Optical properties and microstructures of Pd-free Ag–Au–Pt–Cu dental alloys

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Nine experimental Pd-free Ag-Au-Pt-Cu dental alloys containing 10 at.% Pt and 10-35 at.% Au were prepared and their optical properties and microstructures were investigated by means of spectrophotometric colorimetry, optical microscopy, and electron probe microanalysis. All the alloys were annealed at 850 °C and mirror-polished to observe their reflectance curves in the visible spectrum and three-dimensional color coordinates. All the alloys were composed of a major phase of Ag-Au-rich matrix and a minor and almost colorless Pt-Cu-rich phase. It was found that the color of the alloys was substantially controlled by the Ag-Au-rich matrix and that with increasing Au/Ag atomic ratio from 0.130 to 0.996, the yellow-blue chromaticity index b * increased from 8.0 to 14.4, giving a pale yellow color. This systematic increase in yellowness was caused by a continuous shift of the absorption edge of reflectance curve toward longer wavelengths with increasing Au/Ag atomic ratio.