FREEZE-DRYING AND SCANNING ELECTRON-MICROSCOPY OF SETTING DENTAL GYPSUM

Objectives. The initial and final forms of reactive gypsum products ha ve been photomicrographed previously. However, the purpose of this pro ject was to document the microscopic morphology of setting dental ston e at various stages during the reaction. Methods. Two dental products, a conventional (Type IV) die stone and a fast-setting (Type III) ston e, were investigated. At selected times ranging from 1 min to 24 h aft er mechanically mixing the stone under vacuum, the conversion of calci um sulfate hemihydrate to a dihydrate was suspended by immersion into liquid nitrogen. Water was immediately removed by freeze-drying the sp ecimen to prevent any further reaction so that the specimen could be r eturned to room temperature for examination in a scanning electron mic roscope (SEM). Results. Crystal formation appeared to be nearly comple te 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 tim es include the nucleation and growth of small needle-like crystals on and near the larger prismatic-shaped hemihydrate crystals, the concurr ent decrease in size and number of the hemihydrate crystals, and the p rogressive entanglement of the growing dihydrate crystals. Significanc e. The two-step process of suspending the reaction, then freeze-drying the specimen made it possible to observe and document the intermediat e stages during crystal growth of dental stone. These observations sho uld be helpful in understanding the structural dynamics of crystal gro wth during the setting of gypsum dental products. This procedure shoul d be applicable to the study of other water-based dental materials.