New Insights into the Cleaning of Paintings

The Nonideal Action of Binary Solvent
Mixtures on Oil and Alkyd Paint:
Influence of Selective Solvation
and Cavitation Energy
Stefan Zumbühl , Ester S. B. Ferreira, Nadim C. Scherrer,
and Volker Schaible
Abstract. Cleaning strategies in restoration and conservation often involve the use of solvent
mixtures, despite the fact that the interaction between these and the objects under treatment is
not always well known. Binary solvent mixtures behave nonideally; that is, the properties are not
directly proportional or related to the mixing ratio. This fact contradicts the trivialized solubility
charts, for example, the Teas chart, known and applied in practical restoration. The description of
the dissolving action of binary systems therefore requires additional information such as the solvation
and solubilization mechanisms involved. The effects of binary solvent mixtures on oil and alkyd
paints were systematically investigated, with particular focus on the swelling action and leaching of
binding media components. In total, 75 binary mixtures made from the solvents n- hexane, toluene,
chloroform, diethyl ether, acetone, and ethanol were experimentally applied to free paint films by
immersion. The solvation was studied by application of the linear solvation energy relationship
based on Catalán’s solvatochromic system. Extracted components were quantified gravimetrically
and characterized with Fourier transform infrared spectroscopy, direct temperature- resolved mass
spectrometry, and gas chromatography mass spectrometry. Selective solvation is observed both in
oil and alkyd paints, but to a variable extent. The nonideal action of solvent mixtures, however,
arises primarily from a lowering of the cavitation energy within the liquid by the formation of associates.
This effect is thus a solute- independent process, which raises the entropy of mixing. As a
consequence, many of the solvent mixtures tested had a far more detrimental effect on the paint films
than the pure solvents.
Stefan Zumbühl and Nadim C. Scherrer, Art
Technological Laboratory, Bern University of
Applied Science Fellerstrasse 11, CH- 3012 Bern,
Switzerland. Ester S. B. Ferreira, Scientific Laboratory,
Swiss Institute for Art Research Zollikerstrasse
32, CH- 8032 Zurich, Switzerland. Volker
Schaible, Department of Restoration, The Stuttgart
State Academy of Art and Design, Am Weissenhof
1, D- 70191 Stuttgart, Germany. Correspondence:
stefan.zumbuehl@bfh.ch. Manuscript
received 19 November 2010; accepted 24 August
2012.
INTRODUCTION
The use of solvent mixtures for surface cleaning in restoration and conservation
is widespread despite the lack of knowledge on the true consequences of such a treatment.
Recently, azeotropic solvent mixtures have been proffered in particular (Saunders,
2008; Augerson, 2005). It is well known that binary solvent mixtures behave
nonideally, which means that the properties of the mixture are neither proportional nor
related to the mixing ratio. The solubility of a material is controlled by the solubilization
of the solute and the molecular stabilization of the solute (dissolved solid) within
the liquid phase. There is a clear difference in behavior between a solvent mixture and
either of the pure solvents as both their solvation properties and their cavitation energy