dk nkf - Nordisk Konservatorforbund Danmark

The industrialisation of canvas production
in Denmark and its implications for the
preservation of Danish nineteenth century
paintings
Introduction
Certain canvas paintings are prone to respond
dramatically to moisture introduced accidentally or
as part of a conservation treatment. The response, a
general or more or less localised marked shrinking,
typically produces flaking of the ground and paintlayers
in tent-like formations parallel to the directions
of the treads in the canvas. (fig 1)
Aspects of this well-known phenomenon have been
discussed in a number of papers over the years, and
it has generated research into the various elements of
primed painting canvases, their interaction and their
response to changes in humidity, as well as into the
fundamental mechanical properties of woven fabrics.
A number of factors in the fabric, size and priming
have been identified or suggested as significant to
the response of a painting at moisture absorption or
desorption.
General response patterns of canvas, primed canvas
and paintings at changes in relative humidity (RH)
have been examined by several authors [1]. The
mechanism by which swelling of the cellulosic
fibres and yarns at high moisture content (RH >80-
85%) leads to contraction of the woven fabric was
discussed by Hedley and Bilson [2] emphasizing
the swelling of the yarn diameter, not a longitudinal
shortening of the yarns, as the dominant factor
in the shrinkage. The model explains how tightly
spun yarns and closely woven canvases will shrink
more as there is less intrinsic free space to take up
for the swelling fibres in a compact yarn, moisture
absorption produces greater swelling of the overall
yarn diameter thereby accentuating the crimp of the
ceciL Krarup andersen, troeLs FiLtenborg,
annemette b. scharFF and miKKeL scharFF
transverse yarns in the weave and thus inducing the
shrinkage in that direction. A closely woven canvas
with little separation between the yarns will by the
same mechanism shrink more easily than a loosely
woven canvas. The significance of the morphology
or geometry of the fabric to the shrinking process is
further demonstrated by the fact that, as a rule, most
contraction will occur in the direction parallel to the
yarns with the greater crimp – often the warp yarns.
However, an additional – less predictable - type of
response, called relaxation shrinkage, is triggered by
the release of internal stress in the fabric, incorporated
during its manufacture.
The modifying effect of (warm) size application
on the mechanical properties of a canvas, tending
towards more isotropy in the response of the fabric,
has been shown [3]. And the role and properties of
the size and ground layers have been discussed in a
number of publications [4]. The cohesion within the
ground itself has been investigated [5] showing the
significance of the hygroscopic capacity of a ground
as a factor affecting its adhesion to the canvas.
The occurrence of marked shrinkage and the
accompanying tenting of the ground and paint
layers are high in especially 19 th century paintings,
and the technological background for this has been
inferred from early on. As it happens, the big shift in
linen manufacture took place c. 1820-50 in Britain
when power-spinning and power-weaving of flax
were established. It has been generally accepted that
the yarns and fabrics produced by the mechanisation
were often more tightly spun and woven than their
equivalents made by hand.
39