Appearance of polished 18 karat gold surfaces: linking aesthetics to surface features

Impeccable appearance of surfaces is a key constraint in high-end jewelry and watch manufacturing. Based on long tradition in the industry, appearance of products is assessed by trained individuals through visual inspection in designated intervals on the production line. Despite the high efficiency of the visual quality control in detecting aesthetic defects, it suffers from a major shortcoming. In many cases when the expert declares the appearance of an artifact to be unsatisfactory, she/he is unable to specify the surface defect that has caused the aesthetic imperfection. In other words, the visual control cannot provide a correlation between actual properties of the surfaces and the responsible manufacturing process to their aesthetic consequences. Addressing this industrial problem requires developing a methodology for assessing aesthetic defects on the polished surfaces based on an understanding of the surface phenomena. This research work builds its approach towards the study of aesthetics of surfaces around an existing framework that proposes four parameters of color, gloss, translucency and texture as measurable surface properties, which can be linked to appearance. The current study allowed implementation of this general framework to the case study of polished gold surfaces by developing necessary application-related definitions and measurement techniques. Measurement techniques that were employed here granted the possibility for quantitative assessment of surface and near surface properties of ternary gold-silver-copper alloys in terms of topography, chemical composition, microstructure, mechanical properties and optical properties. Based on findings of those measurements, two leading surface properties that could cause a defect in aesthetic appearance of polished gold surfaces were recognized to be roughness and chemical composition. Quantitative correlations could be drawn for 18 karat gold alloy between the magnitude of two well defined physical and chemical properties of the surface (surface roughness and near surface chemical composition) with color characteristics of the surface (through CIELAB $L^{*}$, $a^{*}$ and $b^{*}$ values) and reflection properties (through calculation of total integrated scattered light). Finally, this work highlighted the significance of roughness evaluation at different length scales and also the importance of considering the chemical composition at the surface and near the surface. It was also shown that both of these parameters are modified during the common fabrication process of a watch or jewelry piece.