Coloured papers of De La Rue - Lin Yangchen
©Lin Yangchen

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Coloured papers were reserved for the highest echelons of the coconut hierarchy, the "general staff" or dollar values. The $1, $2 and $5 each had its operational compass. The black and red on blue looks psychedelic, almost radioactive—not an inappropriate maverick image for the "youngest" of the senior officers. The $5, in the position of command, exudes the refinement of two shades of green and the authority of a splendid shade of red. The $2 (not shown) shares the colours of the $5 but on white paper, signifying its lieutenant-general status.

The coating on this sample of blue paper harbours residual particles of blue and green colouring (Fernbank 2013). I have seen green paper carrying blue particles as well. The orange paste is from a forged Okugawa seal. The focus-stacked image was generated using a Keyence industrial microscope configured with partial annular illumination to provide shadow information.

I used Raman spectroscopy to try to identify the substances in the blue and green particles on coloured papers of the coconut definitive. In Raman spectroscopy, the substance of interest is zapped with a laser. The photons from the laser are scattered when they hit the molecules. Most of the photons show no change in energy, but a small number of them transfer some of their energy to the molecules or vice versa. The quantity of energy transferred depends on the chemical bonds present. This effect was named after Indian physicist Chandrasekhara Venkata Raman, who first reported it in 1928 together with fellow physicist Kariamanickam Srinivasa Krishnan.

The 532 nm laser is generated by a convoluted process full of bombastic chemical formulæ. First a 808 nm laser is produced using aluminium gallium arsenide. The light is aimed at a crystal of a neodymium-doped yttrium compound, which increases the wavelength to 1064 nm. This in turn goes into a crystal of potassium titanyl phosphate oriented in such a way that its anisotropic structure causes the frequency to be halved to 532 nm.

Pigments Checker Raman database (Caggiani et al. 2016)
Raman spectroscopic library of natural and synthetic pigments (Bell et al. 1997)
Raman spectroscopic library of mediæval pigments (Marucci et al. 2018)
RRUFF mineral database (Lafuente et al. 2016)


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