Effect of aging on color properties of gold alloys

In this study, (L, a, b) values of the gold alloys, measured according to the CIELAB color standardization system, were applied to the color scale and color groups are determined depending on the percent gold content. Later, experiments were conducted to investigate the effect of aging process (applied to enhance the mechanical properties of yellow, white, red, and red green alloys of 8, 9, 10, 14, 18 and 22 carats at 250, 300, 350, and 400°C for 1 hour) on the color properties of gold. Following the aging process, L, a, b values of each sample were measured and the chroma values (a² + b²) calculated. It is recognized that the gold concentration on the surface increases during the surface cleaning step takes place after the aging process, in relation to the oxidation of alloying elements on the surface of gold alloys. Thus, the color of all types of alloys approaches to the characteristic color of pure gold as the heat treatment temperature increases. It is also determined that the aging temperature must be definitely below 300°C for the white gold alloys for which the chroma value is very critical (6 < chroma < 10). Chroma values of green gold alloys increase with increasing temperature, while those of red gold alloys seem to be unaffected.