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It is well known that the dimensional change occurring in an impression material is an important factor affecting the fitting accuracy of the final restoration. However, no method of directly measuring the dimensional changes in impression materials, from immediately after initial setting, with accuracy and ease has yet been firmly established. In the direct method which necessitates measurement pressure, force may be applied to impression materials which have great plasticity immediately after initial setting, and a flow may thus be created producing dimensional changes. The indirect method is more advantageous in this respect, but its measurement with the passage of time is complicated, and it is difficult to apply under clinical conditions.In the present report, a method divised by the authors for measuring changes in dimension directly, without the application of pressure, is described, and will hereinafter be referred to as the "non-pressure direct method." The method was used to measure dimensional changes in powdered alginate impression materials available on the market at 100% relative humidity and in water. Materials and MethodsThe materials used are shown in Table 1. The last letter "d" in each code means "dust -free ." Although not mentioned in the Table, VA treated with an anti-dust agent and VA with a partially changed filler were prepared. Each material was mixed with water according to the manufacturer's instructions. As for the materials for which no instruction regarding the water-to-powder ratio (g/ml) was given, each powder was weighed with a prepared measuring cup to determine its ratio.Using the pieces of equipment shown in Fig. 1, a 15-mm-tall discshaped specimen with a diameter of 30 mm was made. A steel net frame with a diameter slightly smaller than that of the specimen (Fig. 1-B) was attached to a stainless steel plate with a small piece of magnetic material buried in the center (Fig. 1-A). Over this was placed a ring of a prescribed size (Fig. 1-C), and this ring was then filled with