Paper and Its Uses/Chapter 9
CHAPTER IX
COATED PAPERS AND BOARDS
Coated papers comprise those to which, after manufacture as paper, a mineral coating, white or coloured, is applied, in order to produce a smooth unbroken surface for the reception of fine printed work. Art, chromo, enamel, and surface-coloured papers are all coated after the body paper is made.
Art papers may be made of rag, esparto, chemical wood, or chemical and mechanical wood, or a mixture of any of the fibres. The body paper is carefully made, its ultimate state being kept in mind, and it is fairly well sized, but without a high glaze. The surface is kept so that the coating will cover properly and the adhesive be fully effective in holding the mineral. The operations comprise coating, drying, and finishing. The coating is carried out on a compact machine. A mixture of china clay, glue, and water is supplied at a constant level to the feed trough of the machine, from which it is transferred to paper by means of a roller and felt; oscillating and stationary brushes rub the coating into the paper, filling up all inequalities and leaving a smooth film on the surface. The purpose of the coating is to give a perfectly smooth surface, obliterating entirely the marks of the machine wire and felts, and to do this effectively the consistency of the mixture is regulated so that it may enter the minute depressions and deposit sufficient matter to take a good finish. An ingenious overhead railway carries the web forward in a series of loops supported on a series of rods, hot air driven forward by mechanical fans effecting the drying. If the paper is two-sided art, it is reeled and the operations repeated on the other side of the paper. As the coating is slightly thicker at the edges of the web, these edges are trimmed off, and the web goes forward for one or more journeys through the super-calender rolls. Dull art and papers with a specially high finish receive slightly different treatment, the surface in all cases being made perfectly smooth in order that the finest half-tones may be printed successfully.
Chromo papers are usually coated on one side only, and the body paper is stouter than that used for art papers. Used largely for lithography, the paper must be as free from stretch as possible. This is obtained as described in the chapter on the reduction to pulp, by using soft fibres, sharp beater knives, and cutting up quickly, this treatment producing what the papermaker knows as "free" pulp, as distinguished from "wet" pulp, which, owing to prolonged treatment, combines with some of the water and actually becomes "wet." The surface of chromo papers may be dull or highly glazed.
Surface coloured enamelled papers are used largely by box-makers, for labels for packets of various commodities, and also as end papers for books. The coating and body paper are thinner than for art papers, the colour is obtained by the use of a pigment or an aniline colour, and the coating and after-treatment are exactly as in the case of art papers. Flint-glazed surface papers are used for the same purposes as surface-enamelled papers, and have a hard burnished surface obtained by a stone burnisher travelling wards and forwards across the surface of the paper as it emerges from the calender rolls.
Boards may be coated in the same way as paper, provided the boards are not too thick. The thicker qualities are either coated on a modified machine, the looping being impossible, or coating by hand is resorted to. The boards are obtainable as one- or two-sided, with different degrees of surface, and with different coloured coatings. Coated boards are sometimes made by pasting coated papers to ordinary middles, and finishing by plate rolling.
Thin box boards for use as cartons for small goods, such as cigarette packets, are coated with a coloured coating in the manner already described.
Coloured cloth-lined cards are first manufactured as pasteboards, and are afterwards coated on the cloth side with the coloured coating, two applications being necessary in many cases to obtain the desired thickness and surface. Plate-glazing is the means of imparting the ordinary surface to this class of cards.