Many different techniques have been developed for the reproduction of watermarks. They all depend on the same basic feature: paper is thinner, and therefore more translucent, where it has been imprinted with the watermark design on the mould. The following is not a complete list of the techniques that have been developed but at least provides an impression of the various processes that can be involved.
Most professional reproductions of watermarks are beta radiographs, which remains the best method for a number of reasons. A beta radiograph is an x-ray of the paper, which is exposed to a radioactive isotope. The paper is placed between the isotope and photographic film. This process results in a high quality image of watermark, chain lines and wire lines. It is the thickness of the different parts of the paper that is being measured rather than their relative translucence; therefore, unlike the results of most other techniques, the image does not suffer from interference from media such as ink, that are more opaque than the paper. Its problems are easily summarised: it is expensive, requires lengthy exposure times (at least one hour) and a darkroom, and uses a radioactive source (Carbon 14).
A related and more rapid process is electron radiography, which makes use of electrons emitted by lead rather than beta particles.
This method was developed by Thomas Gravell. Dylux® is a photosensitive paper made by duPont, its primary use being in the printing industry. For a detailed description of Dylux® paper see this article by Robert Allison. Dylux is coated in a yellow dye that is sensitive to UV light but is desensitised by exposure to blue daylight. The watermarked paper is placed between a blue daylight source and the Dylux paper. The light passes more easily through the thinner parts of the paper (i.e. watermark, chainlines, wirelines), and is removed when those portions (only) have been desensitised, which takes about 7 minutes. When the paper is then exposed to UV light it turns blue except for the imprint of the watermark. Robert Allison's article on the production of contact prints provides substantially more detail. This is faster and cheaper than beta radiography, and does not require darkroom conditions, but the resulting image suffers from media interference. A paper by David Gants discusses how computer graphics packages can be used to improve Dylux images.
This is essentially a method of contact printing, using high-speed film. The film is placed under the paper in a darkroom, exposed to light for a few seconds, and then developed. The image captures both the watermark, chainlines, and any other variations in the paper surface, but also any written or printed material on the leaf. More details may be found in David Schooner, 'Techniques of Reproducing Watermarks: A Practical Introduction', in Essays in Paper Analysis, ed. by Stephen Spector (London, 1987), pp. 154-67
Phosphorescence Watermark Imaging
This method uses a combination of UV and IR light on a pigment plate. A phosphorescent pigment plate will phosphoresce when exposed to UV light, but this effect is cancelled by IR light. The plate is excited then placed underneath the paper which is to be reproduced. Both IR and UV lights are then shone on the paper, and the image of thinner parts of the paper will appear darker on the plate after a few seconds. A photographic image may then be made of the glowing plate. The resultant images are not of the highest quality, and show some interference from certain media (including many types of ink), but the method is cheap and extremely rapid. A more detailed description of the method can be found in Carol Ann Small, ‘Phosphorescence Watermark Imaging', in Puzzles in Paper, ed. by Daniel Mosser, Michael Saffle and Ernest W. Sullivan II (London, 2000), pp. 169-81.