How Is Ink Made? Getting Colour to Stay Where It Is Put

Every ink is a colourant dispersed or dissolved in a vehicle, with additives adjusting behaviour:

• •A pigment, an insoluble solid ground extremely fine and held in suspension, which generally resists fading better because the crystal protects the molecules, or a dye, dissolved in the liquid, which gives brighter and more transparent colour and fades faster
• •A vehicle, which carries the colourant and then leaves, by evaporation in water and solvent inks, by absorption into the substrate, or by drying through oxidation and polymerisation in oil-based printing inks
• •A binder, which holds the colourant to the surface once the vehicle has gone, and is what separates ink from a suspension that would simply rub off
• •Surfactants and wetting agents, controlling how the ink spreads on a surface and whether it feathers along paper fibres
• •Humectants in inkjet inks, which stop the nozzle drying between uses, and biocides, since water-based ink is a growth medium
• •Viscosity modifiers, since a fountain pen, a ballpoint, a screen press and a newspaper press each require a completely different flow behaviour

The two historically dominant writing inks work in opposite ways. Carbon ink, used in China and Egypt from at least the third millennium BCE, is simply fine soot suspended in water with a gum binder, which sits on the surface without reacting. It is essentially permanent, since carbon does not fade or change, and it is not waterproof and can be washed or scraped off, which is why it suited papyrus and why documents written in it could be corrected. Iron gall ink, dominant in Europe from roughly the fifth century until the twentieth, is made by reacting tannic acid extracted from oak galls with iron sulphate, producing a soluble complex that darkens on exposure to air as it oxidises. It penetrates the fibres and binds chemically rather than resting on top, which makes it extremely difficult to erase and is exactly why it became the standard for legal and financial documents. The cost is that the ink is acidic and contains free iron, which catalyses the breakdown of cellulose, so over centuries the ink corrodes through the paper, a conservation problem affecting an enormous body of manuscripts including Leonardo's notebooks and Bach's autograph scores.

Gutenberg's real innovation included the ink as much as the press. Water-based manuscript ink beads up on metal type and transfers badly, so a printing ink had to be oil-based, viscous, tacky and quick to set. The solution, adapted from oil painting practice, was lampblack ground into boiled linseed oil, sometimes with resins, producing a stiff paste applied with leather balls and later rollers, which wets metal, transfers cleanly under pressure and dries by the oil oxidising and polymerising into a solid film. Modern printing inks are specialised by process: offset lithography uses tacky paste inks that must also interact correctly with water, since the process relies on the printing areas accepting ink and the non-printing areas accepting water; flexography and gravure use fluid inks drying by evaporation; screen printing uses thick inks forced through a mesh; and ultraviolet curing inks contain photoinitiators that polymerise the whole film in a fraction of a second when illuminated, which allows printing on non-absorbent materials.

Three consumer inks each solved a specific mechanical problem. The ballpoint, made practical by Laszlo Biro in the late 1930s, required an ink thick and slow-drying enough not to leak past the ball and not to dry in the tip, achieved with an oil-based paste that dries mainly by absorption. The rollerball uses a liquid water-based ink that flows more freely and writes with less pressure at the cost of soaking into paper more. Inkjet printing demanded something harder still: an ink that survives being heated to hundreds of degrees in microseconds or fired by a piezoelectric pulse, forms a consistent droplet, and dries almost instantly on contact without clogging a nozzle measured in microns, which is why inkjet inks are complex formulations and why the printers are frequently sold below cost to be recovered on cartridges. Security inks form a category of their own, including inks that change colour with viewing angle, fluoresce under ultraviolet light or react to solvents, used on banknotes and documents precisely because reproducing them is harder than reproducing an image.

Ink is a pigment or dye in a vehicle with a binder, and the vehicle must carry it and then leave by evaporation, absorption or oxidation. Carbon ink is soot in gum, permanent but removable; iron gall ink binds chemically into the fibres, which made it the standard for documents and also means its acidity and free iron slowly corrode the paper. Printing required an oil-based tacky ink that wets metal type, which was as much of an invention as the press itself.