New Insights into the Cleaning of Paintings

number 3 • 73
Another key finding of this work was that the groups of
solvents consisting of aliphatic alcohols (methanol, ethanol,
propan- 1- ol, etc.), aliphatic acyclic ketones (acetone, butanone,
etc.), and the lower aliphatic esters (n- butyl acetate, ethyl propanoate,
etc.) generally fell within the class of low- moderate
swelling power. Since these solvents are common staples for practical
varnish removal, this finding should be somewhat reassuring
to most practitioners who rely heavily on these solvents. By the
same token, it must be recognized that oil paint may still be very
sensitive to solvents that fall into the low- moderate swelling category
and possibly even some of those in the low swelling group.
The initial interpretation of the swelling data obtained on
paints 16 and 17 (Phenix, 2002b) consciously avoided using the
Teas chart because of the now well- established, intrinsic theoretical
shortcomings of the Teas fractional solubility parameter
system (Phenix, 1998). Plotting the complete set of swelling results
(DA max
% values) obtained for paint 16 in pure solvents
and solvent mixtures, categorized as in Table 1, highlights some
of the difficulties with this mode of graphic representation. It
becomes apparent immediately that the area of special sensitivity
of oil paints to solvents is appreciably more complex than
the single zone of high swelling suggested by Hedley (1980).
Solvents capable of causing significant degrees of swelling, for
example, those having DA max
values of 12% or greater for paint
16 (high- moderate, high, and very high swellers), are spread over
a large part of the Teas chart, covering a broad range of polarities,
extending from the aromatic hydrocarbons and apolar mixtures
of ethanol and white spirit through the chlorinated solvents
(Teas fractional dispersion force parameter (f d
) values of ~60–70)
to the strongly dipolar solvents such as the amides and cellosolves
(f d
values of ~40–50). The areas of low and low- moderate
swelling (DA max
< 12%) correspond mostly to the aliphatic hydrocarbons,
aliphatic acyclic ethers, aliphatic alcohols, aliphatic
acyclic ketones, and some of the esters (Figure 4).
Rather than the single zone of highly active solvents represented
by the Hedley- Stolow peak swelling region, it can be seen
now that high and very high swelling solvents and solvent mixtures
lie in several zones across the Teas chart. One of the more
significant observations on the results as a whole, when presented
using the Teas fractional solubility parameter diagram, is
that scattered between and very close to the zones corresponding
to high- moderate, high, and very high swelling pure solvents and
solvent mixtures, there are several solvents that cause just lowmoderate
swelling on the paint film tested. These isolated regions
of low- moderate swelling arise from the fact that the aliphatic
acyclic ketones and the aliphatic acyclic esters populate the central
area of the active part of the Teas chart, between about f d
=
48 and 65, yet the aliphatic acyclic ketones and esters are just
low- moderate swellers compared to the high- or high- moderate
swelling solvents all around. This anomaly reflects some of the
limitations of the Teas fractional solubility parameter chart:
solvents from families with quite different solubility properties,
for example, ketones and amides, are clustered close together in
similar regions of the chart. Solvents having similar Teas solubility
parameters can have very different swelling powers: compare,
for example, acetone (f d
= 47, f p
= 32, f h
= 21; DA max
≈ 9%) with
N- methyl- 2- pyrrolidone (f d
= 48, f p
= 32, f h
= 20; DA max
≈ 30%).
Irregularities such as this, which become even more evident when
considering mixtures of solvents, illustrate the limitations of the
Teas fractional solubility parameter diagram as a reliable map
for safer cleaning practice, to the extent that in the eyes of some
“this system is not an adequate tool for restorers” (Zumbühl,
FIGURE 4. Teas solubility diagram, showing for paint type
16, zones corresponding to different degrees of maximum
swelling: pure solvents and solvent mixtures.