Traitement Zachery des turquoises : méthode d'identification simple fondée sur la microchimie

Abstract

International audienceZachery-treated turquoise is turquoise made less porous, with a slightly better brilliance, without the use of resin or polymer (Figures 1 to 3). It cannot be identified through classical gemological observations. So far, only an identification criterion based on chemical analysis using laboratory instrumentation (EDXRF, but other similar techniques could work) has been proposed: Zachery-treated turquoise contains much more potassium (K) than its natural counterpart (Fritsch et al., 1999). We have developed a micro-chemical technique to see this difference in K content without using large instruments. We propose to use a small drop (about 1 mm diameter on the surface of the turquoise) of picric acid (concentrated solution in water) put in reaction with the unprotected surface of turquoise. For natural turquoise, there is no real reaction, only rarely a few yellow flakes, and rarely a few short yellow needles (Figure 6). For Zachery-treated turquoise, numerous long yellow needles or groups of needles form as soon as the drop dries out, due to the formation of a potassium compound (Figure 6; see also Table I). This reaction is proof of Zachery treatment for this gem. The test must be practiced under a binocular microscope, to affect the smallest possible area, in an unexposed zone of the gem. As most fashioned turquoises receive a surface treatment after polishing, this reaction does not work if the surface material is not removed. This is achieved with a droplet of diluted nitric acid (30% volume in water). The picric acid can then be put on the exposed turquoise surface, with the reaction as described above. After clearing the needles on treated material, often a yellow or white stain remains, which can be easily cleaned. For rough, the yellow stain disappears after putting a drop of the nitric acid solution on it, and then rinsing with water. For fashioned material, the white stain can be removed by simply washing with water using for example an old toothbrush. Because this technique requires the use of chemicals that are rather concentrated, this test must be practiced with care, in compliance with local regulations on the relevant chemicals