Series editors' preface - Wood Tools

390 Conservation of Furniture
wood preservative treatments on furniture is
highly recognizable. Certain timbers such as
cedar, rosewood and sandalwood have characteristic
odours as do certain finishing materials
(e.g. turpentine). The taste of true oriental lacquer
on the tip of the tongue, once learned, is
also unmistakable.
Techniques such as those described above
can help to give a sense of the object and
especially its surface as something that may
contain a wide variety of finish materials, dirt
and grime rather than a singular material and
this is important.
Microscopic examination
The basic principles of microscope optics set
up for transmitted light operation have been
described by Catling (1981) and McCrone
(1987). For a detailed explanation of special
methods in light microscopy see McLaughlin
(1977). A useful introduction to some of the
many applications of light microscopy is given
by Simpson and Simpson (1988). An introduction
to microscopy using electrons, X-rays and
acoustics is given by Rochow and Tucker
(1994).
Stereo microscopy Stereo light microscopy is a
useful and powerful technique that can be
used to examine surfaces in detail, to monitor
the results of solvency tests and to assist the
removal of samples for analysis or other forms
of microscopy. The most useful type is one that
will swing out over the surface being studied.
It is not necessary to take samples so the surface
remains in context, unlike most forms of
microscopy where sample removal is required.
The addition of photographic equipment and
zoom lens can make this an extremely powerful
examination, documentation and treatment
tool.
Incident light microscopy Understanding the
nature of a finish and problems associated with
it can often be achieved through incident light
microscopy, also referred to as reflected light
microscopy. This usually requires the extraction
of a sample but video microscope techniques
can be used directly on the surface
without sampling. Rather than transmitting light
through a thin section on a microscope slide,
incident light microscopy uses a ‘thick section’
that is illuminated from above with the light
being reflected back into the objective.
Viewing a sample under the microscope with
incident light does take some getting used to,
whether it be normal or fluorescent light. A
properly sectioned sample will reveal the stratification
of a surface, showing its history the
way a road cut through a mountain side will
reveal geological strata.
Except for the video microscope, which is
relatively expensive, the technology required is
within reach of the average conservator. A
fairly basic microscope is all that is needed to
use this technique in one’s every day work.
Used equipment from universities or the biomedical
industry is inexpensive and satisfactory.
The results observed with incident light
microscopy can be a compelling component of
examination that will give a more objective
basis to an evaluation than would otherwise be
possible.
In incident microscopy using normal light,
illumination is generally achieved through
external lighting such as fibre optics or an
internal source that transmits light down the
objective’s cylinder. An external source is most
effective when positioned at a 45° angle to the
sample. The fibre optic system allows the
viewer to manipulate the angle of the light
which can help make components like metals
or pigments stand out. However, an internal
source has an advantage of being out of the
operator’s way and giving consistent lighting.
Normal light shows up pigments, metallic particles,
gold leaf and other parts of a decorative
surface but varnishes can be difficult to detect
with normal light microscopy since they
appear amber in colour, creating insufficient
contrast with the wood part of a sample. It is
useful to move back and forth between normal
and fluorescent light sources to help bring out
and interpret information.
Incident light fluorescence microscopy uses
a specialized light and filtering system to render
visible the components of a coating that
would otherwise go unnoticed. Resinous surface
coatings absorb light at different frequencies
across the spectrum. For example, light
typically passes through resins with minimal
absorbency. As a result, layers of resins in cross
section will look fairly clear with a normal, or
white, light source. However, enough of the
ultra violet light spectrum is absorbed by natural
resins to cause emission in the visible