Series editors' preface - Wood Tools

Figure 9.2 A good quality loupe (right) is invaluable
for examination, whist optivisors (stereoscopic magnifiers
mounted on a headband – magnification 2.5 and 5.5
(shown centre and left) can be used to provide lowlevel
magnification for examination or during treatment
also reveal in the surface the pattern of the
underlying structure. Silicon rubber moulds of
the surface, which provide a reversed pattern
in relief may also help to show up tool marks.
Low-level magnification by means of simple
low power (5–10 power) magnifiers such as
a good quality hand lens (as recommended by
Hoadley (1990) for the purpose of wood identification),
traditional jewellers loupes, linen
testers, ‘opti-visors’, or surgical loupes (see
Figure 9.2) can quickly provide information
about a surface that would not easily be seen
with the unaided eye. Inexpensive box like
magnifiers, which often include a light source
and stereo binocular microscopes can usefully
extend the range up to about 30 magnification
or more and video microscopes can
extend the magnification range up to about
200, though the field of view and depth of
field are often unhelpfully small at such large
magnifications. Linen testers have the advantage
of letting in plenty of light and of maintaining
the correct focal length and thereby
providing a sharp image when in contact with
the surface being viewed, something that may
not be possible, however, if the surface is
severely degraded.
As magnification increases, depth of field
and field of view tend to be reduced. The focal
length of instruments such as surgical loupes
and stereo binocular microscopes is of particular
importance in determining the working distance
and amount of room that the conservator
Conservation preliminaries 387
has available to manipulate tools and equipment
in the space between the magnifying
instrument and the object being examined. A
zoom facility is especially useful on instruments
with higher magnification as it makes
the process of moving from inspection of one
area to another easier and safer if one can
quickly zoom out to establish position and
then zoom in to view detail. The ability to
maintain a fixed position also becomes more
important as magnification increases. To
enable a stereo binocular microscope to reach
across the surface of a large piece of furniture,
a large, heavy, rigid stand becomes necessary.
Such stands may cost several times the price of
the original instrument. Even the most expensive
surgical loupes (which are fixed to the
operator rather than to the bench or the floor)
can seem like a bargain in comparison.
Nevertheless the stereo binocular microscope
is a most useful item of equipment especially
when provided, as it should be, with the
means to attach a 35 mm SLR type camera.
Also, as larger magnifications are used, it
becomes necessary to augment the supply of
natural light. Fibre optic lighting is most convenient
because it is unobtrusive in the working
area, is ‘cool’ (i.e. does not greatly heat the
surface) and readily puts the light where it is
needed as it allows the light to be varied in
direction, angle and intensity.
Access to view the internal condition of
wood is frequently denied although it may
often be possible to infer this from external
signs, such as the presence of flight holes.
Access at joints, breaks and gaps in the surface
may help and further information can be
obtained by probing with a mounted needle or
dental tool. For viewing inside larger inaccessible
structures an endoscope may sometimes
be useful.
Natural, incandescent, halogen and other visible
light sources will give useful information
under both raking and reflected light conditions
on the visual qualities of a finish (Figure
9.3). However, no gross examination of a surface
is complete without studying it under ultra
violet light. Examination of surfaces under UV
can be most useful in discovering the presence
of fluorescent materials in the finish, particularly
natural resins (Plate 2). UV can be useful
for identification of a variety of adhesives coatings
and media, ivory, bone, antler and wood.