Series editors' preface - Wood Tools

246 Conservation of Furniture
Number of molecules with
a given kinetic energy →
T 1
Figure 6.2 Not all molecules at the same temperature
have the same kinetic energy. This is diagrammatically
represented in a Maxwell Distribution graph which plots
the fraction of molecules with a given kinetic energy
against kinetic energy. The kinetic energy of a molecule
is directly proportional to the temperature. In order to
undergo a reaction, molecules must possess an energy
that is equal to or higher that the activation energy E a.
At a low temperature (T 2), only a few molecules have
sufficient energy, thus the reaction will proceed, but at
a slower rate. At a higher temperature (T 1), more
molecules are able to exceed the activation energy (E a),
thus the reaction proceeds at a faster rate. Thus there is
an increase in the rate of reaction as temperature
increases
Even if molecules with sufficient energy to
overcome the activation energy barrier do
collide, reaction may still not occur. Reaction
takes place at particular sites on molecules and
the correct areas may not be in contact on
collision. This becomes important with large
molecules which have only a few reaction
sites. A catalyst is a substance which alters the
rate of a chemical reaction but remains chemically
unchanged at the end of the reaction. It
is generally believed that this results from an
effective lowering of the activation energy
barrier. This may occur through an orientation
of reactant molecules on the catalyst itself. A
catalyst does not alter G for reaction.
Examples of catalysts include iron and some
other heavy metals, which, when present on
the surfaces of objects and the insides of
buildings, may catalyse the conversion of
sulphur dioxide to sulphuric acid.
The concepts introduced here are more fully
explained in the publication Chemistry for
Conservators (Moncrieff and Weaver, 1992),
especially in Book 2 of that series. Further
discussion can be found at the desired level
in many books of physical chemistry (see for
example Brown, 1964) and in an excellent
introduction to the concepts and applications
E a
T 2
Energy →
of chemical thermodynamics by Johnstone and
Webb (1977).
Chemical reactions involving objects almost
invariably mean deterioration. Such reactions
may be endothermic (heat absorbing) or exothermic
(heat releasing) but in either case a
definite amount of activation energy must be
supplied to start the reaction. This can occur
either by heating or by illumination.
6.2.2 Light
Wherever objects are on display, light is a
source of great potential damage. Though its
action is confined mainly to the surface of
most objects this is often of the greatest importance,
especially on painted and decorated
surfaces. All organic material is at risk under
light and even some glasses, such as those
with a high manganese content, may change
colour after prolonged illumination. Frequently,
the effects of light are aggravated by,
or are only achieved in, high relative humidity
and in the presence of oxygen. Paper,
cotton, linen, wood, parchment, leather, silk,
wool, feathers, hair, dyes, oils, glues, gums
and resins can all be affected and in fact about
the only classes of objects which are not
generally susceptible to light are stone, metal
and ceramics.
White light falling on a surface will be partly
absorbed and partly reflected and in some
cases a further part may pass directly through
it. The characteristics of a surface, especially its
colour, are perceived by the amount of light at
each wavelength which is reflected (section
5.8.1). The light absorbed may, through a huge
variety of chemical reactions, lead to photo
oxidation of the material. Molecules of different
types and complexity are usually involved
and after the primary photochemical reaction a
whole series of reactions may be initiated
which are not necessarily dependent on light.
Further information on the mechanisms of
deterioration relevant to specific materials is
given in the following chapters.
Objects may appear for a considerable
period to be unaffected by light and may then,
sometimes only after many years, show significant
signs of degradation. The early changes
brought about by light during the ‘induction’
period may mainly affect impurities or irregularities
in polymer structure which effectively