Series editors' preface - Wood Tools

406 Conservation of Furniture
visible rays into yet longer infrared wavelengths.
The former effect is of course visible
while the latter is not. Fluorescence photography
starts with a source that contains the wavelengths
which the subject will transform into
longer wavelengths and usually an exciter filter
is placed over the source that will allow
through only those wavelengths which will
cause fluorescence. For UV fluorescence a
Wratten 18A filter can be used and for IR fluorescence
a Corning 9788 filter is suitable. For
UV fluorescence photography a Wratten 2A or
2E filter is placed over the camera lens to block
reflected UV and colour reversal daylight film
used to record the results. The barrier filters
suggested for IR fluorescence photography are
Wratten 87, 87C or 88A with Kodak High
Speed Infrared film.
Silver halide emulsions are also sensitive to
X-rays, gamma rays and charged particles emitted
by radioactive substances. Some of these
rays penetrate visually opaque materials to
varying degrees to show up internal structures.
Radiography covers techniques of recording
the subsurface features of objects. X-rays,
which have wavelengths 1/100th to
1/100 000th that of visible light, are produced
by bombarding an electrode with a high voltage
stream of electrons. For radiography, the
object to be recorded is placed between the Xray
tube and the film. The differential absorption
of X rays by the object’s internal structure
is recorded on the film as a projection shadow
graph. The energy of X rays, which is quoted
in Kilo volts, can be adjusted according to the
density and thickness of the structures being
examined. Generally higher energies are used
for thick and dense materials and lower energy
‘soft’ X-rays are used for thinner low density
materials. Good contrast can be achieved with
wooden structures by selecting the proper Xray
kilovoltage and exposure time (Gilardoni et
al., 1994). Xeroradiography which uses an Xray
source employs a recording process similar
to the photocopying process. The technique
requires a selenium developing plate which
provides a wider latitude in a single exposure.
In addition, it offers enhanced contrast and can
either be printed in positive or negative.
Magliano and Boesmis (1988) have demonstrated
this technique for panel paintings and
Willisch (1989) has examined among other
materials wooden and ivory objects. Advanced
X-ray computer tomography provides the synchronized
movement of a focused X-ray beam
around the object to be examined. Electronic
images, corrected for overlaps and distortions,
of transverse or longitudinal sections of the
objects are created (Martius, 1992). Because
radiographs show a limited indication of depth,
stereoradiography may be useful in determining
the location of a specific feature in three
dimensions (Gilardoni et al., 1994).
Bernstein (1991), Brachert (1986), Gill et al.
(1990), Levitan (1987), Van Der Reyden and
Williams (1992) and others have shown the
value of X-ray radiographs in revealing construction
and other information such as the
extent of damage caused by insect infestation
that is not otherwise readily accessible.
The camera
There are many different cameras available for
different purposes and no one camera is ideal.
Some are specialized to perform a small range
of tasks that would be impossible with other
types of equipment. Others are very versatile
and can cover a wide range of photographic
assignments without being ideal for any. The
four main types of camera are the view camera,
the direct viewfinder camera, the twin lens
reflex and the single lens reflex. The advantages
and disadvantages of each type are comprehensively
and concisely reviewed by
Langford (1997). A list of some factors to consider
when choosing a camera is given below.
• Purposes for which the camera is required.
Would it be better to have one camera for
everything or to use more than one type
depending on need?
• The format – large format gives better definition
and enlargement, wide range of
camera movements for image control availability
of specialist film and easy film
change. Small format cameras are less intrusive,
quicker to use, have greater depth of
field and generally faster lenses.
• Film types available.
• Focal length of lens, means of focusing and
availability of other lenses and filters.
Whether the camera sees what the eye sees
should also be considered.
• Shutter speed and aperture control and
means of metering and controlling exposure.