Liquid inks are employed in gravure and flexo printing, while paste inks are used in letterpress and lithography. Screen inks are intermediate between paste and liquid inks.
Basically, all printing inks are made up of
. colourant . vehicle (or varnish) . solvent . additives
ColourantsColourants are grouped into
In printing inks, pigments are used almost exclusively, save with flexo inks
Pigments, by their chemical nature, are further divided into
Furthermore, there are metallic pigments, pearlescent pigments, fluorescent pigments, and others more.
Pigments are usually referred to by their Colour Index name or formula number (e.g. P.Y. 12, CI No. 21090 = Pigment Yellow 12, formula number 21090).
Dyes (or Dyestuffs)
are soluble in the material they are used in. They are rather scarce in printing inks, but they are of some importance in flexo inks and for some special applications, e.g. heat transfer printing, invisible (i.e. fluorescent) inks, and cheque security inks.
Some of these dyestuffs, called basic dyes, are quite popular because of their high tinctorial strength and brillant shades, but they soon fade when exposed to light. They also need some mordant, which helps to make the prints more resistant to water.
As basic dyes usually are soluble in water, they are a bit more frequent in water-based inks then elsewhere. One of the most popular dyes is eosine, which is well known from the teachers' red fountain pen ink; it also shows a strong yellow-green flourescence. (Explain, why the usually visible and fluorescent inks are this way round, while the other would be impossible.)
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