Series editors' preface - Wood Tools

78 Conservation of Furniture
In the gravimetric method, wood is weighed
then dried to constant weight in a convection
oven at 103 ± 2 °C. The MC is then calculated
from the formula given earlier. This method
assumes that the oven is completely dry and
that only water is lost from the sample. It provides
an absolute measure of moisture content
at one point in time.
The mass of an applied load can be measured
as an electrical signal by a device called
a load cell. Typically these devices measure
20 mm 10 mm in size. A wide range of load
cells is available that can be used to provide a
continuous record of changes in mass of large
or small furniture objects as water is absorbed
or desorbed from the atmosphere. Although
they can be extremely sensitive, load cells cannot
readily provide information about the location
of moisture in an object or the absolute
amount present.
The Karl Fischer reagent, consisting of pyridine,
sulphur dioxide, iodine and methanol,
can be used to measure the moisture content
of wood, and a wide variety of other materials,
by titration. It gives the best results of any of
the standard methods but is not practical for
large samples (Kollman and Hockele, 1962).
Two types of moisture meter are routinely
used to determine moisture in wood. They are
the electrical resistance type and the dielectric
type.
Dry wood is an effective electrical insulator
but water is a conductor of electricity. As the
moisture content of wood increases its electrical
resistance decreases (approximately halving for
each 1% increase in MC between 6% and the
FSP). The electrical resistance meter, measuring
the flow of direct current parallel to the grain
between two electrodes inserted into the wood,
is commonly used to measure moisture content
over this range. Below about 6% MC, resistance
metres are not reliable because resistance is too
high (> 10 11 Ohms). Above about 24%, MC readings
become less reliable, partly due to loss of
sensitivity as resistance falls and partly due to
polarization and heating effects. AC metres and
those using short repetitive pulses rather than
continuous current are able to overcome these
shortcomings to some extent.
As electricity takes the path of least resistance,
the electrical resistance meter tends to
measure the highest MC in the area between
the electrodes. However, by insulating the
probes along their length except for the penetrating
tips that are to serve as the electrodes,
resistance metres can be used to measure the
magnitude of moisture gradients in wood by
measuring the MC at different depths from the
surface. Resistance metres require pin electrodes
to be inserted into the wood so their use
on presentation surfaces of furniture is not
acceptable. However, a possible way round this
is to take measurements on wood of identical
size, species and finish that is kept with the
object and allowed to reach the same equilibrium
conditions of RH and temperature. Given
a suitable meter, electrodes can be left in situ
and readings taken as required. The English
firm of Hutton and Rostron have developed a
small, cheap resistance electrode that can be
inserted in multiples into structural timbers and
used to monitor moisture content remotely by
PC. This system is employed in the Royal
Pavilion, Brighton, where it is used to monitor
moisture levels in the fabric of the building as
part of the decay prevention strategy.
The resistance of wood increases with
decrease in temperature, and correction for
temperature is therefore required. Some meters
can achieve this automatically through a temperature
probe connected to the meter. The
change in resistance of wood is also to some
extent dependent on species. However, variation
between species is not as marked with the
electrical resistance meter as with the dielectric
type of moisture meter.
Dielectric moisture metres use alternating
current (AC), usually at radio frequencies and
are of two types, the capacitance type and the
power loss type. The capacitance type measures
the dielectric constant of wood. The
more common power loss meter measures the
rate of energy absorption as the product of
dielectric constant and loss factor. At a given
frequency, the dielectric constant increases
with increasing moisture content, increasing
density and increasing temperature. Power loss
generally increases with wood moisture content
and with temperature. Usually a concentric
arrangement of electrodes placed on one
surface is used by both types of dielectric
meter. This is normally fixed for a particular
meter and the field generated penetrates to a
standard depth so that the reading obtained is
more or less an average value. Dielectric
metres are normally calibrated to read between