Oil-Based Printing Ink

Mix lampblack or soot into water with a little gum, and brush it onto three surfaces: unglazed wood, paper, and a piece of clean polished metal.

Expect it to sit evenly on the wood and paper, and to bead and streak on the metal.

Now press a sheet against the inked metal and look at the impression.

Record what you see. This failure is why Gutenberg could not simply borrow the ink that already existed, and it is a wetting problem, not a pigment problem.

Heat linseed oil gently and hold it hot, outdoors or under strong extraction, until it visibly thickens and strings from a stirring rod. Cool a drop on a cold plate to judge the body.

Expect it to become noticeably more viscous and slightly darker.

You are partially polymerising the oil — building longer molecules, which raises the tack and shortens the drying time.

Hot oil is a serious fire risk. Never fill the pot more than a third, keep a lid to hand to smother it, never add water, and do this outside on a stable heat source. Oily rags self-heat and can ignite — spread them flat to dry or drown them.

Mull lampblack into the cooled varnish on a slab with a muller or a flat stone, working it repeatedly.

Compare a barely-stirred sample with a thoroughly ground one by drawing both down on paper with a card edge.

Expect the ground ink to be denser, blacker and smoother at the same pigment loading.

Grinding does not make the pigment smaller so much as it separates clumps and wets each particle in oil. Unwetted agglomerates scatter light and look grey, and they also print as specks.

Make three inks: thin, medium and stiff, by varying how far you boiled the varnish or by thinning a portion.

Ink a metal surface with each and take an impression.

Expect the thin ink to flood the letter shapes and fill in enclosed counters, and the very stiff ink to skip and to pluck fibres off the paper.

There is a working window in between, and printers moved within it by season — stiffer in summer heat, softer in a cold shop. The ink is not a fixed recipe; it is adjusted to the conditions like a sourdough.

Put one inked sample in a sealed jar with as little air as possible, and leave an identical one in open air. Test both by touch every few hours.

Expect the open sample to set hard while the sealed one stays tacky far longer.

Nothing evaporated — the film took up oxygen and cross-linked. That is why a printed sheet becomes permanently dry rather than merely sticky, why the ink film sits on the surface rather than soaking through, and why the reverse of a page does not show through.

It also explains the oily-rag fire warning in step 2: the same reaction, in a crumpled rag, generates heat with nowhere to go.

Printing borrowed it from painting. The oil varnish that made metal type printable was already in use by Netherlandish painters, who had adopted oil binders for their depth and slow working time. Gutenberg's workshop did not discover a new chemistry so much as recognise that a painter's medium solved a printer's wetting problem. Most invention looks like this — a solution carried across a boundary rather than made from nothing.

The proof is still legible. Surviving copies of the 42-line Bible show a dense, glossy black that later centuries struggled to match, partly because the ink carried a high proportion of pigment and, analysis suggests, notable amounts of copper and lead. Whether that was deliberate or an artefact of the materials to hand is still argued. What is not in doubt is that the film has survived five and a half centuries without fading — a durability specification nobody wrote down and everybody met.

Why lampblack, of all pigments. Carbon is chemically inert, does not fade in light, is opaque in thin films, and is free — it is soot. Nearly every black in this corpus, from vine black to India ink, is the same element collected differently. For a material that must be legible for centuries, unreactive matters more than beautiful.

The idea generalises to every printing process that followed. Match the ink's chemistry to the plate's surface and to the drying you need: stiff oil paste for letterpress and offset, thin solvent inks for gravure and flexography that dry by evaporation at speed, UV inks that cross-link under a lamp in milliseconds. The lithography blueprint in this batch takes the principle further still, using an ink that adheres to grease and refuses water — the entire image is decided by ink chemistry rather than by shape.

Honest limits. Oil inks are slow: sheets had to be hung or interleaved, and set-off — one sheet marking the back of the next — was a constant hazard. They yellow slightly with age, most visible in whites and pale tints. And boiling linseed oil is genuinely dangerous, which is why varnish-making became a separate trade rather than something every printer did in the back of the shop.