Invisible Ink

Inks developed by heat

Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of 'heat fixed' ink, any acidic fluid will work. As a rule of thumb, the most secure way to use any particular ink is by diluting it - usually with water - near to the point when it begins to get difficult to develop.

* Cola drink (diluted)
* Honey (diluted)
* Lemon, apple or orange juice
* Milk
* Onion juice
* Semen - Sir George Mansfield Smith-Cumming, head of the British Secret Intelligence Service noted of his agents that "Every man carries his own stylo".
* Soap water
* Sugar solution
* Urine
* Vinegar or wine
* Cobalt chloride turns blue when heated and becomes invisible again after a while (if not overly heated)


The writing is made visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100 W light bulb is less likely to damage the paper.