I have mentioned a light wire roller which presses the wet film more or less into shape just before it leaves the long trough to be dried. As long ago as the thirteenth century in Europe the fact that devices could be impressed upon the undried film by thinning it and making it more transparent where touched appears to have been known, and from about that time onwards “watermarks” have fortunately been applied in the same manner, namely, so as to come in the middle of the first leaf of the pair forming a folio. It is also fortunate that the “chains” or wires forming the rollers have always been laid in the same way; it is certainly the obvious one, but obvious ways are not always adopted. The result is that by observing the direction of the strong chain marks and the fine “laid” marks between them much information concerning the folding of the original sheets can be obtained.
In very early papers these rules cannot be safely followed, because early chain marks as well as early watermarks were not produced in the same reliable way. The paper being made in moulds with wire netting at the bottom, the impression came below the paper instead of above it as in the case of the roller, and also the sizes of the sheets were more likely to differ.
Watermarks are the semi-transparent devices which show on certain pages of a printed book. They are to be seen on most papers of the fourteenth to the eighteenth centuries, and there has been a good deal written about them, especially abroad. The devices were outlined in wire and set in the bottom of the trough or on the wire roller I have just described, so that the lines are impressed upon the pulp just when it is in its most sensitive condition. The pressure from the wire device thins the pulp wherever it is touched, and so when dry the device shows lighter than the rest of the paper.
These marks should be called wire marks rather than watermarks, and the French word for them, “Filigranes,” is more correct than ours. The difference in tint between a watermark and the rest of the paper is so distinct that a photographic negative placed under one will render a capital photograph of it, far better than any drawing, but it needs a long exposure.