Series editors' preface - Wood Tools

232 Conservation of Furniture
A vast array of stains is now available for
wood and it is sometimes difficult to choose the
most appropriate one. Stains may be classified
according to their reactivity, transparency and
the vehicle in which they are applied. Different
stains have various advantages and disadvantages
with respect to the quality of their effect,
their permanence, the extent to which they raise
the grain of the wood, penetration, speed of
drying and so forth. These materials may be
referred to as transparent stains which are nonreactive
colorants without pigment, translucent
stains which are non-reactive colorants with
pigments, and chemical stains which are
reactive colorants without pigment. These
groups of stains are further considered, in
relation to their application, in Chapter 13.
5.8.5 Identification of pigments, dyes
and stains
The colour of pigments under daylight and
under ultra violet light are two macroscopic
methods that may be of use. For routine
identification of pigments on a day-to-day
basis excellent microscopic and chemical
methods are available. However, for nondestructive
analysis of pigments in situ, for
quantitative assessment of very small samples
and for some more specialized applications,
advanced instrumental methods of analysis
may be required.
Some pigments fluoresce under UV light
and some will also affect the natural UV
fluorescence of binding media. Natural madder
lake fluoresces pink, zinc white fluoresces
greenish-yellow and cadmium yellow, orange
and red pigments also fluoresce under UV (De
la Rie, 1982). The false colour of some
pigments under infra red light may serve as a
guide to their identification and has been
comprehensively investigated by Moon et al.
(1992).
Microscopic techniques frequently provide
positive identification of pigments on the basis
of their size, shape and optical behaviour in
polarized and crossed polarized light. Microscale
sampling and identification of most
artists’ pigment particles has been organized in
flow chart fashion arranged by each colour
(McCrone, 1982). McCrone now recommends
the use of Meltmount, a mounting medium
free from polychlorinated bi-phenyls (PCBs).
The application of microchemical tests to
pigment identification has been described by
Plesters (1956). De Keijzer (1988) has analysed
modern synthetic blue pigments using microcrystallization
and colour change on acidification.
Complex mixtures of pigments or
impurities found in paints may result in
competing reactions from the chemistry
outlined in the laboratory manuals cited above.
It may therefore be necessary to identify
unknown materials by alternative methods.
Organic pigments have been analysed by
Pey (1989) and Schweppe (1989) using thin
layer chromatography (TLC) and by Wouters
and Verhecken (1989) and Pey (1989) with
high pressure liquid chromatography (HPLC).
Hofenk-de-Graaf (1969) also describes
methods by which organic dyes can be
analysed. TLC, HPLC and mass spectrometry
have been used to identify xylindein, a green
coloration produced by the fungus
Chlorociboria (Michaelsen et al., 1992, see also
Blanchette et al., 1992). The use of indigo as
a wood stain has been identified by several
spectroscopic methods (Buchholz, 1991).
Solution and high order derivative
spectrophotometry has been used to identify
the large number of organic pigments that
have come into common usage in the twentieth
century (Billmeyer et al., 1982; Risti-Solaji,
1990). Wakeford and Wardman (1989) has
used derivative reflectance spectroscopy to
identify organic blue pigments from tints
prepared in titanium white. Guineau (1989)
reports that samples as small as 5 μg could be
positively identified with Raman spectroscopy.
Varlashkin and Low (1986) has demonstrated
the use of the non-destructive technique, infra
red photo thermal beam deflection spectroscopy
(PBDS) to identify black inks on paper,
and the components of a dagger. McMillan
and Hofmeister (1988) provides an overview
of infra red and Raman methods used to
identify minerals.
The elemental composition of pigments
from painted passages can be suggested by
study of the X-ray image though one should
be extremely cautious in arriving at such
conclusions based on X-ray observations alone
(see Rees-Jones in Van Schoute (1986) for
information on the X-ray absorptivity of
specific pigmented paints of varying thickness).
Gold leaf, being so thin, is usually trans-