Mercury and lead pigments - 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. Remarkably, cinnabar has also been a Chinese medicine for almost 2000 years. Almost 50 Chinese medicines still contain cinnabar, including infant sedatives (Huang et al. 2007, Liu et al. 2008, Yen et al. 2008). Experiments have shown that HgS pigment on paper 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).


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.

The duty plate ink of the BMA MALAYA $2 shown above was analyzed using Raman spectroscopy (Lin 2018c). 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 calibrated with a 585.25 nm neon-argon-hydrogen discharge and a wafer of pure silicon. The laser power at the surface of the stamp was 414 μW. Raman scattering was dispersed through a diffraction grating of 1800 grooves mm−1 and collected by a charge-coupled device sensor cooled to −120 °C (top right corner of photo) 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 (download raw data). It closely matches a reference spectrum (black) of mercuric sulphide in the spectral library of Caggiani et al. (2016). A separate Raman spectrum of an uninked part of the stamp did not have these peaks.

Heavy metal (in this case mercury) as revealed by a backscattered electron image from a scanning electron microscope. There is at least one known record of the use of mercuric sulphide by De La Rue. In one of its 19th-century recipe books, ink 611 is described as mercuric sulphide shaded with yellow chromes.


Display panel for the 36th Asian International Stamp Exhibition in 2019.
Design by the author.



Energy-dispersive X-ray spectroscopy reveals that pre-war and post-war reds seem to have been printed with mercuric sulphide, while low-value BMA reds (above) and red BMA overprints (see separate article) were printed with red lead or minium (lead (II,IV) oxide, Pb3O4). Red lead has been used since antiquity (Chaplin et al. 2004) and is found in some outdoor paints and lead-acid batteries. The X-ray lines for sulphur (2.310) and lead (2.346) are very close, so the peaks were closely examined in the original vector plots to determine which line they are closer to. Some of the mercury peaks are weak and partially buried by the larger sulphur peaks close by. The barium and sulphur peaks on some of the stamps may be from barium sulphate, which was widely used as a pigment dilutant (Allen & Lera 2013) and paper brightener. The compound has been found in the coatings of KGVI stamps of other British colonies (Glazer 2004).


Possible sulphuretting of red lead, with pitting that might have been caused by chemical change. I initially thought this was the obscure 'blacking of red overprint' forgery (Cameron 1950).

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

It seems that mainly two heavy red pigments were used for the coconut definitives: vermilion before and after the war, with an intervening period of red lead just after the war. It could be because red lead was easier to make and cheaper. The inks based on red lead, however, do not have the intensity, opacity and purity of colour of vermilion, judging from my samples. This is probably why red lead seems more readily influenced by the paper to appear more scarlet or more carmine. It seems possible to tell apart red lead and vermilion by eye if they are compared side by side. This could be why De La Rue returned to vermilion after the war. In fact, the BMA $2 high value was already being printed with vermilion, as described earlier.

Acknowledgements
I am grateful to Lester Kok, Zhang Lulu, Xiong Qihua, Lim Kim Yong, Eddie Yong, Wulf Hofbauer, Li Zhen and Peter Cockburn for their contributions to this article.

References


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