Paintings are usually classified by the binders, such as oil or tempera painting (tempera binders are proteins or polysaccharides). However, both oil and tempera paints have sometimes been used in parallel and on the same paintings by Old Masters such as Sandro Botticelli or Leonardo da Vinci. The analysis of paintings of the Bavarian State Painting Collections (Alte Pinakothek) in Munich revealed that in most cases, oil and tempera binders (mostly egg) have been used in the same paints. [1] This raises the questions how oil and protein binders can be mixed and how this can be used to adjust paint properties according to the needs of the artists.
Application of colloid chemistry to the problem [2] revealed that several microstructures can be formed with oil, egg and pigments: in addition to oil-in-water emulsions (i.e. fatty tempera, so-called tempera grassa), capillary suspensions can be produced, or pigments can be coated with proteins prior to mixing with oil. In a cooperation of the Karlsruhe Institute of Technology, the University of Pisa and the Doerner Institut, the influence of the microstructure on the rheological and chemical properties of paints has been systematically studied. [3,4] The study confirmed that the paint properties depend mainly on the type of microstructure rather than the chemical composition of the samples. Mixing egg into an oil paint with a brush creates capillary suspensions, with the aqueous phase forming a percolating network connecting the pigment particles. The high surface tension of water turns the paints very stiff by raising the yield stress. However, this network is destroyed during brushing of the paint, which results in comparably low viscosities during application despite high yield stresses allowing pronounced impasto. Protein-coating of the pigments also reduces high-shear viscosities and therefore paints can still be applied easily even if pigment concentrations are increased. It is important to point out that higher pigment-binder ratios result in more stable paints: with less binder, degradation phenomena like discolouring, darkening and cracking as well as wrinkling can be reduced.
Literature:
[1] P. Dietemann, U. Fischer und D. Karl unter Beteiligung von U. Baumer und C. Steuer, Die Bindemittel der Florentiner Malerei, in: Florentiner Malerei – Alte Pinakothek. Die Gemälde des 14. bis 16. Jahrhunderts, hg. von A. Schumacher mit A. Kranz und A. Hojer, Deutscher Kunstverlag, Berlin 2017, S. 92-105. [2] P. Dietemann, W. Neugebauer, L. Lutz, C. Beil, I. Fiedler, U. Baumer, e-Preservation Science 2014, 11, 29-46. [3] O. Ranquet, C. Duce, E. Bramanti, P. Dietemann, I. Bonaduce, N. Willenbacher, Nat Commun 2023, 14, 1534. [4] O. Ranquet, C. Duce, G. Caroti, P. Dietemann, I. Bonaduce, N. Willenbacher, ACS Appl. Polym. Mater. 2023, 5, 4664−4677.
