The first option you can find on Internet, if you don't want to make the invisible ink by yourself, is a kit made by 2 pens: an invisible ink pen, that allows to write with a particular ink invisible to our eyes and an UV-pen, a normal blue pen that, by one side, writes as a normal pen and, by the other, it contains a little UV-lamp that makes visible the ink.

This invisible ink could be made by organic substances that, when excited by an UV-light source, emit a visible blue light, with a higher wavelength (> 400 nm).

Some sites present them as products for students who desire to have "easy life" at their written exams with paper sheets or dictionary pages full of information invisible for professors.

HP (Hewlett Packard) offers on the market a invisible ink printer for reserved and delicate documents; the text can be seen only under an UV lamp, with a bright red colour.

In many other cases, there are 3 kinds of invisible ink we can also make at home and use in fountain pens, for example:

1) The invisible ink, after writing, is made visible with some heating of the paper sheet,
The drying process causes the concentration or the oxidation of the ink that assumes a dark colour.
Apple juice, diluted Coca-Cola, red wine or vinaigrette can be used and developed by a hot iron.

A traditional invisible ink is a cobalt (Co) salt solution (CoCl2, CoSO4, Co(NO3)2 ) that are red and transparent at room temperature but not visible when on the paper, in the very thin layers typical of writing.
These inks become blue and visible when the paper sheet is heated, as typical of cobalt salts, when their solutions are concentrated by heat, because the Co++ ions emit a different light when they lose part of the H2O molecules bound to them at the crystalline state or in concentrate solutions.

2) Other inks become visible when some chemical products, like acids or bases, are sprayed on the paper to develop the ink, turning it into a visible compound.
Some invisible inks are vinaigrette, CuSO4 or FeSO4 solutions, NH3 solutions, starch, lemon juice and they can be developed by NH3 and Na2CO3 solutions.
Given that I'm a chemist, I can briefly explain you some of the chemical reactions involved. These solutions, in fact, make visible Fe and Cu as hydroxides or carbonates, gray and brown for Fe(OH)2 and Fe(OH)3, respectively and blue or green for Cu++, or dark-blue complexes between NH3 and Cu++:

CuSO4 + Na2CO3 -> CuCO3 (solid, blue) + Na2SO4

CuSO4 + 4 NH3 -> Cu[(NH3)4]++ SO4- (blue soluble complex salt

Instead, FeSO4 precipitates as grey hydroxide, Fe(OH)2 (but the air oxidizes it to Fe(OH)3, brown-red ), with NH3 and Na2CO3 solutions because these are basic:

FeSO4 + 2 OH- -> Fe(OH)2 + SO4-
(OH- coming from the alkaline hydrolysis of the carbonate or from the weak basic reaction of NH3).

Another invisible ink is AgNO3, whose silver insoluble salts (white) tend to decompose and form dark gray metallic Ag:

AgNO3 + NaCl -> AgCl (white, insoluble) + NaNO3

2 AgCl -> 2 Ag (metallic, black micro-powder) + Cl2

3) The substances revealed by UV or IR light can be used on paper, glass and on plastic surfaces.
You can find them in commerce and are organic chemicals, covered by patents; so, it's difficult to identify them and their chemical action.

Learn more about this author, Aldo Bonincontro.
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