Ink and paper are the two raw materials used in the printing process. Both have been continually refined and improved by succeeding generations. So many combinations of ink, paper and process are now available that it is therefore increasingly important for the printer to choose the right one.
InkIn the fifteenth century, when Gutenberg first printed from movable type, the ink he used was made of boiled linseed oil with resin, soap and lampblack. Modern inks are made of pigments and binding materials (like resins) combined in a varnish with solvents. The ingredients can be varied to give certain desirable characteristics to the ink, such as improved drying or good rub resistance. Apart from all the different colours available, there are many different types of ink, corresponding to different printing processes, finishing requirements, toxicity restraints (as in food packaging) and drying methods.
Drying inkThere are four main methods of drying ink: evaporation, chemical curing, penetration and oxidation.
Evaporation is used when inks contain volatile solvents such as paraffin. Evaporation is a very fast and effective method of drying, and is used for 'liquid' inks such as those used for gravure or flexography.
Chemical curing : dries' ink by linking the pigment molecules in the ink thereby solidifying it. This is done by adding a catalyst immediately before printing to start the solidification process. A similar method uses ultraviolet light to 'cure' the ink.
Penetration is used with paper and board, where the ink dries by being soaked down into the paper, rather like blotting-paper. It obviously cannot be used on plastic or foil, which are nonabsorbent.
Oxidation is a method of drying where the ingredients absorb oxygen from the air. This causes the ink molecules to join together, so that the ink film slowly solidifies. It is a slow process and, when the printing is on paper or board, is used in conjunction with penetration.
Specifying inkUnlike paper, the type of ink is not normally specified by the client, but is chosen by the printer after consideration of the process being used, the machine, the paper and whether any special finishing operations such as varnishing or laminating are involved.
Four-colour process inks are normally made to a standard within a particular country, so that a four-colour set will give the same result regardless of the printer. It is vital that the repro house works to the same ink standard as the printer.
Special colours used to be mixed by hand by the printer to match a swatch supplied by the client. Now a more exact method is available in the form of the Pantone Matching System (PMb), a proprietary system consisting of a book of colour swatches with reference numbers which relate to inks that the printer buys ready-made or can mix to exact instructions. This has made the matching of colours a much more exact process.
Metallic inks contain fine metallic powders such as aluminium, copper or bronze to give a gold or silver effect, or produce metallic blues, reds, greens and so on.