Series editors' preface - Wood Tools

cause inaccurate readings. Wood that has been
treated with any carbon containing material,
e.g. consolidants, hydrocarbons etc., is also not
suitable for carbon-14 dating. Samples can be
taken by drilling a small hole 1 ⁄4 inch deep into
the object to clear the surface, followed by a
smaller drill into the clean area. The frass from
the second drilling can be collected on cleaned
aluminium foil which can be folded into a
small envelope. All equipment should be carefully
cleaned with distilled water. Carbon-containing
solvents should be avoided. The wood
dating institution should be supplied with all
relevant information relating to the object and
how the sample was obtained.
Carbon-14 dating can be used in determining
the age of furniture woods, however, there
are some limitations to the processes. In dating
a 50 mg sample only those 50 mg of material
will be dated and not the entire object
(Weaver, 1982). A longitudinal sample, for
example, taken from furniture made of a tree
that lived from 1200 to 1600, can provide a
date in the direction of any of those 400 years.
Only sapwood samples can provide more
accurate dating about when the wood may
have been used, since the intake of carbon-14
stops when a tree is felled. The intake of carbon-14
of the heartwood, of course, stops
when it is being formed in the still living tree.
Even when a carbon-14 date is provided for
furniture wood, it does not necessarily provide
an answer about when the piece was made
(Hall, 1987; Pearson, 1987).
Carbon-14 dating, however, can be a useful
tool in dating organic furniture material despite
the destructive nature of the method.
Interpretation of data should be carried out
with the greatest care by scientists, conservators
and curators alike (Aitchison and Scott,
1987; Bowman, 1990; Chase, 1972; Coles and
Jones, 1975; Stenhouse and Baxter, 1983).
Dendrochronology Dendrochronology is literally
the dating of wood. Trees which grow in
temperate climates grow in the summer,
adding a layer of new wood to the outside of
the trunk each year. In autumn growth slows
down and in winter stops altogether. In many
cases this leads to an obvious annual ring due
to the difference in size and/or wall thickness
of cells in the autumn and spring wood. Trees
grown in the tropics do not have annual rings.
Conservation preliminaries 395
Some may have growth rings but these are
caused by spurts of growth in the wet season
which may occur only once every few years or
several times in one year. The annual rings of
temperate trees may be counted to ascertain
the age of the tree. In some species it was
found that regular patterns of ring widths occur
in certain years in each tree examined. It was
then realized that this could be used to date
wood which had been converted in buildings,
furniture, paintings and archaeological material.
The trees which give these definite patterns
are those which respond to some limiting factor
in their environment such as rainfall or temperature.
In oak, for example, a dry summer
will give a narrow ring, a wet one a wide ring.
Growth rings may also vary in dimension
under influence of altitude, soil fertility, volcanic
eruptions, fungal or insect attack and
pollution (Schweingruber, 1983). Obviously, if
the ring widths depend on rain fall they will
not be same in, for example, England,
Germany and Italy. Chronologies are therefore
worked out for particular trees for particular
areas. Master curves exist for most main wood
producing regions (Baillie, 1984; Fletcher,
1977; Fletcher and Tapper, 1984; Klein et al.,
1987). Wood sections with sufficient annual
growth rings are compared against master
curves, climatological time charts, compiled
from past and present growth-ring-forming
trees. About thirty tree rings are minimally
required for dating a characteristic sequence,
however, accuracy increases with increase of
growth rings. In general an average of 120 tree
rings are considered to provide reliable results
(Schweingruber, 1983). A smooth end-grain
surface has to be prepared with a sharp knife,
rather than by sanding. The wood surface can
be examined and measured directly under low
power magnification of a binocular microscope.
X-ray radiography of coniferous wood
samples enhances the contrast between earlyand
late-wood considerably by recording the
variety in their density on X-ray film
(Schweingruber, 1983). These filmstrips then
can be analysed by a micro-densitometer
which feeds information into a computer. The
computer averages and compares the pattern
against master curves resulting in dating of the
sample. Other X-ray radiography techniques
such as computer tomography (CT) also record
density variations (Reimers et al., 1989).