Series editors' preface - Wood Tools

(a) (b)
to changes in the medium used that are different
from the original medium and may extend
considerably beyond the original areas of loss
(Figure 8.7). When varnish is removed from a
surface during cleaning it may be left in the
hollows of the texture of a paint film and may
show up later as a mottled darkening of the
surface. Such varnish may prove difficult to
remove without causing damage to the paint.
Previous treatments to improve clarity of a
surface may have included rubbing with oil,
leaving a sticky film which collects dirt.
Mechanical deterioration of paint includes
scratches, dents and abrasion. Old wear and
natural abrasion of a painted decorative surface
through use is common in furniture and often
considered appropriate and part of its history
(Figure 8.8). The degree to which this sort of
deterioration of a decorative surface is considered
acceptable and what sort of treatment is
in order may vary from collection to collection.
However, the evidence found in a surface
coating about an objects history is valuable and
should not be discarded simply to obtain an
ideal appearance for display. For further information
on historic binding media, the technology
of paints and the defects of paint films see
Masschelein-Kleiner (1985), Martens (1968) and
Hess (1965).
8.9.4 Transparent top coatings –
varnishes
Changes in the appearance, mechanical
properties and solubility of coatings result
Deterioration of other materials and structures 337
Figure 8.8 Old wear and natural abrasion: (a) detail of natural wear on the arm of a painted chair, Pennsylvania,
c.1750; (b) detail of natural wear on a painted blanket chest, Pennsylvania, c.1788
from a combination of chemical and physical
interactions between the coating and its
environment. Coatings are usually readily
oxidized. Light, heat and moisture are important
contributing factors. The most common
visual change is a reduction in transparency.
This is particularly true with natural resins.
Changes in the optical qualities of the coating
may include colour changes such as yellowing
and darkening, changes in gloss and loss
in transparency. Cracking, checking and
crystallization may contribute to these
changes. The adhesion of transparent coatings
to the underlying decorative surface may fail
causing it to de-laminate and flake off. This
can also cause a significant change in the
visual properties of the surface and leave
portions of it exposed to the environment.
SEM photographs of aged resin coatings on
wood are shown in Figure 12.8c,d. As
coatings age, the molecular structure is likely
to be altered chemically, particularly due to
photochemical reactions involving oxygen.
Reduction in molecular weight and average
length of polymer chains are common consequences
of this process. Crazing or craquelure
is a common manifestation of oxidation, and
will cause the varnish to refract or bend light
differently than when fresh, causing a visual
change. While an aged coating may appeared
to have ‘dried out’, it is actually a much more
complicated combination of non-reversible
physical and chemical changes. The notion of
‘feeding’ such a dried out surface to restore its
vitality is inappropriate and misinformed.