Series editors' preface - Wood Tools

416 Conservation of Furniture
under another light source. This is called
metamerism. It is possible to calculate whether
a lamp will distort colours by one of two methods.
The first of these is the CIE colour shift
method (CIE, 1974) and the second is the NPL
Crawford Spectral band method (Crawford,
1959, 1960). In the CIE method the test lamp is
compared with a reference lamp of the same
correlated colour temperature to determine the
(average) shift in position in colour space of
eight test colours. A lamp with a colour rendering
index (R a) of 100 has perfect colour
rendering. The lower the colour rendering
index the worse the shift in colour that is
observed. Pure, unfiltered daylight has a colour
rendering index of 100.
Although lamps of both high and low colour
temperature can have good colour rendering,
if they are used together the eye will adapt to
one or other of them making the other(s)
appear wrong. Tungsten lights appear to give
a yellow cast and fluorescent lights a blue cast
whereas on their own any of these sources
(provided it had good colour rendering) could
give satisfactory viewing. It will make matters
simpler in the long run to decide on a
restricted range of light sources of known qualities
for use in the studio. Issues that should be
considered in relation to lamp choice besides
colour rendering and colour temperature are
ease of control, the extent of radiant heating
produced by the lamp, cost of installation and
maintenance and personal preference.
The temperature of the work space needs to
be regulated for comfortable working, and for
the safety of the objects and for the success of
many conservation operations temperature
needs to be maintained fairly constant in the
range suggested in Chapter 6. An architect
should be consulted for advice on how best to
deal with the building to achieve efficient control
of temperature. This will involve some calculations
on the thermal mass of the building,
the probability of overheating in summer
months and the need for heating in winter. In
an environment where flammable solvents may
be in use, safety is paramount and no naked
flames should be used for heating the space or
for heating water. Electrical convection heaters
tend to stir up any solvent vapours that may be
present and are therefore not a good choice.
Central heating radiators are a good choice as
long as the boiler is remote. Electric oil-filled
radiators are also satisfactory. Thermometers
and hygrometers (or thermohygrographs)
should be installed to monitor the space to
ensure that satisfactory conditions are being
maintained.
Extraction
Extraction may be of two types, the dilution
type and the local exhaust ventilation (LEV)
type. Dilution extraction removes stale air and
provides a supply of fresh air but allows any
contamination, although diluted, to become
mixed into the general room air supply. For
efficient removal of noxious vapours removal
at source using local exhaust ventilation is necessary.
Using this system a hood and duct are
situated close to the work and noxious vapours
can be extracted away from the person using
hazardous materials. Flow rates measured in
cubic metres per second or cubic metres per
hour give a good guide to the efficacy of such
systems. Low level solvent use, such as occurs
in retouching for example, requires extraction
of 0.5 cubic metres per second extraction at
source (Hughes, 1987). Other processes, such
as the use of an air brush, may need
0.6–1.0 m/s and may require work to be done
in a spray booth or fume cupboard, for which
well defined standards (e.g. BS DD80) exist.
Systems to achieve this can be portable, freestanding
or fixed. Systems that are fixed in
space are obviously less flexible than portable
systems (Figure 9.6). Such systems are noisy,
as is any system in which a large volume of air
is being moved at speed although devices to
reduce the noise level can be fitted. With any
such system careful thought needs to be given
to where the air is going to and where it is
coming from. Contaminated air should be
vented well clear of the building and away
from areas where people may be working. In
the long run we should be working to avoid
the need for such systems by using less hostile
materials in our work. A disadvantage of fixed
systems is that not only contaminants but also
warm and possibly humidified air is extracted
out of the building causing air to be replaced
with (possibly) cold dry air. If air is being
extracted out of the building, the volume of air
extracted has to be balanced by a similar volume
of fresh air coming in to the room. It is
therefore important when installing such systems
to think very carefully about the source