Varnish, a thin, transparent final coating applied after the painting
has dried, is important in other ways. The accumulation of dust and
dirt can obscure the paintings aesthetic qualities - bright colours
become muddied, dark shadows become greyish and lose their depth.
A protective varnish means that a painting can be cleaned by the proper
removal of the varnish on which the dirt resides. When an attacker
spray-painted Picasso's Guernica, the conservators only needed to
carefully remove the varnish layer, and with it the spray paint, and
no permanent damage was done.
Without this layer, the dirt would become embedded in the actual
paint surface, and cleaning would be a complicated and painstaking
task, if feasible at all. This is especially true with works of
art painted in the era before electricity when candles, coal stoves,
and fireplaces produced smoke and soot which easily dirtied a painting's
surface. That, and the fact that some older varnishes were made
from questionable materials that darkened quickly, means that any
older canvas should be examined by a quality conservationists to
see if it needs cleaning.
 Varnishes
were often applied to paintings built up with glazes - thin, translucent
layers of paint which, when used to good effect, produces depth
and richness of space and colour. The danger of cleaning is that
when the varnish is removed, an inexperienced or improperly trained
conservator may remove some of the paint with it. This is another reason
to choose carefully a quality, knowledgeable craftsman to restore
a painting to its original beauty.
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