The volumetric contraction of dental gypsum materials on setting.

O F ALL the materials associated with dental practice and technic, those containing gypsum products are probably used most extensively. In keeping with this fact, a great deal of research has been directed toward problems involving the physical behavior of these materials. It is felt, however, that knowledge is lacking on some of the more fundamental behavior characteristics. The manifestation of this deficiency is the presence of many phenomena which exist when dental gypsums are used. An illustrative example would be the hygroscopic expansion of dental casting investments containing gypsum as a binder. Since there is no established theory on the mechanism of this action, it is just as difficult for the dental manufacturer to compound an adequate material as it is for the dentist to use it properly. The purpose of -this paper is to resolve some of these difficulties by studying one very fundamental characteristic of these materials, namely, the volumetric behavior of dental gypsums during the process of setting. All dental gypsum products which include plaster, stone or hydrocal, the improved stones, and the binder of most gold casting investments are essentially composed of the hemihydrate form of calcium sulfate (CaSO41/1⁄2H2O). When mixed with water, the hemihydrate is converted to the dehydrate form of calcium sulfate (CaSO,-2H2O) and this process is synonymous with the observed setting action. Specifically associated with this reaction is the paradox which exists with respect to the volumetric behavior of the mixed mass. It is common knowledge that an expansion accompanies the setting process; yet, a contraction should occur based on theoretical considerations as is shown in Table I. Skinner' wrote of this anomaly and explained that the expansion is only apparent and is possibly caused by the thrusting of the gypsum crystals in the direction of their growth during setting. It is the specific purpose of this paper to determine whether a volumetric contraction does exist and, if so, how it can be explained in the presence of a measurable expansion. A thorough search of the dental and chemical literature was of no assistance except for one article by J. H. Van't Hoff2in which the author measured a contraction of plaster of Paris during setting. However, the method of measurement as well as the objectives of the experiment precluded any adaptation of the results to the problem at hand. Accordingly, an apparatus was designed and tests were run to measure the true volumetric behavior of dental gypsum materials on setting.