Mercuric sulphide ink chemistry - Lin Yangchen
©Lin Yangchen


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The darkening of certain inks over time is often attributed to sulphuretting, where lead compounds react with pollutants such as sulphur dioxide in the atmosphere. There is, however, another process of darkening in red inks made of mercuric sulphide (HgS), commonly known as vermilion or cinnabar and often found in old paintings. Experiments have shown that HgS can react with salt (NaCl), possibly deposited by fingers, to form the compound Hg3S2Cl2, which on exposure to light decomposes into white mercuric chloride (Hg2Cl2), sulphur (S) and a black form of HgS called metacinnabar (National Gallery Company 2002).


Using Raman spectroscopy, the ink used for the BMA MALAYA $2 duty plate shown above was identified as vermilion (Lin in press), which yields a distinctive Raman signal. Raman spectroscopy has previously been used successfully on stamps (e.g. Chaplin et al. 2002, 2004; Najjar 2013; Zhou et al. 2016), but this study is to my knowledge the first to apply the method to Malayan material. Due to its expense and relative difficulty of use, Raman spectroscopy is used much less often in philatelic expertising than methods such as UV-Vis-NIR spectroscopy and X-ray fluorescence. Raman spectroscopy, however, has a better ability to identify specific molecules (Clark 2005).

The ink was irradiated with a 785 nm infrared laser through a 50× microscope objective to precisely target the area of interest, using a HORIBA Scientific LabRAM HR Evolution Raman microscope. The incident laser power was 414 μW. Raman scattering was detected by a charge-coupled device sensor cooled to −120 °C to reduce noise in the signal.


Raman spectrum (red) of a single sample of the red ink, stretched along the y-axis to accentuate the peaks. It closely matches a reference spectrum (black) of mercuric sulphide (Caggiani 2016). The original Raman peaks of the stamp (download raw data) are weak because of strong fluorescence and/or the presence of other constituents; ink is often a complicated mixture of pigments, dilutants and drying agents (White 1979). A separate Raman spectrum of an uninked part of the stamp did not have these peaks. R graphics by Lin Yangchen.



These white and black deposits could be mercuric chloride and metacinnabar.

Photomicrograph showing possible deposits of white mercuric chloride, black metacinnabar and yellow sulphur particles. Imaged using a plan semi-apochromat microscope objective and charge-coupled device sensor with Peltier thermoelectric cooling and 14-bit analogue-digital conversion.

In the past, many collectors tried to clean up their stamps using talcum powder. Unfortunately it doesn't help at all and causes further damage in the form of an oily deposit.

Acknowledgements
I am especially grateful to Lester Kok, Zhang Lulu, Xiong Qihua, Lim Kim Yong and Eddie Yong for their contributions to this article.

References


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