X-rays in philately.

Abstract

As is well known, radiography provides a means for non-destructive analysis in many fields outside of medicine and medical research. Perhaps the industrial applications have received the most publicity, but x-rays have been similarly employed in such fields as archaeology, paleontology, geology, biology, conchology, zoology, and entomology. One of the best known non-medical and non-industrial uses of radiography is in art museums, where paintings are examined for analysis of the paint, canvas, paper, or wood base. Quite recently similar investigations have been made in the field of philately, or stamp collecting. A postage stamp consists basically of a design printed on a small piece of thin paper. It is important to the collector to be able to differentiate various characteristics of stamps. These include: (1) the method of printing, such as intaglio, letterpress, offset, photogravure, or other process; (2) the type of paper—woven, laid, ribbed, pelure, etc.; (3) watermark, if any, in the paper. Such information is essential in cataloguing stamps for collections or sales. Many stamps are of high value, selling for hundreds and even thousands of dollars if in good condition and genuine. Years ago clever counterfeiters created reproductions of quite a few of these high-value stamps. Nowadays the unscrupulous artisan will not forge an entire stamp, but he is able to increase the value of a genuine stamp by repairing torn or thin areas of the paper, adding paper to narrow margins (Fig. 1), replacing perforations that have been torn off, and chemically removing cancellations. He also will redraw the design where damaged (Fig. 2). Such repairs frequently are impossible of detection by the naked eye, the microscope or ultraviolet light. Hence, such alterations may be overlooked even by the advanced philatelist. The application of x-rays has made it possible to study the minute details of the design, paper texture, watermark, and cancellation of the postage stamp. One can facilitate determination of authenticity by comparing the radiographic image of a genuine with that of a questionable stamp. Repaired areas will usually show as clearly on the radiograph as would a fracture of a long bone. Such analysis is made possible by the use of three different technics: low-voltage radiography, autoelectronography, and electronography. Each procedure reveals certain aspects of the stamp not readily apparent with the others. Technics Low-Voltage Radiography: Because postage stamps are printed on such thin paper and offer so little absorption, x-rays of very long wave length must be employed. We have found that a beryllium window tube operated at 6 kv. is most satisfactory. The designs and cancellations of stamps are printed with inks whose color is derived from metallic pigments such as mercuric oxide, lead oxide, etc., or from organic substances such as carbon and the aniline dyes.