Pictures and paper in harmony
The surface of the paper is crucial to picture quality – the smoother, whiter and glossier it is, the easier it is to obtain an image close to the original photograph. Of course, good picture printing is not merely a matter of maximum likeness to the original. You can print images on all types of paper surfaces, as long as you know what you want to achieve, if you do things right from the start, and know the scope and limitations of the job.
The finer the screen the more detailed the picture and its nuances. But this applies only up to a point. On an uncoated paper the dots swell more (dot gain) and become less even around the edges than on a coated paper with a uniform, smooth surface. So when printing on uncoateds the gaps between the dots have to be bigger, i.e. the screen must be coarser, otherwise the dots will merge and the image will be murky.
A multi-coated paper can handle screens of more than 200 lines/inch (80 lines/cm), while you should not exceed 120 (48) or 133 (52) for uncoateds (depending on surface smoothness).
Ink spreads more evenly on the smooth surface of a coated paper than on the rougher surface of an uncoated paper. A common misapprehension is that uncoateds need more ink because they absorb more. But that is a sure path to dark and murky images and longer ink drying times. The way to improved picture printing quality on uncoateds is to increase back cylinder pressure. For fully coated papers the pressure should be 0.15 mm. For uncoateds it should be 0.25 mm.