New Insights into the Cleaning of Paintings

28 • smithsonian contributions to museum conservation
APPLICATION EXAMPLE
The picture depicting Saint Leonardus of Porto Maurizio
was executed probably soon after Leonardus’s beatification in
the year 1797. However, after he had been proclaimed a saint
in 1867, the painting was altered in order to reflect Leonardus’s
new status: the nimbus was added, as well as a secondary inscription,
“St. Leonard.” At the same time the primary inscription was
overpainted together with the whole background area. The exact
location of both inscriptions within the painting’s structure was
resolved by means of optical coherence tomography and is described
elsewhere (Targowski et al., 2010).
This study focuses on the application of optical coherence
tomography to resolve the problems that emerged during the
conservation of the painting. The overpaint was lying on three to
four layers of varnish. It was decided that the overpaint would be
removed while attempting to preserve the old varnishes underneath.
Since, as the result of this treatment, the primary inscription
was to be revealed, the aim of the research was to find the
most safe and satisfactory treatment procedure. Cross- sectional
views (OCT B- scans) were used both to assess noninvasively the
integrity of the primary layers covered by the overpaint (Figure
1, left- hand side of the OCT images) and to examine the condition
and thickness of these layers revealed as a result of the overpaint
removal (Figure 1, right- hand side of the images).
During the first stage of the experiment, small trials of overpaint
removal were performed with different solvent compositions
in the background area of the painting. The results were
evaluated with OCT. The composition of isopropanol, water,
and ammonium (9:1:1) applied with cotton swabs gave the
most satisfactory and even results: the overpaint was removed
completely, and the varnishes were preserved. However, further
overpaint removal on the larger areas of the background proved
less successful. OCT cross- sectional images revealed little holes
in the varnish layer, which had started to occur before the overpainting
was removed completely. Therefore, another set of OCT
monitored trials was performed in order to evaluate the effect of
using the same solvent composition, but with different removal
procedures, such as rolling with a cotton swab, brushing, exposing
to poultices, removing mechanically with a scalpel, and
FIGURE 1. Trials of overpaint removal with different solvent compositions with the results evaluated by means of
OCT. Consecutive layers are visible in the OCT tomograms: 1, original paint layer; 2, three to four layers of varnish; 3,
overpaint layer. (top) Poor result obtained by removal with a mixture of ethanol and white spirit (1:1). (bottom) A satisfactory
result obtained by removal with a mixture of isopropanol, water, and ammonia (9:1:1), where the overpaint
was evenly removed and the varnish was preserved.