Oil paintings vary hugely, but there are discernable patterns in their behaviour,
and this has led in turn to a repertoire of reliable treatment types. They are usually
painted on canvas, wood panel or board, with a primary levelling (and sometimes tinted)
The artist’s paint consists of ground powder pigments suspended in a drying oil,
that is, a plant oil that naturally sets chemically in atmospheric oxygen - a process
that the pigments themselves can sometimes hasten.
Some time after completing it, the artist would normally varnish their work to re-establish
the vivid colours that were visible when the painting was still wet. This would also
have the benefit of protecting the surface against minor abrasion, UV damage and
atmospheric pollution. Pictures have often been revarnished several times – it can
be a useful way of reviving the surface –and a substantial layer may have built up.
A layer of dust and other airborne grime quickly begins to gather on the varnish
A painting’s problems can be roughly divided between two groups: age degradation
and accidental damage.
Ageing is best thought of by contrasting the relative youthfulness, over time, of
a picture's constituent parts. Varnish is badly affected by UV and oxidation, and
is effectively a sacrificial component of the picture’s makeup. Natural resin varnishes
start to yellow and grow brittle within only a few years of drying; after half a
century, they take on a golden glow and at least the outer portion will have grown
brittle and can look crazed. Twentieth-century restorations would often be done using
varnish as a medium, older repaints being carried out in oil paint.
The canvas will last a little longer. A fine linen canvas may be up to 100 years
old before chemical degradation within its cellulose polymers start to noticeably
affect its strength and flexibility. It gradually becomes less able to hold the paint
layer in plane and to withstand the occasional scratch or knock. Regular keying out
of the wedges becomes critical in order to avoid permanent bulges or hollows forming
in the surface. Contrastingly, wood panel supports are often very long-lived, if
well cared for, although original wood-to-wood joints can be a problem long-term.
Cradled panels need checking and servicing periodically.
Consolingly, the paint itself is the most long lasting part of the structure. Linseed
oil forms a matrix which helps protect the pigments from fading, and usually hardens
into a tough polymer. Cycles of fluctuating humidity and temperature can be enough
to stress the paint film and cause hairline cracks, which may widen in time. Many
pigments – particularly those from before the age of science –are naturally-occurring
minerals that will themselves have been millions of years old before being prepared.
However, manufactured artists’ colours can be more varied, some being resilient and
long-lived, others prone to problematic alteration.
Accidental damage can happen more easily to an aged, degraded painting. As mentioned
above, old canvas will dent and break more readily than when it was new if struck
accidentally by an object. Although water ingress should in theory affect an old
textile less than a new one, the problem of water damage to canvas paintings is greatly
influenced by the differential between the reaction of the textile and the paint
and preparation. Aged paint is less flexible, and the bond between canvas and paint
gradually weakens over time, so an old picture can easily be badly affected. Varnish
becomes less and less able to cope with either mechanical damage or water, until
even innocent wipings with a damp cloth can sometimes cause problems.
Cooke and Sons has many decades of experience with dealing with the diverse problems
that age degradation and accidental damage can create. We can advise on the paths
that can be taken to achieve the finished look that the client seeks, as well as
suggest strategies to preserve the painting into the future.