To recognize gold, it is possible to use a chemical identification method based on strong acid, which allows distinguishing pure gold from other common metals.
The acid kit consists of various vials and a touchstone, also called a "touchau". To use it, one firmly rubs the jewel on the stone, then applies a few drops of the acid corresponding to the assumed carat of the jewel (9 carats, 14 carats, 18 carats or 22 carats).
If no reaction occurs, the jewel is gold and of the corresponding fineness.
If the trace disappears, the jewel is not gold.
If the trace turns red, the degree of purity is lower than that of the acid used.
The nitric acid in the vials must be handled with extreme caution.
Wearing appropriate gloves and glasses is strongly recommended.
A genuine touchstone is necessary to obtain an indisputable answer.
Note: the acid can also be applied directly to the jewel after scratching its surface.
Information and visuals collected from the website of the Bureau des Poinçons de la Douane - Garantie de Paris
https://www.douane.gouv.fr
https://www.goldinfo.fr/
