In my second novel, on WW1 naval intelligence, I made reference to the secret or invisible inks used by intelligence agents during and leading up to WW1. The use of invisible inks can be traced back as long ago as the 4th Century BC and was still occasionally used by British SOE agents during WW2. George Washington is reported to have used a secret ink during the American War of Independence. There is evidence of their use by spies during the Cold War and, as you will read later, they are still in use today by businesses and government agencies on public documents.
Early invisible inks tended to be made from natural products, readily available. The idea was that the ink would be colourless and invisible until heated or treated with another agent, such as a chemical, to reveal the original message. Milk or plant products were deemed ideal and often lemon juice was used. WW1 intelligence agents were usually supplied with a specially designed invisible ink by their handlers. The preference was to provide the ink in the form of an innocent object that would not attract suspicion during a search. British agents tended to hide their ink powder in a shaving stick. The writing was then read by exposing the paper to an iodine vapour. However, all agents had to be prepared to use any substance readily available. It is said that during WW1, some British agents even had to resort to human semen at times! The practise was ceased after complaints about the smell. During WW2, German agents even used substances that had to be mixed with their own blood to produce the ink.
At the start of WW1, German agents were using lemon juice as an invisible ink, but this was quickly discovered by the Secret Service Bureau. The Germans then came up with Argyrol, a light brown powder comprising a silver salt of protein soluble in water and used as a medicine or antiseptic. It was used to combat gonorrhoea! The German spymasters smeared a paste of it over a pair of the agent’s socks and the agent would produce the ink by rinsing out the socks in water. Indeed, agents from all countries often used impregnated clothing as a source of invisible ink right through to the 1950s.
Today, a variety of invisible inks are commonly in use for non-nefarious use. A common example is as a security marker to protect property in the event of burglary. The ink is only revealed under ultra-violet light. Certain official documents also contain messages only visible to aid processing of the document and in this way avoid cluttering the visible parts of the page to the person completing the document.