The search for the perfect secret ink preoccupied the Secret Service Bureau. In 1915 Cumming asked scientists at London University to come up with something suitable. To C’s ‘delight’ they recommended the use of semen. There would be an inexhaustible supply if the agent didn’t over do it, and it was hard to detect because it didn’t react to iodine vapour. At least one of C’s agents appears to have experimented with this perfect ink. ‘Our man in Copenhagen…evidently stocked it in a bottle – for his letters stank to high heaven’, Frank Stagg, a member of the Bureau’s staff recalled; ‘we had to tell him a fresh ‘operation’ was necessary for each letter’.
The Service’s first scientific advisor, the physicist Thomas Merton came up with a more practical form of secet writing: ‘Write on glass or on any hard material with a silver point. The trace will be entirely invisible, but it can be made visible as follows. Make up two solutions A and B.
- A: Metol 5 grs, citric acid 5 grs, acetic acid 15 grs, water 100 cc
- B: Silver nitrate 10 grs, water 100 cc.
Mix ten parts of A with one part of B and with 100 parts of water. This is the developer. It can be used only for 10-20 minutes after mixing.’
Formula for German invisible ink passed to the US Office of Naval Intelligence by the French in 1918.
It was Merton who discovered German agents were soaking their clothes in the chemicals they needed to make invisible ink – a mixture of nitrate, soda and starch – before drying them carefully. To constitute the ink the clothes were simply soaked water.