The Testing Department (MI9c) devised increasingly cunning tradecraft in its work on postal censorship. As German spymasters grew ever-more sophisticated in their attempts to communicate with their agents in Britain, and as German prisoners in Britain and their officers abroad also developed more inventive ways to interact, the Testing Department kept pace. In addition to experimenting with secret ink, it also investigated ways that civilian correspondence might wittingly or unwittingly reveal strategically sensitive information such as evaluations of public morale or the availability of foodstuffs. Ink, however, accounted for a great deal of its work. The earliest inks exploited the chemical qualities of everyday organic substances. Fruit and vegetable juices, milk, saliva and urine all featured regularly. Not only were they easily available, they were easy to transport without raising suspicion and were similarly easy to make visible, to develop as one would a photograph. The application of heat, as with an iron, quickly reveals these organic compounds. Another technique consisted of dusting paper with graphite, ashes and other pigmented powders, such as cupric oxide. The key to the choice of substance is its ability to adhere to greasy or sticky inks. MI9c documents note that milk, for example, can easily be made perceptible in this manner.

Paper also determines the degree of an ink’s invisibility. MI9c categorised paper as being of three basic textures (glazed, thin and tissue) and four basic surfaces (smooth, vellum, rough and linen-faced). Frequently, letters or postcards held at oblique angles to the light revealed the indentations and scratches from an applicator’s point, showing the presence of a message or, with careful examination, the message itself. Highly glazed paper, for example, was the most revealing, so special ‘prisoner of war paper’ was issued to German prisoners to help censors quickly sift through their correspondence. In 1916, the government considered introducing a similar scheme for use by the general public. A paper was developed so that the application of any liquids caused permanent damage to the integrity of the surface, revealing any attempts to write with secret ink. Because it consisted of several layers, dry impressions were also revealed.