Dampening of paper.
Printing poses two different problems insofar as paper is concerned.
The first one concerns the proper distribution of ink on the sheet of paper when pressure is exerted by the press. If this is not the case the ink will only show up on the top of the uneven surface while any hollows will stay white. In the past high-pressure printing was almost always used since the paper was quite stiff and showed the lines left by the wire cloth and the felts. The paper used for printing woodcuts was dampened for much the same reason. Unfortunately the uneven dampening of the paper was such that the sheets turned out to be of all different sizes and this made the registering process very complicated.