Bull's Head and Mermaid - The Bernstein Project - Österreichische ...

Imaging Technologies for Watermarks
For nearly all watermark compendia until the middle of the
20 th century watermarks are just tracings allowing cheap
and 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 Bernstein partners will be briefly explained.
Rubbing
(Ill. 1 a, b, c) In order to make a rubbing of a watermark a
soft pencil and 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 and 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 and to put them together for a complete image of the
watermark.
Other methods
Three different X-ray technologies for watermark and paper
structure imaging exist and have now been used for more
than 20 years. These technologies are: beta radiography,
electron radiography and soft-X-ray radiography. Their use
depends strongly on the watermark source and 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
and secondly due to the evenly exposed films. Depending
on the intensity of the radiation and 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 and their Collections
1960. The 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. The resulting images look very
sharp and clear. Here, a metal foil is irradiated by a high energy
X-ray source causing the electrons from the metal foil
to break free. These electrons are then absorbed by the
penetrated paper differently depending on its thickness and
structure. An X-ray film placed on the opposite side of the
paper is exposed by them thus making watermarks and paper
structures visible. The technology offers the possibility
to collect several images from an incunabulum in a single
pass. One of the Bernstein project partners (National library
of the Netherlands, Koninklijke Bibliotheek, The Hague)
processes usually five or more incunabula with a single radiation
impulse exposing up to six films per incunabulum. The
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
and 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 and most
appropriate technologies for watermark imaging in the area
of art history. The necessary devices are transportable without
difficulties and can be positioned directly in museums
or private collections. The method works in a very low-energy
(7–10kV) radiation range and 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 and electron radiography soft-Xray
images can have different exposure intensities within
the film. This is caused by the air layer between the object
and the radiation source, which interferes with the radiation.
The difference between distances S1 and S2 (cf. Ill. 4)
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