The purpose of this study was to explore the possibility of reusing for rapid burnout type gypsum-bonded investment materials which are frequently used in dentistry. The study added burned out investment materials into new investment at a rate of up to 80% and examined the physical and mechanical properties, x-ray diffraction analysis and fitness of castings made by recycled investment materials and investment material particle morphologies by SEM. The following results were obtained: 1. The strength of the trial reused investments satisfied the specification requirement for gypsum-bonded investment when the burned out investment materials were added at a rate of up to 20%. 2. X-ray diffraction analysis detected calcium sulfate anhydrate after the burning-out of investment materials, but not before their burning-out. 3. Based on the present fitness study, the recycled investment materials which contained burned out investment materials at a rate of up to 30% could be utilized in dental laboratory procedures.

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This study examined the setting and thermal expansion of 8 commercial quick-heating, gypsum-bonded investments. The results were as follows. 1. When measurements were made following the manufacturer's instructions, setting expansion was 0.11% to 0.64%, thermal expansion was 0.72% to 1.22% and total expansion was 1.30% to 1.66%. 2. When the setting expansion was measured for 120 minutes, it increased by 0.24 to 1.21%, and thermal expansion decreasaed by 0.08 to 0.26% compared to measurements obtained following the manufacturer's instructions. Although the total expansion of one investment was lower, the total expansion of all of the others increased by 0.20 to 0.98% compared to that obtained by following the manufacturer's instructions. 3. When the specimens were stored at a relative humidity of 100%, in a room, and in a desiccator, thermal expansions were lower than those obtained by following the manufacturer's instructions in all cases. The rates of decrease in specimens stored at 100% humidity, in a room, and in a desiccator, were 0.02 to 0.21%, 0.11 to 0.23% and 0.15 to 0.38%, respectively.

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In this study, Ag-Au-Cu-Pd system alloys consisting of 5 or 10 mass% indium were experimentally developed. The mechanical properties in terms of tensile strength, proof stress, elongation, and Vickers hardness were evaluated. The corrosion resistance was evaluated by the amount of released element and the potentiodynamic polarization profiles. Tarnish tests in 0.1 mol/L sodium sulfide were also carried out. The results were compared with commercial silver-palladium-gold alloy and examined comparatively. The mechanical properties of these alloys were as follows: proof stress; 427〜552 MPa, tensile strength; 537〜673 MPa, elongation; 5.1〜8.6%, Vickers hardness; 170〜209. The transpassive potentials of these experimental alloys were 168〜248 mV, and the amounts of released element were 14〜130 μg/cm²・7 days. The addition of indium to Ag-Au-Cu-10 mass%Pd system alloys effectively improves the tensile strength and tarnish resistance. These findings imply that the 37.5 Ag-25 Au-15 Cu-10 Pd-2 Zn-10 In-0.5 Ir alloy is applicable in dental practice.

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This study examined the effect of the transparency of experimental cements on the color of indirect resin composite. Experimental resin cements with five different transparencies were fabricated using 0, 0.05, 0.10, 0.15 and 0.20 mass% TiO₂ powder. Disk specimens (14 mm dia.) were prepared from indirect resin composite [1 mm thick, CERAMAGE, Shofu, (incisal and body color)] and from two different-colored (lighter-and darker-colored) resin substrates (2 mm thick, DIE Color Checker, Shofu). A layer of cement (100 μm thick) was placed between the overlaid resin and the substrate. Color differences (ΔE*ab, n=3) between the specimens with and without cement layers were determined using a spectrocolorimeter. The results showed that there was little change in the transparency of the cement when the colors of the overlaid resin and the substrate resin were both lighter or both darker. However, when a darker substrate and the incisal-colored overlaid resin were used, a decrease in the transparency levels of the cement layer appeared to influence the color of the overlaid resin. The color of the overlaid resin material was influenced not only by the transparency of the cement layer but also by the color of the substrate and overlaid resin materials.

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