The behavior and measurement of hygroscopic expansion of dental casting investment.

SINCE the discovery of the phenomenon of hygroscopic expansion of dental casting investments when subjected to an environment of water, there has been considerable work carried out to investigate its behavior. The ultimate goal is to develop a technic based on this property capable of compensating for the casting shrinkage of dental gold alloys. The result of these efforts has been the establishment of two technics which involve the introduction of water to the investment by a surrounding wet asbestos liner or by immersion of the entire inlay ring into a water bath. Results obtained with these technics have indicated that a successful casting is dependent on an almost overwhelming number of variables. Previous investigators have used various methods of measuring hygroscopic expansion which afford different results in accordance with the restriction offered to the expansion, as well as other less significant factors. Degni,1 Docking and Chong,2 Landgren and Peyton,3 and Delgado and Peyton4 have summarized, commented on, and investigated these methods. The general conclusion is that hygroscopic expansion is affected by the method of measurement and test when initial readings are made as soon as possible after mix. When measured from the initial set time, the expansion values of the various methods are more closely in agreement with each other. It has been felt that these latter values are more indicative of the actual enlargement of the mold cavity, since they are recorded when the investment is more rigid and can hold its shape. Accordingly, the term "effective" expansion has been applied. However, for lack of further, more conclusive, information, it is difficult to accept a starting point of such arbitrary nature. In addition, the methods for measurement of hygroscopic expansion designed to determine its compensating abilities as well as its behavior as influenced by these variable factors have been based on linear changes as measured in one direction. These measurements, as well, are dependent on factors whose effects are manifest in a significant variability of test results. As a result of these difficulties, a different approach to the problem was initiated. It was postulated that the amount of water taken up by the investment might be related to the subsequent expansion, and a procedure for adding a measured or controlled quantity of water was devised. Furthermore, since the previous methods of measurement have been based on changes occurring in only one direction, consideration was given to measuring the expansion in three directions simultaneously. It was felt that this measure would establish a more