Fig. 32.
Fig. 33.
The Dandy-roll.—When it is required to produce a design or name, termed a water-mark, upon the paper, this is done by means of a roll called the dandy-roll, which consists of a skeleton roll covered with wire-cloth, upon which the design is worked by means of very fine wire. If the paper is required to be alike on both sides, without any specific pattern or name upon it, the roll is simply covered with wire-cloth, the impressions from which upon the moist pulp correspond with those of the machine-wire on the under surface. By this means paper known as "wove" paper is produced. A dandy-roll of this character is shown in Fig. 32. "Laid" paper, as it is termed, is distinguished by a dandy-roll having a series of equidistant transverse wires on the upper surface of the wire cylinder, as shown in Fig. 33, the effect of which is to produce parallel lines on the paper, caused by the pulp being thinner where the moist paper is impressed by the raised wires, which renders the lines more transparent than the rest of the paper. The dandy-roll, which is usually about 7 inches in diameter, corresponds in length to the width of wire on which it rests, and is placed over the wire-cloth between the suction-boxes. The journals of the roll turn in slits in two vertical stands, one behind the machine frame and the other in front of it. The roll, however, rests with its whole weight on the wire, and revolves by the progressive motion of the wire. The stands which support the roll prevent it from being influenced by the lateral motion of the wire. By thus running over the surface of the pulp when the wire is in motion, this roll presses out a considerable quantity of water, at the same time rendering the paper closer and finer in texture. Dandy-rolls of various lengths, and bearing different designs or patterns, are kept at the paper-mills, and great care is exercised to preserve them from injury.
Water-Marking.—Dr. Ure describes the following processes for producing a design for a line water-mark:—1. The design is engraved on some yielding surface in the same way as on a copper-plate, and afterwards, by immersing the plate in a solution of copper sulphate, and producing an electrotype in the usual way, by which all the interstices become so filled up as to give a casting of pure copper. This casting, on being removed from the sulphate bath, is ready for attaching to the wire gauze of the dandy-roll. 2. The design is first engraved on a steel die, the parts required to give the greatest effect being cut deepest; the die, after being hardened, is forced by a steam hammer into some yielding material, such as copper, and all of this metal which remains above the plain surface of the steel is subsequently removed by suitable means; the portion representing the design being left untouched would then be attached to the wire-gauze as before. Light and shade can be communicated to the mark by a modification of the above process, for which purpose an electrotype of the raised surface of a design is first taken, and afterwards a second electrotype from this latter, which consequently will be identical with the original surface. These two are then mounted on lead or gutta-percha, and employed as dies to give impression to fine copper-wire gauze, which is then employed as a mould. Thus absolute uniformity, such as could not be attained by the old system of stitching wires together, is now attained in bank-notes by the adoption of the above method. It may be mentioned that when the moulds were formed by stitching the fine wires together to form a design, no less than 1,056 wires, with 67,584 twists, and involving some hundreds of thousands of stitches, were required to form a pair of £5 note moulds, and it was obviously impossible that the designs should remain absolutely identical.
Sometimes water-marks are produced by depressing the surface of the dandy-roll in the form of a design, which causes the paper to be thicker where the design is than in the rest of the sheet of paper. This modification was invented by Dr. De la Rue.
De La Rue's Improvements in Water-marks.—By one method, patented in 1869, dandy-rolls, having a surface of embossed wire-gauze, are used; the indentations in the gauze are inwards, causing a thickening of the paper where they are brought in contact with it. These thickenings correspond in form to the configuration of the design or water-mark. The inventor has also affixed wire to the surface of such dandy-rolls so as to form projections, in order to thin the paper where the projections come in contact with it, by which means light lines are obtained in the water-mark, strengthening the effect of the thickened opaque design.
Fig. 34.