Perfectly clean, and especially unsized paper, refuses color like the prepared stone, when it has been wetted thoroughly so that it is saturated. But here, too, mere water is not a complete preparation. Under strong pressure it is forced away readily from the paper, the printed places are dried and the color adheres. If the pressure is not sufficient to force all the water away, the impression will be imperfect. The tougher the printing-color is, the more will it resist the dampness and the greater must be the pressure. Experience has taught me the following:-- (1) Every paper not spoiled with fat will permit itself to be prepared, like the stone, with water so that it will take no color. In the case of entirely clean, unsized paper, water alone is sufficient. Mucous, gummy, and acid substances increase its power. Unsized printed paper need merely be dipped in water, laid on a stone, and coated with oily color, and the printed parts will all take the color while the rest of the paper remains white. (2) Any great pressure will remove this preparation and the whole paper will take color. (3) The oil color must be very thin and fluid, because a tougher one will take hold of the fibres of the paper and tear them off. The foregoing experiences applied to the theory of the print itself lead to the following conclusions:-- (_a_) The paper to be used for printing must never be too wet, because the most powerful pressure could not remove the water sufficiently. (_b_) Paper that is too wet is prone to adhere to the stone with its printed parts, which are likely then to tear away easily, thus damaging or ruining the work. This happens the more readily if the pressure be not sufficient. If the scraper or the stone is not uniform and even, it is very prone indeed to tear at the places subjected to the least pressure, because there, where the water has not been sufficiently squeezed away, the paper remains soft and fragile, while the pressure still is great enough to grip the fibres of the paper. (_c_) Therefore the paper must be only slightly dampened if the color is very tough, partly to prevent tearing, partly to oppose no undue obstacle to the reception of the color. (_d_) Paper dampened too much stretches in printing and produces uneven and dirty impressions. (_e_) The quality of the water is not important so long as it is not dirty or putrid, in which latter case it may infect the paper and rot it. (_f_) Just how much the paper must be dampened can be learned only from experience, because papers vary very much and in the case of sized papers it depends chiefly on the kind of sizing. On the average, we may calculate one wet sheet to eight dry ones in sized papers and one wet one to ten or twelve dry ones in unsized papers.