New Insights into the Cleaning of Paintings

number 3 • 57
FIGURE 11. Differential scanning calorimetry plot of two paints
made with 100% hydrolyzed linseed oil with lead white and red
iron oxide pigments, respectively. The peak around 50°C for the red
iron oxide paint reflects a phase transition of some fatty acid that
does not appear in the lead white paint.
Acetone is the most aggressive solvent, to the point that the
paints decompose after treatment. The toluene-treated paints
were very embrittled and decomposed with the application of
mechanical force. Water and hexane, both rather mild solvents,
did not affect the paint films. Previously reported treatments of
red iron oxide paints made with 25%, 50% and 75% hydrolyzed
oil showed that the effects of hydrolysis became significant for
paints with over 50% hydrolyzation (Tumosa et al., 2005).
The paint made with lead white and totally hydrolyzed
linseed oil would be unaffected by the solvents based on the
differential scanning calorimetry data obtained. The smalt was intermediate
to the red iron oxide and white lead paints in behavior.
The loss of these low molecular weight compounds, regardless
of whether this occurs by evaporation, migration within the
paint, or removal by solvents, can embrittle a paint film and promote
cracking as well as loss of color saturation (Tumosa et al.,
1999; Erhardt et al., 2001).
occurs over time. Improper temperature on a hot table may do so
as well since the volatility of the fatty acids becomes significant
above 70°C to 80°C (Erhardt et al., 2000).
Migration of fatty acids, for example, may occur from an
area with little binding capability, such as one containing earth
colors (iron oxide–based pigments), to an area where with considerable
binding, such as one with lead compounds or zinc
oxide. The graph shown in Figure 11 is a differential scanning
calorimetry plot for two paints made from hydrolyzed linseed
oil with lead white and red iron oxide pigments, respectively.
Both paints form coherent films, but upon heating, a low melting
phase transition in the red iron oxide paint occurs that is absent
in the lead white paint. As indicated by the low-temperature
phase transition at 50.5°C, this phase should be quite mobile
and capable of migration. It is also apparent that lead-containing
areas bind that mobile phase since this phase transition is absent
in the lead white paint.
Table 1 shows the reactions of selected paints made with
100% hydrolyzed linseed oil and different pigments, such as
red iron oxide, burn umber, smalt, and raw umber. These paints
mimic the worst case since all the ester bonds are hydrolyzed.
The paints were immersed in the solvent for 5 minutes.
CONCLUSIONS
In conclusion, early hydrolysis of oils can lead to the formation
of free fatty acids that can alter the initial chemistry and
mechanical properties of oil paint films. The diffusion of small
molecular weight compounds through the films of a painting may
be quite extensive, with metal ions influencing drying rates and
fatty acid anions (or neutral compounds) affecting stiffness or
flexibility as well as forming accretions. Removal of these small
compounds by solvents can embrittle a paint film and promote
cracking. Loss of these small compounds by any mechanism will
also result in a loss of color saturation. However, the nature of
these compounds has yet to be determined.
This study has shown that there still are other questions that
require answers. For example, the drying of alizarin paint is accelerated
if the paint is applied on a copper substrate, but it is
practically not influenced when the paint is applied over a lead
white paint. Also, alizarin in a linseed oil with litharge showed
an unexpectedly slow drying. Furthermore, the presence of inert
materials decreases the mechanical properties of the paint films
significantly.
Table 1. Reaction of paints made with 100% hydrolyzed linseed oil and different pigments
after immersion with selected solvents for 5 minutes.
Solvent
Pigment Acetone Toluene Hexane Water
Red iron oxide decomposed embrittled coherent coherent
Burnt umber decomposed embrittled coherent coherent
Smalt decomposed embrittled coherent coherent
Raw umber decomposed embrittled coherent coherent