An investigation into the compatibility of some irreversible hydrocolloid impression materials and dental gypsum products. Part I. Capacity to record grooves on the international standard die.

Some alginate impression materials produce rough, chalky, soft surfaces on gypsum casts. This is recognized by various standards, which include tests for an alginate's compatibility with gypsum, and its capacity to record details and impart these to a gypsum cast. These qualities were looked at in this study. The method used closely followed that laid down by International Standard ISO 1563-1978(E) for alginate impression materials. Ten alginates and seven gypsum products were tested. Using the criteria specified by this Standard, it was found that none of the materials tested was able to record the finest (0.020 mm) line on the test block. However, two elastomeric impression materials tested in the same manner also failed to reproduce this line, as did all the gypsum products when tested directly against the block. These findings cast some doubt on the efficacy of utilising such a fine line in the classification of these materials. Only thirty-four of the seventy possible combinations were able to reproduce the 0.050 mm line. Eight combinations reproduced the 0.075 mm line as the finest line, and the remainder (thirty-eight) failed to record any of the lines. Some of the alginate materials were treated with fixing solutions of varying concentrations of potassium sulphate and alum. The results obtained varied, sometimes producing marked improvement, sometimes deterioration in the surface quality of the subsequent gypsum casts, despite the fact that none of the manufacturers of the alginates recommended the use of fixing solutions.