The Papers used in Printing.
Generally speaking one can make a distinction between paper that will allow, when printed, for a relief, due to the plate used in printing ( intaglio plates) and the paper that is used for printing planographic work. The former is "tallle-douce" paper. This paper will bear the "plate mark" and will be pressed into the intaglio lines of the plate. In fact, all of the papers which are used for embossing or goffering belong to this category. T aille-douce paper must be used when damp and must bear up under the dampening and soaking procedures. The second type of paper, which is used in planographic work, serves the purposes of lithography, serigraphy, offset and, to a certain extent, woodcut.
1. PAPER FOR ENGRAVINGS:. The paper used for printing engravings must always have specific characteristics, above all, because it is compressed by the press and will only print correctly if the paper penetrates the lines of the plate. This paper must therefore be very elastic. It must take well to being compressed and have an homogenousness and a fairly strong body ( i. e .a fairly low density in comparison to its mass) .The paper surface must be soft to the touch but it must not be finished nor, above all, must it be coated. In selecting a paper to print on, the printer must choose in function of the relief he will need as determined by the plate to be used. Quite obviously finely etched plates will not require the same kind of paper as a deeply cut engraving. Fine etching requires soft, silky, and light paper whereas engraving calls for a heavier, more resistant, and elastic sheet of paper. Furthermore, one must bear in mind the thickness of the plate itself since a one millimeter thick plate ( 0.039 in ) can easily be absorbed by light laid paper, but if the plate is thicker one must have recourse to a thicker and more resistant sheet of paper. The smoothness of the paper to be used must be chosen in function of the type of lines to be printed. Very fine lines must be printed on a very soft and homogenous surface such as a satin finish laid paper. Thicker lines can be printed on slightly grained paper.
Paper used for printing engravings must be very absorbant, permeable, only slightly sized and never coated. This is because engraving paper must often be dampened in the printing process. Their wettability must be such that they will resist traction and compression when damp, and that they will go back, to their former state after drying. It is not absolutely necessary that the paper be very thick but the pulp used must be well mixed. Pure rag paper is the paper that will take best to dampening. It might be mentioned that rice paper, Japan paper and some Dutch papers ( in particular the Van Gelder (koperdruk) vellum paper for intaglio printing) are extremely absorbant and therefore should be dampened with a sponge. The new types of paper containing pulpex can, on the other hand, be used for intaglio printing without being dampened even though they take well to dampening .
Since the paper used for printing engravings must show up colours without artefice or brutality it is best that it be of a natural white colour not unlike the creamy white of linen or cotton. The different whites obtainable in rag paper are subtly varied and thus the printer will have to choose the tonality best suited for the engraving to be printed.
2. PAPERS USED IN PLANOGRAPHIC PRINTING. Although this paper does not have the same characteristics as the papers used in printing intaglio plates it must still be chosen in function of certain criteria.
The paper used in lithography must be absorbant in order that it take on both the oily inks and the water. Usually such a paper has a mat finish which is not at all fluffy. It should also have a very even satin finish since a grain that is too pronounced will refuse solid colour applications and may damage the half tone areas. In the past litho paper was laminated after being dampened so as to polish the surface. This kind of work was done sheet by sheet between two polished zinc plates. After this calendering the sheets of paper were pressed in packets of 25. In chromolithography the sheets were calendered in order to give the greatest possible precision in registering. Coated and sized papers were much used in commercial lithography as well as simili Japan paper, which is smoother and more rigid than real Japan paper (the latter being preferred for quality work). For map printing a special type of paper was used which was insensitive to humidity.
In offset printing the paper used must be both well squared ( in the direction of the machine) and very flat. Furthermore, such paper must have an average humidity when used and, although it must be absorbant, it must not be porous. Actually the more absorbant varieties of paper are not the ones that are more permeable on the surface. The "chromecoat", for example, is extremely smooth and yet very absorbant while blotting papers are less so. This is due to the fact that the capillary action of well laid paper is finer but greater and thus it takes an ink more readily than other papers. Offset paper must not be magnetized nor must it be very acid.
In serigraphy the levelness of a sheet of paper is a fundamental requirement. An irregular surface or a surface that is too grainy will cause problems in printing. The paper is chosen in function of the type of ink to be used ( a great variety of them are used in serigraphy! ) .Water base inks require a thick paper, very slightly sized if not at ail, which must in no way be coated. The paper must be capable of good absorption but must also dry without being deformed. The papers used for oil base inks should be chosen in function of the effect to be obtained: absorbant mat finish paper will give a mat finish whereas heavily sized smooth paper which is also coated will result in a shiny finish. Shiny inks must be printed on shiny papers.