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Imaging Technologies for Watermarks For nearly all watermark compendia until the middle of the 20 th century watermarks are just tracings allowing cheap <strong>and</strong> quick accumulation of large stocks. This tracing method doesn’t allow to capture the whole paper structure of an object – apart from the already mentioned deviations of the tracings from the original watermarks. Many other imaging technologies for watermarks exist. All of them have already been published. In the following, the imaging methods used by the <strong>Bernstein</strong> partners will be briefly explained. Rubbing (Ill. 1 a, b, c) In order to make a rubbing of a watermark a soft pencil <strong>and</strong> a piece of thin paper are needed. A Plexiglas plate serving as a stable underlay is placed underneath the sheet of paper which contains the watermark <strong>and</strong> the thin sheet of paper on top of it. Rubbing the pencil on the thin paper creates a copy of the watermark. In folio-size papers the watermark is found in the middle of the leaf. Watermarks in quart or octave formats were usually cut into several parts; in this case the rubbings have to be reassembled again. Rubbing is an easy method to collect these several parts <strong>and</strong> to put them together for a complete image of the watermark. Other methods Three different X-ray technologies for watermark <strong>and</strong> paper structure imaging exist <strong>and</strong> have now been used for more than 20 years. <strong>The</strong>se technologies are: beta radiography, electron radiography <strong>and</strong> soft-X-ray radiography. <strong>The</strong>ir use depends strongly on the watermark source <strong>and</strong> the circumstances under which the images are to be taken. Beta radiography (Ill. 2) Beta radiography can provide very high-quality images of watermarks, primarily because of the good contrast <strong>and</strong> secondly due to the evenly exposed films. Depending on the intensity of the radiation <strong>and</strong> the films used the exposure time varies usually from 2.5 to 8 hours for a single copy. This technology was tested in the mid-forties in the U.S.A (cf. Kaiser, Neue Erkenntnisse, p. 203). D. P. Erastov published the first beta radiographies of watermarks in V Watermark Imaging Technologies, Watermark Collectors <strong>and</strong> their Collections 1960. <strong>The</strong> British Museum in London has been applying identical procedures from July 1966 onwards. Researchers in Copenhagen have been working with a modified beta radiography procedure since January 1967. Electron radiography (Ill. 3) Very good results in watermark imaging are achieved by electron radiography. <strong>The</strong> resulting images look very sharp <strong>and</strong> clear. Here, a metal foil is irradiated by a high energy X-ray source causing the electrons from the metal foil to break free. <strong>The</strong>se electrons are then absorbed by the penetrated paper differently depending on its thickness <strong>and</strong> structure. An X-ray film placed on the opposite side of the paper is exposed by them thus making watermarks <strong>and</strong> paper structures visible. <strong>The</strong> technology offers the possibility to collect several images from an incunabulum in a single pass. One of the <strong>Bernstein</strong> project partners (National library of the Netherl<strong>and</strong>s, Koninklijke Bibliotheek, <strong>The</strong> Hague) processes usually five or more incunabula with a single radiation impulse exposing up to six films per incunabulum. <strong>The</strong> technology is very appropriate for imaging watermarks from books. Electron radiography requires with quite strong X-ray sources (200–250kV) which necessitates extensive radiation protection. In contrast to beta radiography, the electron <strong>and</strong> the soft-X-ray radiography have shorter exposure times which make them less time consuming. A single radiation lasts between 1–2 minutes depending on the used X-ray films. Soft-X-ray radiography (Ill. 4) Soft-X-ray radiography is one of the safest <strong>and</strong> most appropriate technologies for watermark imaging in the area of art history. <strong>The</strong> necessary devices are transportable without difficulties <strong>and</strong> can be positioned directly in museums or private collections. <strong>The</strong> method works in a very low-energy (7–10kV) radiation range <strong>and</strong> the short exposure times make an efficient work possible. For the exposure of the film by soft-X-ray, the film has to be placed directly underneath the paper containing the watermarks. In comparison to beta <strong>and</strong> electron radiography soft-Xray images can have different exposure intensities within the film. This is caused by the air layer between the object <strong>and</strong> the radiation source, which interferes with the radiation. <strong>The</strong> difference between distances S1 <strong>and</strong> S2 (cf. Ill. 4) 67
Ill. 1 a, b, c: rubbing Ill. 2: Beta radiography Ill. 3: Electron radiography 68 1 Book / incunabulum 2 Radiation beams / C14-source 3 Folio with watermark 4 Film 5 Object slide / table 1 X-ray source 2 Book / incunabulum 3 Electron transmitter / Metal foil 4 Folio with watermarks 5 Film 6 Object slide / table
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