Mixing ink is a small exercise in measurement followed by a large exercise in judgement. A formula gets you close, but colour on a slab, colour on the roller and colour on dry paper are three different things, and the gap between them is where most matching problems live. Understanding what the ink is made of makes the judgement much less mysterious.
Letterpress ink is a paste of pigment, vehicle and additives. The pigment provides the colour and its particle size and transparency decide how the ink covers. The vehicle — traditionally a varnish made from linseed oil or a resin — carries the pigment and controls how the ink behaves on the press. Additives adjust tack, drying and flow. Almost every problem blamed on "bad ink" is actually a problem with one of those three.
What is actually in the tin
| Component | What it does | Symptom when wrong |
|---|---|---|
| Pigment | Colour, opacity | Weak colour, or a shade that shifts as it dries |
| Varnish / vehicle | Carries pigment; sets the tack | Ink too stiff to transfer, or too soft and misting |
| Drier | Speeds oxidation of the oil | Ink still wet after 24 hours, or skinning in the tin |
| Reducer or extender | Lowers tack and strength | Ink bleeds at the edges, or prints grey |
| Wax or slip compound | Prevents set-off between sheets | Sheets stick in the stack |
Opacity is the property most often misunderstood. An opaque ink covers whatever is beneath it. A transparent ink lets the paper show through, which is why transparent white is used to lighten a colour without making it chalky. Mixing an opaque white into a colour moves it toward grey and flattens it; mixing transparent white keeps the hue alive and just reduces its density.
Mixing by weight, on a clean slab
Mix on glass or polished stone, never on paper. Work from a formula expressed in parts, and measure by weight rather than by eye, because a difference of 2 parts in 100 is visible in the finished print. A simple kitchen balance reading to 0.1 g is enough for a small tin.
- Start with the lightest colour and add the darkest to it, never the other way round.
- Add pigment in small increments and mix to a completely even paste before judging the colour.
- Compare the mixture against the target under the light you will actually use — daylight if possible.
- Roll a thin film onto a strip of the actual job paper and let it dry before making a final judgement.
- Record the parts by weight, the paper and the drying result. A formula without those notes is not repeatable.
The dried strip is essential. Most inks dry darker, some dry lighter, and a few shift hue noticeably as the vehicle oxidises — a mixed green can go bluer overnight, and a warm red can lose a little of its fire. Judging wet ink on the slab and printing a run on that judgement is how a job ends up the wrong colour by the following morning.
Tack, and why it changes with the press
Tack is the stickiness of the ink — the force needed to split a film between two surfaces. It matters because ink is split between the roller and the type, and then between the type and the paper. Ink that is too tacky will pick fibres off a soft uncoated sheet, leaving a snowy, mottled print. Ink that is too soft will mist, fill in fine lines and lose sharpness.
- Reduce tack with a reducer or a small amount of varnish. Add a little at a time; tack changes fast.
- Cold workshops raise tack. Ink that behaves in a warm room can pick badly on a winter morning.
- Soft, absorbent cotton stock tolerates tacky ink poorly. Dense, hard-surfaced stock tolerates it well.
- Roller hardness matters as much as the ink. Soft rollers transfer a heavier film and demand less tack.
If a print is picking, either reduce the tack or change the paper — not both in the same test. Adjust one thing, print five sheets, and look. Otherwise you will never know what fixed it.
Drying: oxidation, absorption, and time
Ink dries by two mechanisms at once. Part of the vehicle is absorbed into the paper, which is why an uncoated sheet dries faster than a coated one. The rest oxidises, hardening into a film. Oil-based inks rely mainly on oxidation and can take 12 to 24 hours to become genuinely dry to the touch, and several days to be safe for handling in a stack. Rubber-based inks set faster and stay open longer on the slab, which is why they suit a workshop that prints in short sessions.
Driers are metallic soaps that accelerate oxidation, and they are potent. A small excess will dry the ink on the roller during a run and skin the surface of the tin. Treat them as a controlled addition, not a general-purpose fix, and read the safety data sheet — many contain cobalt or manganese compounds, which carry their own handling requirements.
Cloths used to clean oil-based ink or to wipe linseed-based products can generate heat as the oil oxidises and have been known to ignite. Do not leave them crumpled in a bin. Spread them out to dry, or store them in a lidded metal container, and check local guidance for disposal.
Why the same formula prints differently twice
Colour matching fails for predictable reasons, and almost none of them are the formula. Paper colour and absorbency shift the apparent hue. Ink film thickness changes density more than any pigment adjustment will. Impression depth changes how much ink is transferred and how much spreads. Roller condition changes the evenness of the film. And the light you judge under changes what you think you see.
| Variable | Effect on colour | How to control it |
|---|---|---|
| Paper shade | Warm stock yellows a neutral; bright stock cools it | Always proof on the job stock |
| Ink film | Thicker film reads darker and more saturated | Set the ink bead and roller pressure, then leave them |
| Impression | Deeper impression spreads ink and lightens the edge | Fix the packing and the impression before matching |
| Drying | Most inks darken; some shift hue | Judge the dried strip, not the wet slab |
| Light source | Warm light hides blue; cool light hides red | Match under the light the job will be seen in |
The practical rule is to fix everything that is not colour before you start mixing, then adjust only the ink. If the press settings are still moving while you chase the shade, you are chasing two targets at once and you will not catch either.
Matching to a reference
Commercial colour reference systems give a numbered target with a printed sample, and they are the standard way a customer and a printer agree on a colour. What they cannot do is guarantee a match on every stock, because the reference is printed on one specific paper with one specific ink film. Treat a reference as a target and a tolerance, not a promise, and say so before the job starts.
For critical work, mix slightly heavy and let the press operator reduce with a small amount of transparent white or varnish as needed, rather than mixing exactly to target and then having no room to move. It is easier to lose a little density than to add pigment to a full tin.
Colour also changes with the size of the printed area. A small block of colour reads darker than the same ink laid over a large field, because the eye judges a large area against more surrounding white and compensates differently. Where a job contains both, check both on the same sheet before approving the mix.
Keep a dried printed strip for every mixed colour, labelled with the formula, the paper and the date. Two years later that strip is the only way to reproduce the job without starting from scratch.
You are not matching a colour. You are matching a colour on a paper, at a film thickness, under a light. Get those three wrong and the formula is irrelevant.A printer, on a reprint that came back the wrong shade
Mix carefully, print a test, wait for it to dry, and judge it in the room where it will be used. The formula is the beginning of the job, not the end of it.
