Invisible Writing

well_chuffed, Going Postal
A coded letter from Benedict Arnold, originally written in invisible ink. Handwriting by Peggy Shippen Arnold is interspersed with coded communication in Arnold’s hand

Herbert O Yardley was a a code clerk in the US State Department who took an interest in codes and ciphers. He taught himself as much as he could and he proposed a cryptographic bureau. This ended up in Military Intelligence and he was to set it up. This happened in 1917 and Herbert ran it until 1929 when it was shut down. Herbert , not best pleased at losing his job , then wrote a book called the American Black Chamber about his work. This was not received favourably at all by the Military. Strangely enough , he ended up working for the Canadians in much the same line but was kicked out in 1941 when the Americans and Canadians started sharing information ; the Americans had not forgiven him.

During WW1 there was a lot of German espionage in the USA and Mexico and some of this was conducted using invisible inks. This a potted version of what happened. Suffice to say , it was a bit like the drug cheats in sport today and the testers’ attempts to discover the drugs they use.

Having arrested a woman suspected of being a spy on the Mexico border , they found a blank sheet of paper on her. Was there any invisible writing on it , Herbert’s Bureau was given the job of finding out. Step 1 was to find the most skilful chemist in Washington. Unsure of what was on the paper they first tried heat , sure enough faint traces of writing were visible but it disappeared again when the paper cooled. They heated it again and took a photostat copy and found that the writing was in Greek. Step 2 was to find a Greek scholar , which they did , and then they had the translation.

Step 3 was to request help from England , we sent our foremost secret ink chemist to help the Americans get started. He then proceeded to set up the secret ink laboratory and recruit chemists to staff it. The British had discovered that the Germans were sending three or four copies of each letter to cover addresses , at least one of these was bound to get through. they then worked on reagents to develop the German inks but as soon as they found one , the Germans developed ever more difficult inks to solve.

Now it gets into James Bond/Q territory. One spy had potassium ferrocyanide concealed in a tube of toothpaste , another has his in a cake of soap. The Germans had as far as possible , developed inks that could pass as something else if discovered. Some of the inks had concentrations so low that only spectroscopic analysis could detect the presence of silver in them. The German chemists had by now developed inks that could be concealed by soaking articles of clothing in them. The spy then soaked the garment in distilled water to extract the secret ink.

The Germans then developed inks that could only be detected by reacting it with one particular agent , this made finding it even more difficult. Eventually they found that the Germans had started using distilled water to write the messages but how could it ever be developed ? The Allies were by now working on this together but the reagent eluded them. Now we get to the CSI part , the solution was to use iodine vapour  in a glass case. This was a massive breakthrough because it worked for any secret ink , it showed the pen strokes regardless of the chemicals.

Then disaster , The Germans found out the iodine vapour trick and changed to something else. It also implied that there was a spy , he remained undiscovered , among the Allies. Back to the drawing board. Eventually the scientists discovered that if a letter is written in secret ink , dried , dampened by a brush dipped in distilled water , dried again and finally pressed with an iron , the secret ink cannot be developed by an iodine vapour bath. The reason being that the dampening process disturbs all the fibres on the paper thereby covering up the original pen marks.

This however was stalemate , neither side could develop the secret ink letters of their enemies. The Americans then found that if they streaked a suspect letter with two different chemicals and the two streaks ran together , it was proof that the letter had been dampened and who would do that but a spy.

Eventually the Allies discover the holy grail , a general reagent that worked on all secret inks. This secret was carefully guarded and even in 1931 , Herbert would not give away the formula of this reagent.

The final steps were in opening letters , extracting their secrets and then resealing them and allowing them to continue to the addressee , this involved recruiting forgers and some more unsavoury types of person. This usually involved lots of steam to melt the glues and various ways of recreating seals by taking a cast of them.

The book contains various examples of spies being caught using secret inks , often it was to do with supplying money to finance various shady deals. It would take too long to describe them here but the number of spies active in North and Central America was remarkably high.

This was a very brief history of secret inks during the last year or so of WW1 , the book was published in 1931 so there are no details of such things in WW2 but no doubt , they still used secret inks then.


© well_chuffed 2018

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