Series editors' preface - Wood Tools

256 Conservation of Furniture
humidity. Standard tables (see Thomson, 1986)
and other aids are available to make this a
relatively simple routine calculation (Figure
6.4f ). The traditional version of this instrument,
known as the whirling hygrometer or
sling psychrometer, has a handle arranged at
right angles to the frame containing the
thermometers and can be swung round rather
like a football rattle through 360° so that air
moves past the bulbs. When carefully used,
the accuracy of such devices is adequate to
meet the needs of most furniture conservators
but they should be carefully calibrated at
regular intervals against a more accurate and
reliable instrument such as a dew point
hygrometer (Pragnell, 1989). This service
should be available from the supplier of the
instrument but failing this can be obtained
from an independent laboratory such as SIRA
Test and Certification Ltd. A range of instruments
is also available capable of recording
RH and temperature continuously over several
days, weeks or months (Child, 1993). Such
measurements are a valuable guide when
assessing the suitability of an environment for
display or storage of wooden objects. For
extensive general discussion of moisture and
humidity and their measurement see Wexler
(1965) and the Instrument Society of America
(1985). Measurement of moisture in wood is
discussed in Chapter 2.
RH and damage
Relative humidity influences deterioration in
three main ways: first through high levels or
excessive wetness of the air, secondly by
excessive dryness and thirdly by changes,
particularly rapid changes. Rhythmical daily
changes such as those caused by turning lights
on and off or seasonal changes such as those
caused by winter heating are hard to evaluate
but it is accepted that they are responsible for
stresses that are damaging in the long term.
Damage may be due to dimensional change,
biological activity or chemical reaction.
Constancy of RH around a predetermined
optimum value is the aim in museums and
other institutional collections but before
examining ways in which this may be realized
the major classes of objects affected will be
briefly reviewed.
All moisture-absorbent materials including
leather, parchment, textiles, wood, animal glue
and some other adhesives, bone, ivory, paper
and easel paintings are affected. Skin products,
whether cured or uncured, become very dry
and brittle in excessively dry conditions (RH