Series editors' preface - Wood Tools

which property changes become apparent due
to progressive degradation of the film. In the
first stage, when solvent is still present, the film
is still flexible and has not yet come to a
solvent/environment equilibrium. The solvent
will diffuse through the body of the coating to
the upper portion of the film and evaporate
from the surface, a process that may take
months to complete (Martens, 1968). Solvent
may also diffuse into the layers below the
coating and begin to dissolve soluble low
molecular weight components into the varnish
layer itself. Also, a small amount of some
solvents may be permanently retained. Physical
stress caused by solvent evaporation may result
in fissures which may appear as a very
pronounced craquelure or small fault lines only
visible with magnification. The second stage
(solvent dissipated), when the film has lost all
of the free solvent and is set into a fairly stable
coating is usually the longest stage in the life
of the coating. In the third stage, physical and
chemical changes occur yielding an oxidized
film which may be crazed, brittle, opaque,
discoloured or otherwise altered in properties.
This aged coating is likely to be chemically
quite different from the initial state. Materials
which are substantially changed during this
process would be considered less stable. Some
coatings such as beeswax may change very
little over an extended period of time.
Development of insoluble matter
The tendency for thermoplastic polymers to
develop insoluble matter with time is the rule
rather than the exception. This is generally
due to crosslinking but the formation of
ketonic and acidic groups, which also takes
place in the process of oxidation, tends to
make polymers more polar. For example, the
decreasing solubility in toluene of Acryloid
B82 and Rhoplex AC33 that occurs with time
is apparently not due to crosslinking but rather
to a change in polarity. This is also the case
with the natural resins dammar and mastic and
with the cyclohexanone based coatings AW2,
MS2, MS2A and Ketone N. When fresh these
are soluble in white spirit but require more
polar solvents for their removal as time goes
on. They may eventually require ethyl alcohol
or similar solvent for their removal.
While poly vinyl acetate is highly resistant
to crosslinking, many of the methacrylate
Deterioration of other materials and structures 339
polymers of interest in conservation are highly
prone to becoming insoluble through this
mechanism. These include iso amyl, iso butyl
and n-butyl methacrylates. However, there are
others, including poly (n propyl methacrylate)
and Paraloid B72 (co-polymer of methyl
methacrylate and ethyl acrylate), which are
highly resistant to crosslinking. The most
sensitive materials tend to be those polymers
based on methacrylic esters which have a
tertiary hydrogen atom in the alkyl radical of
the alcohol group.
The development of insolubility seems to
proceed in four stages for polymers that
crosslink on ageing. There is generally an
induction period during which the development
of insoluble material is not noticed. This
may be because the polymer contains
substances (either intentionally or accidentally)
which are attacked more readily than the
polymer itself and therefore protect it. It is also
a consequence of the growth of molecules by
crosslinking to a size sufficient to become
insoluble. It does not need many crosslinks to
form before a high molecular weight material
becomes insoluble. Low molecular weight
materials require many more reactions before
insoluble material is formed. That part of the
induction time that is the result of the growth
of molecules to a sufficient size to become
insoluble is thus inversely proportional to
molecular weight. The exposure necessary
before the first insoluble matter is formed is
sometimes referred to as the gel dose.
In the second stage, following the point of
initial appearance of insoluble polymeric
material but before a high degree of crosslinking
has occurred, most films can be removed
as a mixture of highly swollen gel and
dispersible sol. This relative ease of removal
appears to last up to about five times the gel
dose. The rate at which insoluble matter builds
up follows a regular mathematical law usually
becoming approximately 90% insoluble after
five times the gel dose. If chain breaking and
crosslinking occur simultaneously this will
effectively reduce the percentage of insoluble
material. Nevertheless, the density of crosslinks
in the insoluble portion will steadily
increase and this will therefore become less
able to swell and hence more difficult to
remove. During this period, polymers such as
poly (n-butyl methacrylate) and poly (iso-amyl