min to 24 h after mechanically mixing the stone under vacuum, the conversion of calcium sulfate hemihydrate to a dihydrate was suspended by immersion into liquid nitrogen. Water was immediately removed by freeze-drying the specimen to prevent any further reaction so that the specimen could be returned to room temperature for examination in a scanning electron microscope (SEM).

Crystal formation appeared to be nearly complete at the 20 min interval for the die stone and at the 10 min interval for the fast-setting dental stone. Transitions noted during these times include the nucleation and growth of small needle-like crystals on and near the larger prismatic-shaped hemihydrate crystals, the concurrent decrease in size and number of the hemihydrate crystals, and the progressive entanglement of the growing dihydrate crystals.

The two-step process of suspending the reaction, then freeze-drying the specimen made it possible to observe and document the intermediate stages during crystal growth of dental stone. These observations should be helpful in understanding the structural dynamics of crystal growth during the setting of gypsum dental products. This procedure should be applicable to the study of other water-based dental materials.