Invisible Ink

Observation Post, Volume 1, Issue 32, 24 December 1942, Page 1

Invisible Ink

(By W.O. 11. L. V. Winks.) Invisible ink is not purchasable .in. a stationer’s shop; for, that matter it is not necessary to purchase it at all —you can make your own. A spot of lemon juice will make an admirable invisible ink. Take a perfectly clean nib, dip it into the lemon and write your message on a piece of paper; allow the juice to dry. —and there will be nothing to be seen. Then run a hot iron -over the paper, and the writing will return faint and light brown in shade, but readily legible. There are, however, chemical inks of greater complexity, but not so readily revealed. There are many in use which defy the hot iron; but there are, .of course, reagents to all chemicals. There is another way of detecting writing with invisible ink. Take a clean nib, dip it in water, and begin to write; or dip it in nothing at —merely write with the dry nib on the paper. Your nib will make minute scratches on the paperinvisible to the naked eye, but easily seen under the microscope. Nor is microscopic examination necessary. An iodine vapour bath is an essential appliance in every , censorship office; this is a simple apparatus—a tin oven in which iodine is maintained at the lowest .'temperature-at which iV will remain vapourised. The suspected letter is introduced into the bath; iodine tends to settle on the rough .surfaces, and in a few minutes, when the letter is withdrawn, .it will'be found that minute crystals of iodine have settled along the tiny -rough edges formed by the scratch of the nib. The writing clumsily outlined and with many blurs, comes to light by this ingenious device. SPIES’ USE OF INK Yet even this can be countered. The Germans frequently supply their agents with ’ball-pointed —nibs with a tiny ball instead o< a point. This makes 'no scratches as it passes over the page —it is necessary to write'large in order to make the necessary loops. The resultant letter could pass even the detecting interior of the vapour bath without arousing suspicion. But, on the other hand, if a man were caught with a ball-pointed pen in his possession, then he was on the face of it a spy; there was no other reason for possessing such a thing. Invisible ink, naturally, is not carried about in bottles so labelled. One German spy arrested' i« England carried his supply in a tin originally manufactured to house homely talcum powder! Others carried their ink in their clothing; before leaving their base they would dip handkerchiefs, collars, and even socks in a solution of the chemical. The articles, carefully. drifid, were carried to their destination; then it was only necessary to dip the handkerchief or other article of clothing in a very small quantity of warm water, and to squeeze it out to turn the water into an ink of adequate strength. Until its use was discovered and the necessary reagent supplied to counter-espionage officers, the Germans during the World War made great use of a fluid comprised of naphthol, collodion and acetone in the'proportion of one, twenty, sixty. This mixture was smuggled to their agents; as medical tablets, which were always packed in paper bearing the trade mark of a genuine manufacturer of chemists’ supplies. They were also supplied with point protectors in metal, which fitted over their pencils, and which served as a measure for the quantity of powder necessary for the production of the ink!

DEVELOPING REAGENT

The following is the reagent used to develop the writing: Five grains of sulphuric acid were mixed with fifty cubic centimetres of nitric acid in a litre of water, and added cold to one gramme of sodium nitrate. Fifty grammes of sodium acetate were then dissolved in two hundred cubic centimetres of water. Working with a mixture of twenty cubic centimetres of this second solution with a hundred

cubic centimetres of the first solution, the complete paper was then dipped in the mixture until all the letters appeared, and was afterwards washed in distilled water and dried between sheets of blotting paper. THE CONCEALMENT OF THE INK Lead acetone is a frequent component of invisible ink, but a more popular mixture is one of brandy and milk, these components being' more readily obtainable. Italian spies in wartime were supplied with an invisible ink made of potato pulp and disguised as toothache cure. Eggs figure more than once in the spy records of Eurtipe. There was one case of a lady who lived in France, but only a mile or two away from the Swiss frontier; legitimately enough, she, used to have her eggs sent from a Swiss farmin times of peace, movement over friendly frontiers 13 easy enough, and, a frontier being an artificial line, it is quite natural that local people should move freely from one side to the other. Unfortunately for the lady, the French counterespionage service grew suspicious. She had done nothing to excite their curiosity, but a French agent had discovered that The Swiss farmer was ,a German whose activities had been known in another connection some years before. For weeks, however, they were unable to discover any ilicit -intercourse—except for the consignment of eggs and payment for them, there seemed to be no communication. The 1 cartons in which the' eggs were delivered were examined by the French agents. At last in despair, it was decided to examine the eggs themselves. Within . a few hours laboratory tests had revealed their secret for when they were dipped in a certain solution of gallnut, writing was disclosed on their shells; the' ink was identified as a preparation of tithymal. • ■ - EGGS AS THE MESSAGE CARRIER Yet this method was punitive compared to another which was worked on more than one occasion. A counterespionage officer was -amazed when, peeling the shell from a hard boiled egg, he noticed hieroglyphics in black on the white of the egg, which eventually deciphered themselves into letters. Yet the device is simple enough-and for that matter by no means modern. After the egg is hard boiled and has been allowed to cool, the spy gets a small portion alum and vinegar-both, it will be noted, homily articles arousing no -suspicion. The alum is the more unusual but anyone can legitimately carry an antiseptic block with which to rub the chin when he cuts himself when shaving. The alum and vinegar are mixed together to the consistency of ink; then the message is written on the outside of the shell. As this unique invisible ink dries, there is nothing to be seen; but a few hours later the message, which must be written in large characters, will appear on the white of the egg. What happens is this. There is , of course, iron in allumferrous ammonium sulphate. Commercial vinegar (acetic acid) is just strong enough to perforate the egg shell (calcium carbonate) carrying with it a little iron in solution. After the 'acid has eaten through the shell, it reaches the white of the egg. This contains sulphur —as you will have noticed when smelling a bad egg. The sulphur combines with the iron’ in solution to form iron sulphide, which is black.