Reading separations, and the fifth plate you did not order

What separations show you

A composite view shows the four inks combined and looks like the finished job. Separations show one ink at a time, and they reveal things a composite hides entirely.

Four questions that only separations answer:

1. Is my black text actually 100% K, or is it four colours?
2. Is my spot colour still a spot, or did it convert?
3. Are there plates I did not ask for?
4. Is anything set to overprint that should not be?

Looking at them

Acrobat Pro: Print Production ▸ Output Preview. Tick and untick each separation, hover to read ink percentages under the cursor, and set Total Area Coverage to a number (300%, say) to highlight every area over the limit in a warning colour.

Free, with Ghostscript:

gs -o sep-%02d.tif -sDEVICE=tiffsep -r300 job.pdf

You get sep-01.Cyan.tif, sep-01.Magenta.tif and so on, plus a file for every spot colour, named after the spot. Open them in GIMP or any viewer.

The file list alone answers question 3 before you look at a single image. Two plates expected, five files produced: something converted.

Free, for ink coverage:

gs -q -o /dev/null -sDEVICE=inkcov job.pdf

Prints four numbers per page as fractions of full coverage. Useful as a quick screen across a long document.

The black-text check

Open the Cyan separation and look at your body text. If you can read it, your black is not 100% K.

This happens in three ways, all common:

• Text drawn in RGB black (#000000) converted with a standard profile, which becomes a rich black of roughly 90/70/70/100 rather than 0/0/0/100.
• Text copied from another document carrying its own colour definition.
• A logo whose black was defined as rich black, then reused at 8 mm on a form.

The consequence, from the colour module: four plates never register perfectly, so small text built from four inks gets a coloured fringe. Fix it in the source by setting the text to a 100% K swatch, not by adjusting the export.

Total area coverage

Add the four percentages at any point. Presses cap the total, typically:

• Coated sheetfed: 300 to 330%
• Uncoated: 260 to 300%
• Newsprint and web: 240%

Above the limit, ink does not dry, sheets offset onto each other in the stack, and the press slows down or refuses.

Where does too much ink come from? Usually a rich black built too heavily (60/40/40/100 is 240%, which is fine; 80/70/70/100 is 320%, which is not on uncoated), or a dark photograph converted with a profile whose ink limit is higher than the press’s.

Acrobat’s TAC warning finds it instantly. Free, the inkcov output gives you a page-level total, and a conversion to the correct destination profile prevents the problem rather than detecting it.

The accidental fifth plate

Common sources, in order:

• An RGB object that nobody converted: often a logo pasted from a website, or a chart from a spreadsheet.
• A spot swatch used once, on a hidden or off-page object.
• A Registration swatch used as a black. Registration prints on every plate and is for crop marks only; a headline in registration black prints 400% ink and will not dry.
• A duplicate spot with a slightly different name: PANTONE 2925 C and Pantone 2925 C are two plates to a RIP.

The swatches panel in your layout application usually has a Select All Unused command. Delete them before export, and check the separations after.

Separations on a one-colour job

Worth doing even when you think it cannot go wrong. A “black only” job that separates into four plates is quoted as four-colour, and the difference on a 5,000-sheet run is substantial. The usual culprit is a logo supplied as an RGB or CMYK file and never converted to 100% K, or a greyscale photograph that carries an RGB profile.

The fix is upstream: convert the image to greyscale, set every swatch to K-only, and check again. A two-minute check that removes three plates from a quotation pays for itself immediately, and it is the kind of thing a client notices you doing.

The habit

Open the separations on every job that matters, look at four things (black text, plate count, spot names, overprint) and close them. Two minutes, and it catches the failures that produce a reprint rather than a complaint.