Invisible ink types

For practical reasons, the inks are listed here according to their method of development. It must be understood however that some inks - particularly those of organic origin or those consisting of a mixture of several chemicals - may be made visible by several methods. For example, invisible writing with soap water may be made visible either by heat, reaction with phenolphthalein, viewing under ultraviolet light, or by placing the page inside an iodine fume cupboard.

Inks developed by heat

Detail of a software user manual security feature made with invisible ink.

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. The most secure way to use any of the following substances for invisible ink is by dilution, usually with water, close to the point when they become difficult to develop.

Cola drink

Honey solution, sugar solution (sugar turns into caramel by dehydration)

Lemon, apple, orange or onion juice (organic acids and the paper forms ester under heat)

Milk (lactose dehydrates)

Bodily fluids such as blood serum.

Soap water (carboxylate partially oxidises)

Wine, or vinegar

Cobalt chloride, which turns blue when heated and becomes invisible again after a while (if not overly heated)

The writing is rendered visible by heating the paper, either on a radiator, by ironing it, using a hair dryer, or by placing it in an oven. A 100-watt light bulb is less likely to damage the paper.