Series editors' preface - Wood Tools

336 Conservation of Furniture
they were applied, the thickness of the paint
and the strains caused by the support and any
coating that may be present. All binding media
that become hard enough to act as a paint film
also, eventually, become sufficiently brittle that
they are unable to withstand these stresses and
therefore crack. Cracks in paint and ground
arising from movement of wooden supports
are thin and fine and tend to reflect the
pattern of movement and hence the underlying
grain direction and construction, of the
support. A different kind of crack arises from
forces acting within and between the layers of
paint. Paint films have a smaller volume when
dry than when wet and hence can be considered
to shrink on drying. If the rate or extent
of drying is too great, the ability of the paint
film to resist the stresses imposed by drying
will be exceeded and it will crack. The time
taken for a paint film to dry can be considerable
but will vary according to the type of
medium (e.g. different oils), the amount of
thinners or diluent used, the amount and type
of drier added to the medium, the pigment(s)
present and the state of the underlying layers.
The rule in painting has been to apply fat over
lean, that is slow-drying layers over quickdrying
ones. If quick-drying layers are applied
over slow-drying layers they are very likely to
crack. This has been used in the deliberate
creation of age-imitating craquelure. The
pigment to medium ratio is also important
since the pigment acts as a filler to increase
the modulus of the coating. If too little
pigment is present the film will be more brittle
and more likely to crack. Problems can also
arise if a paint layer was not dry before the
next one was applied or even in some cases
where a mistake was made and retouching
varnish was applied over soft paint so that
corrections could be made before the paint
was dry. Paint applied over an unstable film
such as bitumen for example and covered
over with a thick layer of varnish often splits
open into wide rifts from the pull of drying.
This has been called alligator crackle or alligatoring
(Stout, 1975). Cracks caused by the
inability of the paint film to hold together as
it dries are characteristically wider than those
caused by movement in the support and have
more of the appearance of cracks in a dry clay
soil with the paint in between having a
somewhat lumpy appearance.
Surface deterioration of paint films can take
many forms, some of which are shared with
degradation seen in clear varnish coatings.
Spots on paint can be caused by mould growth
which will cause pitting or staining if allowed
to persist for any length of time. Fly excrement
causes small brown marks on the surface of
paint which by their acidic nature may also be
etched deep into the surface. Woodworm
larval tunnels under a painted surface may
leave only a thin skin that will tend to sink or
collapse in time and mature insects leaving an
underlying wooden support through the paint
are further examples of biological mechanisms
of deterioration. Discoloration (yellowing and
darkening etc.) of paint films may arise through
changes in the medium caused by light but
may also be due to, or made worse by, previous
treatments. Retouching may discolour due
Figure 8.7 Detail of the Garrick wardrobe
(c.1772–1778, made by Thomas Chippendale), before
conservation. Old restoration retouching extends beyond
the area of loss, covering original paint