Objectives, Some of dental casting alloys such as Type 4 gold alloy and silver-based alloys frequently show blackening of their surfaces due to the heating of the alloy during the casting process. The purpose of this study is to develop a gypsum-bonded investment which can prevent the blackening of these castings using a deoxidizer. Methods, Either aluminum particles (0%, 5%, 10%, 15%, 20%) or boron particles (0%, 0.06%, 0.125%, 0.25%, 0.5%) was added to a commercially available cristobalite investment (Newcrivest, Mutsumi plaster industries, Japan) as a deoxidizer. Using these experimental investments, a casting mold was prepared. A Type 4 gold alloy (Casting gold M.C. Type 4, GC, Japan) was cast into the mold and a cast specimen (25x25x1 mm) was prepared. Each casting was cleaned using a brush in water for 2-3 min until the investment is thoroughly removed from the surface. In addition to these as cast specimens, a polished specimen was prepared from the as-cast specimen using #1000 SiC paper. The brightness index (L*) and the chromaticness index (a*, b*) of each as-cast specimen and the polished specimen were then measured using a spectro-photometer (Color analyzer type 307, Hitachi, Japan). Color difference (DE*) between the as-cast surface and the polished surface was calculated respectively to evaluate the blackening of the specimens. Results, DE* significantly varied both with different aluminum and boron contents in the investment (p<0.0001). For the aluminum-added investment, DE* significantly decreased from 42.8 (with no additives) to 12.6 (at 20% Al). For the boron-added investment, DE* significantly decreased from 42.8 (with no additives) to 14.3 (at 0.5% B). Conclusions, This results showed that both aluminum and boron were effective additives to prevent blackening of a gold alloy at its casting.

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