Editing Invisible ink (section)

=== 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. 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 (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 [[Blood plasma|serum]].
* Soap solution ([[carboxylic]] partially oxidizes)
* Wine, or vinegar
* [[Cobalt(II) chloride|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.