The authors successfully developed a new investment material which consists of pure quartz powder and can be hardened without any bonding agent except mixing water. By controlling purity, grain shape and grain size distribution of this silica powder, it can be easily spatulated with mixing water, poured into a casting ring and then hardened or coagulated. This new investment material has many excellent characteristics which arc: high temperature stability, easy removal of investment from the casting, possibility of re-use of the investment, etc. In this study, grain size distribution, X-ray diffraction, setting time, setting shrinkage, thermal expansion, and compressive strength of the new investment were determined, and some casting experiments were performed with it. Judging from the results of many investigations, the best grain size distribution was 0-10 µ 38.1%. 0-40 µ 80.1%, 0-50 µ 85.5%, 0-75 µ 100%. Purification procedure was heating with sodium meta-phosphate, washing by dilute hydrochloric acid, and decantation with a large amount of water. The grain shape was made nearly spherical by dry and wet milling treatment for over 100 hours. The normal W/P ratio with distilled water was 0.30 and the compressive strengths were 6.0 kg/cm2 and 4.6 kg/cm2 the former was the green strength and the latter the fired strength. When invested by a newly developed vacuum investing method, this value increased to 7.6 kg/cm2. By adding colloidal silica and phosphoric acid to the mixing water, the investment flour can be spatulated more easily without losing the high temperature stability and the W/ P ratio can be decreased to obtain a higher compressive strength. In conclusion, not only in the case of high fusing casting but also of medium fusing casting, this new investment material showed numerous superior characteristics and was tested for practical use.
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