Editing Invisible ink (section)

== 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 ===
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.

=== Inks developed by chemical reaction ===

In most cases, these substance changes color when mixed with an [[acid]] or [[base (chemistry)|base]].

* [[Phenolphthalein]], commonly used as a [[pH]] indicator, turns pink in the presence of a base such as [[ammonia|ammonia fumes]] or [[sodium carbonate]].
* [[Vinegar]], is revealed by red cabbage water<ref>[http://chemistry.about.com/library/weekly/aa012803a.htm Red cabbage water] {{Webarchive|url=https://web.archive.org/web/20090124193908/http://chemistry.about.com/library/weekly/aa012803a.htm |date=2009-01-24 }}</ref> Vinegar contains [[acetic acid]] that affects the pH indicator in red cabbage water.  Vinegar may also be developed by heat.
* [[Ammonia]], developed by red cabbage water.
* [[Semen]], developed {{By what|date=November 2024}}.<ref>{{Cite web |date=28 October 1999 |title=''The Independent'' review of ''The Quest for C: Mansfield Cumming and the founding of the British Secret Service'' by Alan Judd |url=https://www.independent.co.uk/arts-entertainment/books/reviews/the-spymaster-who-was-stranger-than-fiction-737707.html |url-status=live |archive-url=https://web.archive.org/web/20190831124549/https://www.independent.co.uk/arts-entertainment/books/reviews/the-spymaster-who-was-stranger-than-fiction-737707.html |archive-date=2019-08-31 |access-date=2017-09-11 |website=[[Independent.co.uk]]}}</ref>
* [[Copper sulfate]], developed by [[sodium iodide]], [[sodium carbonate]], [[ammonium hydroxide]] or [[potassium ferricyanide]].
* [[Lead(II) nitrate]], developed by [[sodium iodide]].
* [[Iron(II) sulfate]], developed by [[sodium carbonate]] or [[potassium ferricyanate]].
* [[Cobalt(II) chloride]], developed by [[potassium ferricyanide]].
* [[Iron(III) sulfate]], developed by [[sodium sulfide]].
* [[Starch]], developed by [[iodine]] solution which turns starch dark blue and the paper light blue.
* [[lemon|Lemon juice]], developed by [[iodine]] solution (ink turns white, paper turns light blue).
* [[Sodium chloride]] (common table salt), developed by [[silver nitrate]].
* [[Cerium oxalate]] developed by [[manganese sulfate]] and [[hydrogen peroxide]]<ref>{{cite web |url=https://www.sciencedaily.com/releases/2006/11/061108103815.htm |title=Cold War Invisible Ink Secrets Unlocked |website=ScienceDaily |access-date=2014-06-10 |url-status=live |archive-url=https://web.archive.org/web/20140320203148/https://www.sciencedaily.com/releases/2006/11/061108103815.htm |archive-date=2014-03-20 }}</ref><ref>{{cite journal |doi=10.1021/ed2003252 |title=Invisible Ink Revealed: Concept, Context, and Chemical Principles of "Cold War" Writing |journal=Journal of Chemical Education |volume=89 |issue=4 |pages=529–532 |year=2012 |last1=MacRakis |first1=Kristie |last2=Bell |first2=Elizabeth K. |last3=Perry |first3=Dale L. |last4=Sweeder |first4=Ryan D. |bibcode=2012JChEd..89..529M }}</ref>

=== Inks visible under ultraviolet light ===
[[File:20 USD in UV light.jpg|thumb|A US$20 bill showing a visible stripe under an ultraviolet light]]
Some inks glow faintly ([[fluorescence|fluoresce]]) when under an [[ultraviolet]] lamp. This is a property of many substances, particularly organic substances and body fluids.

Other inks work in a near opposite way by {{em|absorbing}} ultraviolet light but without fluorescing. When these are used on [[fluorescence|fluorescent]] paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint.

Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.

Examples of inks revealed by ultraviolet light are:

* [[Laundry detergent]]s containing [[optical brightener]]s
* [[Soap]]
* Body fluids, [[Blood plasma|serum]], [[saliva]]
* Sunscreen
* Lemon juice

=== Inks which alter the surface of paper ===

This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will alter the paper surface fibers or sizing.

Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the altered areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution.

Slightly dampening paper with a sponge or by steam and then drying it before writing a message will prevent writing from being developed by this method, but overdoing dampening will result in telltale paper cockling.