Specks in presswork - ink in the non-image area

Press operators are often seen bent over a press sheet examining it under a loupe. One of the things they should be looking for, but often miss, are specks of ink appearing in the non-image area of the presswork.A 20x enlargement of solid ink patches showing small speckles in what should be unprinted paper. The pale thin lines are paper fibers.
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A 200x enlargement of the above image showing the small speckles of ink more clearly. The pale thin lines are paper fibers.
(Click on image to enlarge)
There are several possible causes of this problem.

Tinting (also called toning). This is caused by contamination of the fountain solution by either ink, or some coloring matter from the ink. Since fountain solution is all over the non image area, any coloration will be likewise. It is usually caused by the fountain solution breaking down the ink but it can also be caused by the plate. Usually though, tinting will appear more like a very pale wash of color over the non-image area rather than discrete specks of ink on an otherwise clear background.

Redeposit. This occurs when specks of developed/removed coating are re-deposited onto the plates later in the processing cycle. It's typically due to dirty rollers or contaminated rinse water, but can be exacerbated by hard water in the rinse or improper exit roller pressures (allowing more developer to carry-over into the rinse). These specks of coating adhere to the plate and accept ink and print on press.

Incomplete processing Problems with the mechanics of the plate processor like bad brushes and/or pressure may not scrub the plate well enough to remove the particles of coating from the unexposed areas of the plate. Typically though, there would be a more general toning in those cases (but not always).

In general, if the specks appear only in one color then that press unit is more likely the cause of the specks and it's also more likely that the problem is tinting/toning. However, if the specks appear in all four colors then it is more likely that the plates are the cause of the problem and it's important that the press operator inform prepress about the issue.

From the print buyer's point of view, there will likely always a few specks appearing in the non-image areas of presswork. If this is a critical concern, as in security printing, then it is best to discuss the issue with the print supplier and perhaps agree to what would be an acceptable number of specks per square inch/centimeter.