Measurement and Reinforcement of the Superficial Hardness of Indirect Stone Models

¹ Department of Operative Dentistry, Tokyo Medical and Dental University, Tokyo, Japan

The superficial hardness of the stone models was determined by a scratch hardness tester, newly devised, and compared with the results from the Knoop hardness test. The effect of a newly devised hardening agent for stone models was investigated by using this scratch test. The following results were obtained. This new tester is considered to be more suitable than the testers so far reported to detect the change in the superficial hardness of indirect stone models, which resist careless injuries by a carving knife or by other means. The newly devised hardening agent (10 per cent polystyrene solution in amyl acetate) could increase the superficial hardness of stone models poured in irreversible hydrocolloid impressions by more than 3 times, 30 minutes after application. Application of the hardening agent was most effective when applied more than 3 hours after removal of the models from the impressions, although the increase in hardness was nearly twice as much, even when applied immediately. Application of this hardening agent could fully compensate for a decrease in hardness of stone surfaces due to the influence of the irreversible hydrocolloid, and the stone surfaces thus treated were harder than the stone surfaces that set in contact with the glass surface.