[Correct timing of work on a hard plaster of Paris model].

Of the individuals in Hamburg questioned, 48% waited an average of 1 to 2 hours after making a stone cast before proceeding with work on it; 38%, 1 day; and only 2%, more than 1 day. The material test however showed that 5 or 6 days of storage at room temperature are necessary before optimal values are obtained for compressive strength, ball-thrust hardness, and abrasive hardness. The greatest amount of expansion with type IV stone plaster, removed from the cast after 30 minutes, was measured between 8 and 24 hours at room temperature, after the plaster had been mixed. The least changes in dimension were recorded after a 5-day contractive phase. Given these results, the stone cast should be stored for 5 to 6 days at room temperature before work is resumed on it if precision work is to be done.