Scumming

Also Known As: Greasing, catch-up (specifically when caused by low water), dry-up, plate scum, tinting (closely related but chemically different), background tone

In simple terms: the parts of the printing plate that were supposed to stay clean failed to repel the ink, so the background got dirty.

What causes scumming?

Lithographic printing (offset) relies on the principle that oil (ink) and water do not mix. The non-image area is chemically treated to hold water and repel ink. Scumming happens when this barrier fails.

1) Water Starvation ("Catch-up" or "Dry-up")

If the dampening system doesn't supply enough water to the plate:

• The non-image areas dry out
• Without the protective water film, ink invades the open spaces
• This usually creates streaks or patches of ink in the margins or trailing edges

2) Plate Sensitivity / Wear

If the printing plate is old, worn, or poorly processed:

• The "desensitized" layer (the part that holds water) wears off
• The bare aluminum underneath starts grabbing ink
• This creates a permanent dirty spot that appears on every sheet in the same place

3) Improper Chemistry (Fountain Solution)

The water in a press isn't just water; it's a chemical mix (fountain solution).

• If the pH is wrong or the conductivity is too low, the solution can't keep the plate clean
• This causes a generalized "scum" or all-over tint

4) Too Much Ink / Soft Ink

If the ink is too soft (low tack) or the press operator runs the ink density too high:

• The ink emulsifies (mixes) into the water
• The "dirty water" then washes over the whole plate, leaving a faint tint everywhere

5) Excessive Pressure

If the rollers press too hard against the plate:

• Friction generates heat
• The friction scrubs away the plate's protective coating, leading to mechanical scumming

How to identify scumming in a book

What it looks like

Look for:

• Dirty Margins: Streaks or splotches of faint color in the white borders
• "Foggy" Backgrounds: A photo or text box that should have a clean white background looks grayish or muddy
• Shadows in Type: Fine loops in letters (like the inside of an "e" or "a") start filling in with faint ink, not because of "fill-in" (spread), but because the plate is dirty

Where it shows up most

• Page Edges: Trailing edges where water control is hardest
• Large White Areas: Any open space where the defect has nowhere to hide
• Uncoated Paper: Rough paper generates more dust/lint, which can trigger scumming
• Coated vs. uncoated: The difference in surface holdout between coated and uncoated stocks affects how scumming appears and how visible the background contamination becomes

Simple at-home checks

Check A: The "Eraser" Test

(Note: This works on the plate, but for a printed book, you can try a gentle rub with a clean vinyl eraser).

• If the dirt wipes off the paper easily, it might be loose dust
• If the dirt is firmly printed into the fibers and doesn't erase (without destroying the paper), it is scumming

Check B: The "Loop" Check

Look at the margin with a magnifier.

• Scumming often looks like tiny, irregular specks or a "greasy" texture, unlike a smooth gray screen

Check C: The "Consistency" Check

• Does the dirty patch appear in the exact same spot on the next 10 pages?
• If yes, the plate is scumming in that specific area

Common look-alikes (and how to separate them)

1) Tinting

Tinting is when ink bleeds into the fountain solution, turning the water slightly colored.

• Tinting: Usually a very uniform, light wash over the entire page (like a weak colored filter)
• Scumming: Often streaky, patchy, or localized to specific areas

2) Set-off

Set-off is ink rubbing from the sheet below.

• Set-off: You can often see a mirror image of the text from the facing page (or the back of the previous sheet)
• Scumming: Random dirty patterns, not readable text reflections

3) Toner Scatter (Digital)

In digital printing, loose toner dust can land in white areas.

• Toner Scatter: Looks like fine distinct specks of pepper
• Scumming: Looks more like a smear, wash, or connected grease mark

4) Dirty Paper

Sometimes the paper itself has recycled flecks.

• Paper: The flecks are inside the paper fibers
• Scumming: The ink is sitting on top of the fibers

Impact on book quality and readability

Readability

• Contrast: Scumming lowers contrast by darkening the background. It makes the page look "muddy" and harder to read
• Legibility: Severe scumming can fill in the counters of small text

Image quality

• Highlights: The first thing to go. Bright white highlights in photos become dull and gray
• Cleanliness: The book feels "dirty" or "used," even if it is brand new

Perceived quality

Scumming is a hallmark of "rushed" printing. It implies the press operator didn't stop to clean the plates or balance the water. It looks cheap.

Industry standards and "acceptable tolerances"

Usually acceptable

• Start-up sheets: Sometimes the first few sheets off a press have scumming (catch-up) before the water balance stabilizes. These should be thrown away, but if one slips in, it's a random error
• Micro-tinting: Extremely faint tinting on newsprint or low-grade paper is often tolerated

Usually not acceptable

• Visible dirty streaks: Any distinct streak in a margin is a defect
• Muddy photos: If the "white" points in a photo are gray/colored
• Consistent grease: If every page in the book has a dirty background

What you can do as a buyer

• Check the Margins: This is the easiest place to spot it
• Report it: If the book looks dirty or "grayed out"