What is rich black, and when should I use it?

Black ink on its own is not as black as people expect. Across a large solid panel it looks slightly thin and washed out, because a single ink film can only absorb so much light. Adding the other three inks underneath deepens it considerably, which is why packaging with large black areas is almost always built rich.

The rule that matters for artwork is the opposite one, and it is where files most often go wrong. Small text set in rich black is printed by four separate plates. Any registration shift — and a fraction of a millimetre is normal — leaves coloured fringes around every letter, and at 8 or 9 point that turns crisp type into something that looks blurred. Body text, captions and any fine detail should be 100% K and nothing else, so only one plate is involved and register cannot affect it.

There is a practical ceiling too. Total ink coverage across all four plates has a limit before the sheet stops drying properly and starts setting off onto the sheet above it in the stack. A rich black recipe of 100 per cent in all four channels exceeds what most stocks will take. Prepress will adjust an over-inked black rather than run it.

On leaflets and low-GSM stock the ceiling is lower still, which is one reason pharmaceutical inserts are usually printed in a single colour.