produced much research and some novel deacidifying agents have been pro-
duced. One historically important and effective method was the use of diethyl
zinc. This chemical is a gas at ambient temperature and pressure, and has to
be used in large vacuum chambers. Since the gas reacts with water, the books
first have to be dried for 3 days before the deacidifying gas is allowed to dif-
fuse into the volumes. Next, excess diethyl zinc is destroyed by the addition
of ethanol vapour. Moist carbon dioxide is finally introduced into the cham-
ber to ensure the alkaline reserve is converted to zinc carbonate.
Cyclohexylamine carbonate, morpholine, amines in general and ammonia
itself have all been considered as deacidifying treatments. Many are liquids
with a significant vapour pressure or are gases, which makes them potentially
useful for mass processes. However, all suffer from the problem that the
deacidification produced is not permanent and some cause yellowing of the
paper. None is currently in favour.
6 BLEACHING AND WASHING OF PAPER TO REMOVE
DISCOLOURATION
Discoloured paper can be made lighter by washing or bleaching. Uniform
discolouration is nowadays considered much more acceptable on paper than
it was 20 years ago and consequently, less washing and very much less bleach-
ing is performed. Non-uniform discolouration is aesthetically displeasing and
these techniques may be used to lessen the effect. Washing in water removes
some discolouration but usually only a small fraction of that is possible by
bleaching. Higher temperatures produce faster and more complete washing.
The addition of a surfactant can also help especially with a well-sized
(hydrophobic) paper. Many different washing processes are performed by
conservators because the image materials can be fugitive in water, e.g.some
watercolour paints will become mobile when immersed in water. In such a
case it is desirable to carry out washing while exposing the paint to the min-
imum possible amount of water. When fugitive pigment is present, simple
immersion of the paper in a bath of water may be unsuitable but a process
where the paper is placed on a suction table and exposed to ultrasonically
generated water mist might be much better as the mechanism for movement
of particles through liquid water is reduced.
On ageing, paper components degrade and may produce double bonds in
the constituent molecules. When a few double bonds are present and they are
present as alternating double and single bonds, they are called conjugated
double bonds. When there are sufficient conjugated double bonds they start
to absorb visible light and they become coloured. Organic dyes and most dis-
colouration in paper owe their colour to this phenomenon. If some or even
one of the double bonds can be destroyed, the paper may stop absorbing visible
44 Chapter 3