Offset lithography works on the concept that oil and water don’t mix. An offset printing plate can have the image area and non-image area on the same flat surface (unlike the recessed image areas of intaglio printing or the raised image areas of relief printing) because of the immiscibility of oil and water.
For oil-based custom printing inks to adhere to image areas of the printing plate (for transfer to the press blanket and from there to the paper) and not to non-image areas, the ink/water balance must be correct. Printers learn how to do this. If the balance is not maintained (or, more specifically in this case, if there is not enough water), ink will print in non-image areas. For instance, tails of ink may appear to streak outward from the letterforms of type. Or streaks of ink may run down the press sheet. This is called scumming. Watch for it (check the samples your printer pulls every so often during the press run as a quality check, if you attend a press inspection). Your printer will be checking this ink/water balance throughout the commercial printing process.
Doubling and Slurring
As the press rolls, the plate transfers the image to the rubber press blanket, and then the blanket transfers the image to the paper substrate (hence the term “offset,” because the plate does not print directly on the paper). If the blanket hits the paper a second time (perhaps a light touch), the halftone dots will appear to blur. This is called dot doubling.
If there is a problem with inaccurate press blanket pressure or poor ink tack (the stickiness of the ink that allows it to pull off the plate onto the blanket and off the blanket onto the paper substrate), halftone dots can become elongated. This is called slurring.