©Lin Yangchen


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It seems that mainly two heavy, toxic 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. 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 the author's samples. Fitzhugh (1986) considers red lead to have low tinting strength. 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.

Mercuric sulphide (HgS) is 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 (Spring & Grout 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.

Raman spectrum (red) of a sample of red ink from the BMA $2 duty plate (Lin 2018c), 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 (Lin 2020d) 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 were printed with lead tetroxide or red lead (lead (II,IV) oxide, Pb3O4). Red lead has been used since antiquity (Chaplin et al. 2004) and is also used in lead-acid batteries and anti-corrosion marine paints. 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).

Ultraviolet fluorescence is rarely used to examine red inks on the coconut definitives (above), in contrast to its extensive application to the Australia KGV 1d reds. The fluorescence differs across 25¢ duty plates (upper row under visible light; lower row same stamps under UV) and could offer further clues to their chemical makeup. Cameron (1950) listed “aniline red” and “aniline scarlet” among other red shades of the 25¢ duty plates, although he did not specify whether his terminology was based on chemistry or fluorescence. The head plates above are in the magenta shades with their own set of fluorescence signatures.

Red lead turns brown or black over time on exposure to oxygen, humidity or light (Fitzhugh 1986). The black substance has been identified as lead dioxide (PbO2) in old paintings, but the underlying chemistry is murky. Red lead can also blacken into lead sulphide on contact with other sulphides.

Photomicrograph of the blackened overprint under Köhler illumination through a 20× plan apochromatic objective. The halogen light source is colour-corrected. It looks like the famous lava flows of Kīlauea.

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.

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

References


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