printing plate

In more detail

A press does not mix colours the way a screen does. Each ink it carries has a cylinder of its own wearing a plate: a sheet that takes ink where the marks are and rejects it everywhere else. The paper passes under one plate after another, and the colours land on top of each other.

Splitting a page into one drawing per ink is called separating it. Ordinary colour separates into four; a job may add an ink the printer holds itself, on a plate of its own, and that ink is then written as a single figure — how much of it goes down.

What it does on a real document

A file made for a press states its colours in the inks the press lays down, so the split is settled before the job arrives. Where it is not, somebody at the printing works settles it for you, and the day their answer differs from yours the proof comes back wrong.

Some marks belong on one plate and no other: black laid over a photograph without a drop of ink reaching the three colour plates beneath it. Others belong on every plate at once — the crosses that let the printer lay the plates exactly over one another have to come out on all of them, or they line up nothing.

Where it comes from, and why

Colour printing has worked this way since lithography: one stone, later one plate, for each colour, and a set of marks in the margins to line them up by. The words a file uses are new; the arrangement on the press is a hundred and fifty years old.

See it on a finished document

Inks on plates of their own Several plates at once, each at its own strength The rules behind this word, and the texts they are read from