MORE ON POLYMER-IMPREGNATED TURQUOISE
In the Summer 2007 issue, K. S. Moe et al. ("Polymerimpregnated turquoise," pp. 149–151) documented the spectroscopic properties of a turquoise cabochon and concluded that it had been impregnated with an ultraviolet (UV) radiation–hardened polymer. Comparing the spectroscopic properties of the "filler" with that of Norland Optical Adhesive 65 (a known photopolymer for various technical applications), the authors further concluded that this type of material, which was previously known in gemology only for emerald fracture filling, is also now being used for turquoise.
It should be noted, however, that the radiation polymerization of impregnated turquoise has been known for more than 25 years. H. Breault and A. E. Witt (Turquoise-Plastic-Composite, U.S. patent 4,075,365, issued Feb. 21, 1978) described the impregnation of natural turquoise with a liquid monomer in which polymerization is possible through catalytic, thermal, or radiation initiation. In particular, a monomer consisting of the dimethracrylate of triethylene ether glycol is used for the impregnation, and polymerization is carried out by 60Co gamma irradiation.
In addition, it is known from another U.S. patent (E. Proksch and H. Eschweiler, Porous, Heat-Sensitive Substrates Coated with Synthetic Resins, U.S. patent 3,935,364, issued Jan. 27, 1976) that natural porous stone material can be coated with a liquid synthetic resin (of different components) that is then polymerized and hardened by UV or electron irradiation. Further details, especially the compounds used with these processes, can be found in the patent documents.
Based on information obtained from the trade (H. Zimmermann, pers. comm. 2006, 2007), it appears that several different companies are now engaged in the impregnation and polymerization of turquoise. To assist impregnation of the rough with the liquid monomer, the untreated material is sometimes placed in an autoclave. The exact chemical composition of the monomer is proprietary, but copper sulfate is sometimes added to intensify the color (H. Zimmermann, pers. comm., 2006, 2007).
The polymerization is performed mainly by 60Co gamma or electron irradiation. Over the last few years, BCS Stones GmbH, Harxheim, Germany, has treated several thousand kilos of rough per year by polymerization via electron irradiation. Production figures of other companies are not available at the moment.
Karl Schmetzer
Petershausen, Germany
RAMAN PEAKS IN CORAL
The article "Pink to Red Coral: A Guide to Determining Origin of Color" in the Spring 2007 issue (pp. 4–15) is very informative, and the analysis is very detailed and systematic. However, the article also states that there is a Raman peak at 1123 cm−1 (see figures 14 and 15). Recently, I performed Raman analysis on the same type of natural red coral but observed a peak at 1132 cm−1, not 1123 cm−1. This result also matched well with the Renishaw Raman Library. I would appreciate it if the authors could clarify if the peak value they report is an error or a variance in data.
C. G. Zeng
Nan Yang Gemological Institute
Singapore
REPLY
We want to thank Mr. Zeng for pointing out a typographical error made early in our data record that was carried throughout the text of our article. The last two numbers for the position of this peak were inadvertently reversed. The correct identification of the peak position should indeed be 1132 cm−1 and not 1123 cm−1.
Christopher P. Smith, Shane F. McClure, and David M. Kondo
