Abstract
The principles behind the touchstone have never been investigated in a scientific manner, even though there are many technical and historical accounts of its effectiveness. In this article I investigate whether and to what extent the content of Au-Ag-Cu ternary alloys can be visually ascertained with a touchstone without acid. I show that it is very likely to be true as the touchstone solves several problems (illumination, surface state and curvature, etc.) and makes it possible to carry out a luminance comparison. A colorimetric approach confirms the high level of accuracy attained.
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