As a kid, I used to collect stamps. Cleaning up my place I just found my little collection and I thought to give it a quick Technical Photography glance. Postage stamps are a form of art, no doubt about it. I recall how much I was fascinated by those little images. Technical examination of postage stamps is used for their authentication
Stamps and Phosphors
In the late 50s postage stamps, in countries such as UK and USA, started to be coated with phosphors in order to make postal sorting and cancelling machine automatic. Phosphor ink is invisible to the naked eye but the sorting machines can see it.
There are two Ultraviolet wavelengths used for the examination of stamps. Phosphors are best viewed in shortwave ultraviolet light (254 nm). Longwave light (365 nm) is used to detect alterations, damages and repairs. I selected some of postage stamps out of my collection and took some multispectral images.
In US the first phosphors used for automatic stamps sorting show a characteristic orange glow, just in UV shortwave light (254 nm).
Used by Countries, such as Canada, Mexico and Switzerland, the yellow glow from this phosphor is visible both in longwave and shortwave UV light.
Green glowing phosphors are currently the most common used for automatic sorting.
Phosphors supplemented paper
Some Countries, such as Republic of San Marino, use a paper supplemented with phosphors.
Stamps and Pigments
Analytical identification of stamps’ pigments is part of their authentication process. The UV fluorescence of an old Italian stamp suggest an ink containing madder lake .
Interestingly, the Colombian stamp’s red shows the same behavior of the AIC Photo target red ink, both infrared and ultraviolet fluorescence.
Pigments Checker is for photographers, conservators and scientists interested in technical documentation of paintingss. It has 54 swatches of historical pigments designed for infrared photography, ultraviolet photography and other technical photographic methods for art examination. Check it out!
Pigments Checker is a collection of 54 swatches of historical pigments that have been applied using gum arabic as a binder on a cellulose and cotton watercolor paper, acids and lignin free. This paper is not treated with optical brighteners, it’s slightly UV fluorescent, and it reflects IR. Two cross-hair lines, 0,2 mm (vertical) and 0.4 mm (horizontal) are printed on each swatch of paper before the application of paint, in order to have a means to evaluate the pigments’ transparency in the IR and IRR imaging. Among all the pigments and their varieties ever used in art these pigments collection select the most used ones from antiquity to early 1950’.
References T. D. Chaplin, A. Jurado-Lopez, R. J. H. Clark, D. R. Beech “Identiﬁcation by Raman microscopy of pigments on early postage stamps: distinction between original 1847 and 1858–1862, forged and reproduction postage stamps of Mauritius” J. Raman Spectrosc. 2004; 35: 600–604.  Bristow M. “Scientific Detection of Philatelic Forgeries” Fakebusters II, Scientiﬁc Detection of Fakery in Art. SPIE and McCrone Research Institute: Chicago, IL, 2001; 55–65.  Chaplin TD, Clark RJH, Beech DR. “Comparison of genuine (1851–1852 AD) and forged or reproduction Hawaiian Missionary stamps using Raman microscopy” J. Raman Spectrosc. 2002; 33: 424.  http://www.linns.com/  http://www.filatelia.fi/forglinks/index.html  R. Chenciner “Madder Red: A History of Luxury and Trade” Routledge, Reprint edition (2011). Page 201.  J. Sharkey “Chemigry of Postqe Stamps: Dyes, Phosphors, Adhesives” Chemistry on Stamps, Volume 64 Number 3 March 1987. [ws_table id=”4″]