Extraction of Material from Oil Paints by Solvents
- PDF / 528,873 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 216 Views
Extraction of Material from Oil Paints by Solvents David Erhardt, Roland H. Cunningham, and Sirpa Räsänen1 Smithsonian Center for Materials Research and Education, Smithsonian Institution, Washington, D.C. 20560, U.S.A. 1 Institute of Arts and Design, Lummetie 2, 01300 Vantaa, Finland ABSTRACT Drying oils used to formulate oil paints consist primarily of polyunsaturated triglycerides, along with smaller amounts of mono- and diglycerides, free fatty acids, and other compounds such as sterols. The drying of oils occurs through an oxidative crosslinking process that also produces smaller scission products such as short chain fatty acids and diacids. Compounds that are not attached to the crosslinked oil matrix include glycerin, the unreactive free saturated fatty acids and their glycerides, soaps of fatty acids produced by reaction with metal ions from the pigment, and scission products. As the oil ages, additional soluble material is produced by hydrolysis of glyceride esters. These relatively low molecular weight compounds can be extracted by exposure to solvents, as occurs during the cleaning of paintings. Excessive extraction of material can embrittle the paint film and affect the appearance of the paint. This paper examines the types and amounts of compounds extracted from oil paint films as a function of solvent, exposure time, pigment, and age of the paint. The results indicate that solvents vary more in the speed that they extract material than in their selectivity. The composition of the extracts is consistent with predictions based on the mechanisms of the drying and aging processes. These results confirm the importance of using the least polar solvent possible to clean paintings and to formulate varnishes.
INTRODUCTION Oil paints are made by combining primarily drying oils and pigments, though lesser amounts of other materials may be added as modifiers. Drying oils are plant-derived oils that polymerize upon exposure to air. When drying oils are mixed with pigments and spread out and exposed to air, the resulting layer polymerizes to form a rubbery, durable colored film. These oil paints have been used for centuries to produce works of art. Varnishes are often used to cover the surface of paintings, serving both to protect them and to saturate the colors. Unfortunately, many of the traditional, as well as newer, varnishes darken over time and must be removed. Other materials also obscure or mar the surface of paintings, including accumulated dirt, soot, and later additions of overpaint or retouchings. Almost every imaginable method and material has been used to clean paintings, including caustic soda, alcohol, ammonia, soap, turpentine, bread, erasers, and recently, lasers. Needless to say, not all approaches are equally successful, and many paintings have been severely damaged. Presently, the use of solvents is the most common method of cleaning oil paintings, having been found in most cases to produce the least visible change when used properly. Solvents do affect the original paint, however, and the choi
Data Loading...
