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Shortly after his discovery of the phenomenon of hygroscopic expansion, Scheul found that the exertion of force on the setting investment reduced the hygroscopic expansion. Skinner2 had found that force reduced normal setting expansion. The effect of restrictive forces on hygroscopic expansion has been considered by many investigators and emphasized especially by Docking3 and by Skinner and Degni. 4 We attempted to measure the effect of horizontal longitudinal force on hygroscopic expansion and setting expansion of one gypsum investment in a direction against this force. Measurements were also made on the vertical expansion in a direction perpendicular to the force. The effect of longitudinal force on the rate of expansion in the early stages was also measured. An attempt to determine the hygroscopic expansion under vacuum was not successful. MATERIALS AND METHODSThe investment used contained hydrocal 35 per cent, cristobalite 10 per cent, silica 55 per cent, and chemical additives none. The investment was used at a WIP 32/100; it was spatulated mechanically 30 seconds at 650 r.p.m.; the hygroscopic water was added 5 minutes after mix; and the Vicat setting time was 11 minutes.The apparatus used for testing hygroscopic and setting expansion is simple to operate and has been used in the control laboratory for many years, having been originally designed by Mr. T. E. Moore. It consisted of a glass plate with one fixed metal stop for confining the specimen at one end and a moveable metal slide contacting the dial gauge which confined the investment at the other end. The sides were confined by glass plates. The specimen was 100 mm. long and 1 square inch in cross-section. The dial gauge was calibrated in millimeters, so that the dial read directly in percentage. For regular setting expansion tests, the area was lined with tin foil or a rubber dam material; and for hygroscopic expansion, it was lined on the bottom and two sides with wet asbestos sheet.Water may be added to the surface from burettes (Fig. 2). The unweighted equipment was used for a control test on each mix of investment.Force was applied on the setting investment by placing a cord around the movable stop and passing it over the rests and attaching a container to the looped cord. Various weights were placed in the container, producing longitudinal force on the setting investment. There was no confinement in the vertical direction; and in the transverse direction, the confinement was limited to the small glass slabs.

It is probably essential for the production of sound dental castings to provide in some way for the elimination of the gas in the mold cavity during the casting procedure. The gas permeability of investments should be adequate to allow the gas to pass through the hot investment at the rate that the metal enters the mold. Some measurements of gas permeability have been made, but these tests have been performed on cold investment after the mold had been burned out and cooled to room temperature. Such tests do not provide data pertinent to the actual conditions present when a casting is made. Investments shrink when cooled to room temperature after being burned out, and it is highly probable that this has a decided effect on the permeability. Numerous cracks may form during the cooling and may drastically affect the permeability. It is also possible that the shrinkage causes an increase in density of the investment which might also affect the permeability. EXPERIMENTAL METHODSThe permeability of investments at the casting temperature employing pressures in the range of those encountered in dental casting was measured. The equipment is illustrated in Figure 1. Dry air at a slight pressure (5 cm. Hg) was passed through the monel tube and the investment sample during the burn-out to prevent water and wax vapor from collecting in the cold end of the monel tube. The readings were made by passing air, at a controlled pressure, through the train, which includes the flowrator, a monel tube with adapter (Fig. 2), and the investment sample in the furnace. The investment and the adapter (Fig. 3), were heated in the furnace employing any desired burn-out cycle. The readings were made at the time and temperature at which the casting would normally be made. The compressed air was dried by first passing through a desiccator containing an anhydrous calcium sulfate.t The pressure was held constant by a pressure regulator and was indicated by the mercury manometer. The flow of air was measured by the flowratorl (Fig. 4), containing a floating ball in a tapered glass tube.The flowrator was calibrated by removing the investment specimen and substituting a needle valve. The valve was adjusted to give desired readings on the flowrator, and the gas was collected in a burette. The flow of gas in cubic centimeters per minute at

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