©Lin Yangchen

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Imagine a landscape of deep canyons snaking across the earth’s crust, punctuated by gigantic sinkholes with vertical precipices dropping thousands of feet to the bottom. It sounds like an expedition to Mexico or Mulu, but one needs voyage no farther than one’s stamp album.

You could mistake this for a satellite pass over the fractured karst landscape of the Yucatán Peninsula, with its ancient and mysterious cenotes concealed in lush forest. The main image is one of 23 optical sections of a fluorescence-mode confocal laser scan of the chalky surface of the upper-left corner decoration of a coconut definitive. The 3.60 μm-thick section was visualized by combining the fluorescence signals from simultaneous laser excitation wavelengths of 404.8 nm (1.7 mW), 486.2 nm (0.9 mW), 561.5 nm (0.9 mW) and 638.8 nm (1.6 mW). Laser power was measured at the tip of the optical fibre and totalled 5.1 mW before passing through the objective on its way to the stamp. The dwell time was 3.03 μs and there was no visible damage to the stamp. The fluorescence signals were amplified through gallium-arsenide-phosphide photomultiplier vacuum tubes. The chalky surface is both pitted and cracked. The deep orange spots in the image are particles of unknown composition fluorescing at approximately 595 nm. These are not discernible under normal illumination. The bars at the bottom and right side of the figure show the full z-profiles of the fluorescence along the x and y crosshairs. The resolution on the x-y plane is 0.78 μm, while the z-resolution is 10.26 μm. See Ismail et al. (2020) for a confocal laser scan of tissue paper.

A snowy Christmas

The Straits Settlements $5 top denomination on green paper.

Micrograph in the spirit of the post-war American art movement of Abstract Expressionism, as characterized by quasi-spontaneous blots and brush strokes.

Coconut Pox
Fossilized bubbles. Perhaps the liquid coating mixture was overheated, too thick or stirred too hard.

Transmitted bright-field micrograph, extended depth of focus, height map and 3D reconstruction from 25 micrographs spaced 2 μm apart (total depth 48 μm) using the author's Olympus BHSP microscope.

Some of the largest craters ever seen on the coconut definitive, measuring about 70 μm across. Deposits of the blue pigment used to colour the paper can be seen embedded in the crater floor. The translucent rim raises suspicion of overhangs arising from bubbles in "prehistoric" times when the "crust" was still "molten". The cracks probably formed when the coating mixture shrank as it dried. They propagated along the path of least resistance, which tends to be the line joining the centres of each pair of nearest neighbouring pits. The mechanics are similar to separating stamps along their perforations.

Confocal laser scan of the pits in the previous micrograph, with a field of view of about 250 μm on each side. The pits go as deep as 50 μm, about half the total paper thickness. Confocal scan settings: optical sections 0.50 μm thick, pinhole 1.2 airy units, pixel dwell 1.09 μs, horizontal resolution 0.28 μm, vertical resolution 1.44 μm.

The simulated underside shows the cracks reaching depths of about 30 μm. The confocal microscope was able to resolve the three-dimensional morphology of these narrow cracks where the conventional optical microscope failed. The pinhole in the confocal system eliminated out-of-focus light that would have "polluted" the in-focus light from inside the crack. Signal fidelity is critical here because of the extreme z-gradient.

A 3.0 kV scanning electron micrograph (upper image) of a pit at 1000×, and the 8-km-wide caldera of Tambora volcano (lower image: nasa), where the most powerful eruption in recorded history took place in 1815. The author summited the volcano in 2006. Now he has his own miniature Tambora at home, complete with volcanic fissures, lava flows and fumaroles—and Gustav Mahler's Symphony No. 6.

Altum foraminis
(“deep hole”)
Red-cyan anaglyph created from a stereo pair of scanning electron micrographs with the stage tilted at −5° and 5°. As the microscope projects the image like a telephoto lens, the simple translational movement used in conventional stereo photography does not give a sufficient perspective difference, especially of the sharp and overhanging edges. The relatively large angle of tilt also simulates a close-up view, like that from a helicopter hovering over the sinkhole, its intricately sculpted walls reminiscent of limestone carved and scalloped over millions of years by a subterranean river.

The author's sketch of a scanning electron microscopy setup for looking under the overhanging lip of the pit. The plan was eventually abandoned; it required prolonged electron bombardment that would overcharge the specimen and blow out the image.

Lin (2020c) did a qualitative comparison between chalky and substitute papers regarding pit size and how closely pits were spaced to one another. The author examined six examples each of pre-war and post-war chalky paper and six of substitute paper (above) under the microscope. In addition, I examined two BMA 50¢ stamps, one on pitted substitute paper, the other unpitted. I examined used stamps as mint copies were not available in sufficient quantity. I measured randomly selected pits on the micrographs using a ruler to determine their actual diameters, using a scale bar embedded in a test image taken at the same magnification. This worked better than digital thresholding, which was inconsistent in accurately capturing pit edges even within individual samples. This was due to the watermark, which caused the brightness range of unpitted parts of the paper to overlap with the brightness range of the pits. Furthermore, some large fibers let light through which got picked up by thresholding.

Pre-war chalky paper varies widely in pit spacing; there are stamps with abundant pitting and those with very little. The pits tend to be relatively large, with estimated diameters mostly from 36 μm to 64 μm, although smaller pits are sometimes present. In contrast, post-war chalky paper tends to have smaller and more sparsely spaced pits, mostly between 14 μm and 29 μm in diameter, occasionally up to 43 μm.

Substitute paper is usually sparsely pitted, but is sometimes heavily pitted. The latter condition was first documented by Pollard (2012) on the BMA Malaya 50¢, an example of which is shown above. The pits on substitute paper are similar in size to those on post-war chalky paper (Lin 2020c). The difference in pit size between pre-war and post-war papers could be due to a difference in the viscosity of the coating mix. Meanwhile, variation in pit density probably stemmed from the preparation of the coating mix, which was probably not unlike stirring a pot of creamy mushroom soup; you get clumps of bubbles forming in areas of turbulent flow and smooth creamy soup in calmer regions of the fluid. When the mix was applied to the paper, some parts got lots of bubbles and others almost none.

The heavily pitted form of substitute paper under the microscope.

If you still can't get enough of mysterious sinkholes, take a look at those in Siberia.

I am grateful to David Beech, Benedict Sim, Ernest Cheah, Clement Khaw, Wulf Hofbauer, Li Zhen and Goh Wah Ing for discussions and technical assistance. I also thank the Nikon Imaging Centre and Science Centre Singapore for the microscopy facilities that made this research possible.


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