©Lin Yangchen


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The surface of stamp paper is very interesting, being a composite of different types of paper fibres and coating substances. Although paper classifications for major stamp issues are well established, not much is actually known about exactly how the papers differ from one another. Paper surfaces interact with inks in complex ways that affect the look of the final print; even something as uncomplicated as spray painting is affected by such interactions (Govaert & Bernard 2004).

The coconut definitives started out on a so-called chalky paper. Glazer (1997) documents in detail the manufacture and application of the chalk coating, which was outsourced to Samuel Jones & Co. of London. This coating was more variable on KGVI stamps than is often realized. Calcium carbonate was mixed with varying proportions of kaolin, other aluminium compounds, calcium chloride and calcium sulphate to get the desired consistency (Glazer 2004). It's not unlike mixing concrete; you have to get it just right. Salts of the natural milk protein casein were added to chemically encapsulate the chalk mixture. Solvent was then added to make a slurry that was applied to the paper. Finally the paper was heated to evaporate the solvent and finished by passing it between rollers.

The right side of this composite image shows the blue fluorescence of chalky paper under 365 nm ultraviolet radiation. Calcium carbonate does not fluoresce, so something else is lighting up. The underlying paper fibres could be partly responsible, as cellulose fluoresces white on longwave UV excitation. This is a mint stamp, so it is unlikely to have been tainted with optical brighteners from an envelope. Visible-light contamination can be ruled out, as the UV source was filtered.

The chalk coating yielded a crisp print and vivid colours, as it prevented ink from spreading through the fibre network (Melville 1916, Glazer 1997). It also supposedly made the stamps less prone to reuse (Easton 1949, Watterson 2004). When soaked in water, the casein salts were supposed to dissolve, thereby releasing the chalk and ruining the stamp. Glazer (2004), however, quoted the Crown Agents' former Chief Inspector of Colonial Stamps, Marcus Fauz, as saying that the paper coatings were meant to enable the use of certain inks and had nothing to do with fraud prevention. In addition, Fernbank (2013) mentioned that the coating was made “waterproof” using formaldehyde. Glazer (2004) also found that the chalk coating was often unencapsulated, making it waterproof and resistant to all but a very few dangerously corrosive reagents such as concentrated nitric acid. But mechanical damage is another thing.

Pre-war chalky paper. The coating is often pitted, perhaps due to heating, agitation or gas-producing chemical reactions.

Photomicrograph of pre-war chalky paper under a 20× plan apochromatic objective and transmitted Köhler illumination. The image has a field of view of about 0.9 mm along the diagonal. The surface appears granular. This indicates that ground rock chalk was used rather than the more expensive precipitated calcium carbonate. The microscopic bumps and pores helped it absorb ink evenly.


A scanning electron micrograph of pre-war chalky paper again reveals its microporosity (Lin 2020c).

In 1941, as war raged in Europe and De La Rue was bombed, striated paper (Barker 1996) appeared. It is thin, about 85 μm compared with the 105 to 150 μm of chalky paper (author's measurements using a micrometer screw gauge). The striae are more visible in large contiguous inked areas of intermediate luminance. They look too irregular to have been made by a mesh. Lin & Hisey (2020) postulated that they were formed by cockling.

Photomicrographs of striated paper under transmitted Köhler illumination. The upper image has a field of view of about 1.8 mm along the diagonal, while the lower one is at twice the magnification. The coating is thin but seems granular like chalky paper; it returned a positive silver test and is probably calcium carbonate (Robert Hisey pers. comm.).


Scanning electron micrograph of striated paper.

"Rough paper" appeared around the same time as striated paper. It had even more pronounced fibres on the surface and little or no coating. It was probably much cheaper to make than chalky paper.

Photomicrographs of rough paper under transmitted Köhler illumination. The upper image is at half the magnification of the lower image. The ink is largely deposited in the interfibre space. This is a sign of an oil-based ink repelled by the hydrophilic cellulose, which agrees with De La Rue's reported use of linseed oil as a pigment binder (De La Rue c. 1950).

The first known use of a compound optical microscope for the observation of the coconut definitive can probably be attributed to Rogers (1996), who observed differences in the visibility of fibres between chalky paper and rough paper. Rogers used what he called a “conventional” microscope, which he distinguished from a philatelist’s pocket microscope.

Visions of planet earth: a north American beaver lodge on a winter morning. Rough paper under the scanning electron microscope, showing fibrous uninked areas and smoother inked areas compressed by the letterpress plate.

After the war a "substitute paper" (Barker 1996) emerged. It is apparently not common, reportedly having been used only for certain issues for Ceylon, Hong Kong and Malaya (Lowe 1951). The print is cellophane-like, with the ink forming puddles and accumulating along the edges. The coating looks relatively smooth, without the granularity of chalky paper, and did not absorb ink well.

Another sample of substitute paper. Extensive pitting may occur, although the pits are not quite the same as those on chalky paper. The coating appears thinner than that of chalky paper (Lowe 1951); the underlying paper fibres can often be discerned, especially on worn-out stamps. From the perspective of security printing, the finished product should be consistent. Instead, the economic and political upheavals from the 1930s to the 1950s forced De La Rue and the Crown Agents into detours they could not avoid.

Substitute paper has a “slippery rock” microtopography. This is distinct from both the granularity of chalky paper and the fibrous roughness of the uncoated and thinly coated papers, as also shown by surface profilometry. The unevenness may be partly responsible for the puddling of ink on substitute paper.



It turns out that substitute paper is coated with aluminium silicate or kaolin—Al2O3·2SiO2·2H2O (Lin 2020c). It's the same substance used in the ground layer of Chinese paintings (Fitzhugh 1986) and in diarrhœa medication. The difference from chalky paper is quite clear in X-ray spectra (above). Many other colonial stamps printed during and shortly after the war were found with kaolin or aluminium oxide coatings (Glazer 1997).

Kaolin, like chalk, was plentiful in Britain. According to Robert Hisey (pers. comm.), English kaolin was the best in the world; it was bright white and of high purity. Kaolin makes the paper more opaque and absorb ink more evenly (Glazer 2004). The only thing that will dissolve kaolin is concentrated hydrofluoric acid, an extremely dangerous chemical (Glazer 2004). These stamps were built for the most extreme conditions.

Although chalk was plentiful in the British Isles, such as in the White Cliffs of Dover, it required casein salts as a binder (Glazer 1997). Casein was in short supply during the war (United States Tariff Commission 1945). Milk was strictly rationed and the print industry was prohibited from using it (Norris 2019). Chalk would make a comeback in 1948 (Glazer 2004).


Substitute paper under transmitted Köhler microscopy.

This scanning electron micrograph of substitute paper reminds the author of the landscape that would have greeted the Chang'e probe as it prepared to land on the far side of the Moon. It shows the letterpress effect in one of the upper corner decorations. The smooth areas are where the paper has been compressed and ink deposited by the printing plate.


Chalky paper came back soon after the war, but it was less pitted than pre-war chalky paper.

Scanning electron micrograph of post-war chalky paper with characteristic microporosity. Chalky paper seems less compressible by letterpress than substitute paper, perhaps because the coating is thicker or harder.

This is supposed to be post-war chalky paper, but it seems to be missing its coating. To the naked eye, it looks unnaturally white, on both the front and reverse, as if it has been brushed with Colgate toothpaste. The stamp may have been artificially cleaned to make it more saleable or presentable, or even to try to make it appear mint—the postmark (not shown) appears partially removed. But how? As mentioned earlier, the chalky coating is highly resistant to chemicals. The alternative is mechanical removal but that would almost certainly wreck the print.


No sign of the coating.

Additional paper types have been used for the coconut definitive but rarely noted as such. One of them is the uncoated buff-coloured thin card impregnated with slender brown fibres, used in postal stationery bearing direct prints of the stamp. The surface looks similar to that of rough paper.

The coconut definitive has also been printed on the even rougher aerogramme paper. An aerogramme is all about reducing weight. The paper is as thin and light as they dared to go, so much so that I wonder if they left out the coating for this reason. See Singapore 25 cents for more.

Yet more kinds of paper were involved in the genesis of the coconut definitive. First there was the plate-glazed paper or "glazed card" used for die proofs. The paper, though uncoated, was given a smooth ceramic-like surface by pressing it under heated metal plates. Ink texture on a specimen examined by the author (lot 586, Spink auction in Singapore, 28 October 2017) resembles that on substitute paper, with mottling and outlined edges, but gave crisp detail nevertheless.

In early 1941, De La Rue experimented with what they called "uncoated paper", making some test prints apparently using individual cliches. They might have been testing cheaper paper for wartime use. The rough surface yielded terrible print quality. The duty plate of the 1 cent appears to carry the plate flaw of the dot between the "T" and "R" of "STRAITS". The author examined these samples in the Crown Agents, Philatelic and Security Printing Archive at the British Library.

This so-called "plate proof", "printer's trial" or "printer's waste" on unwatermarked rough paper (Parr 1972, Stanway 2009), here showing Sultan Ismail of Trengganu, is similar in texture and colour to the buff-coloured card of postal stationery. This rare transitional stage between the die proofs and the production stamps was not officially recorded and may have served to clean the plate or remove excess ink; one occasionally finds the stamp of a different country on the same sheet (McClaren 2006)! This piece has two unevenly inked blocks and faint overlapping impressions of additional blocks that suggest plate cleaning. This specimen is unusual as most plate proofs of the coconut definitives are devoid of additional plate-cleaning impressions. These trials were supposed to be destroyed but a few made it out of the 'gulag'. Veteran philatelist Len Stanway noted that nothing was supposed to get out of security printing works, not even off-cuts from sheet edges after guillotining (Lin 2018d).

Plate proofs also exist in a rarer greenish grey paper of apparently similar fibre structure, here seen carrying the plates of Johore. Formerly in the collection of Dr. Abdul Majid Dato Kassim. See Lin (2018a) for further discussion of circumstances surrounding plate proofs. Mudie (2018) and McEwen (2020b) reported similar imperforate test prints apparently made on actual stamp paper further down the production line.

Lin (2018d) proposed a possible source of the paper used for the plate proofs: could it simply have been the wrapping paper for the actual stamp paper? It would be big enough to take an impression of the entire plate. Why else would such a large piece of scrap paper be lying around? Stanway, however, argued that the paper used for test prints had to be of a consistent thickness and surface texture so as to expose plate anomalies, and that wrapping paper would not have been up to the task (Lin 2018d). The literature offers descriptions of the paper but stops short of hypotheses for its origin. Exploration in this direction could bring fascinating insights into the day-to-day goings-on in the printing room.

Acknowledgements
I am grateful to Rein Bakhuizen van den Brink, David Beech, Paul Skinner, Benedict Sim, Ernest Cheah, Clement Khaw, Wulf Hofbauer, Li Zhen and Goh Wah Ing for discussions and technical assistance, and to the Nikon Imaging Centre (Singapore Bioimaging Consortium) and Science Centre Singapore for providing microscopy facilities.


References

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