The Changing Mechanical Properties of Aging Oil Paints
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The Changing Mechanical Properties of Aging Oil Paints Marion F. Mecklenburg1, Charles S. Tumosa, and David Erhardt 1 Smithsonian Center for Materials Research and Education 4210 Silver Hill Road, Suitland, MD 20746 ABSTRACT The stiffening and embrittlement of oil paints over time has been a real concern for those responsible for the long term care and preservation of paintings. This paper examines the effects of time, pigments, relative humidity (RH), temperature and solvents on the mechanical properties of traditional oil paints. In this way it is possible to determine the role of each factor in causing the paints to become brittle. Even after 14 years the oil paints show little evidence that the long term “maturing” processes have begun to slow down. It is shown that there seems to be little correlation between the time paint requires to “dry-to-the-touch” and the longer term mechanical properties. Both low and high temperature levels can increase the stiffness of the paints though the mechanisms are quite different. Considerable hydrolysis of the paints occurs early in their history and the ones that hydrolyze most quickly are the ones that remain the most flexible. INTRODUCTION, THE EARLY DRYING STAGES OF PAINTS MADE WITH DRYING OILS The chemistry of the drying of oil paints is a complex, multi-step process in which oxygen is absorbed, unsaturated ester hydroperoxides are formed, and then the hydroperoxides decompose with subsequent cross-linking. This process called autooxidation has been extensively reviewed [1-10]. In the process of absorbing oxygen, the paint initially gains weight and simple weighing over time allows one to follow the early stages of the drying process. Figure 1 illustrates the weight gain of four different paints made with lead carbonate ground in cold pressed linseed oil, cold pressed safflower oil, walnut oil, and poppy seed oil over a 20 day period. Within this period of time a thin tack free film forms on the surface of the paint. The time to form this film is one definition of the “drying time.” Each of the oils has a different “dry to the touch” period and the letter D placed on each of the plots indicates that time. For example, the paint made with the cold pressed linseed oil takes 16 days while the paint made with the walnut oil takes only about three days. The weight plots level out at that time indicating that a rather impervious film has formed and oxygen penetration is thus limited. The inability of the paint to absorb easily additional oxygen slows the drying of the paint below the surface film. Within about three years, however, most but not all paints having a thickness of about 0.25mm or less will form a cohesive film throughout its thickness. It is what happens beyond those three years that is of interest.
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Lead Carbonate
Weight Change (%)
1 D
0.8
D
Walnut Oil D
D
0.6
Safflower Oil
0.4 Poppy Seed Oil
0.2
Cold Pressed Linseed Oil
0 0
5
10 Time (days)
15
20
Figure 1. The percent weight gain of oxygen versus time for paints made with
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