THE EFFECT OF SUPERCRITICAL CARBON DIOXIDE EXTRACTION ON COLOR RETENTION AND PESTICIDE REDUCTION OF WOODEN ARTIFACTS
SUNG MO KANG, ACHIM UNGER, & J.J. MORRELL
For centuries, religious artifacts such as icons have been carved from indigenous materials, including wood. Wooden works of art have not generally been prone to fungal attack because they have been preserved under dry conditions, but they have been susceptible to attack by a variety of wood-destroying insects. To reduce the risk of insect infestation, many museums began to apply pesticides to the surfaces of these wooden artifacts in the 1940s (Unger 1998). The pesticides included a variety of organophosphate and organochlorine compounds, but one of the most commonly used was DDT (1, 1, 1-trichloro-2, 2-bis (p-chlorophenyl)-ethane). DDT is an organochlorine compound that revolutionized insect control in the 1940s. Although banned in much of the world in the 1970s because of concerns about concentration of DDT through the food chain, this pesticide continued to be used for many years in Eastern bloc countries, including the former East Germany. Reunification of Germany brought with it the challenge of restoring many artifacts that had formerly been treated with DDT.
DDT was generally applied in solvent-based formulations that penetrated to various depths into the wood, depending upon the wood species. The primary goal of its use was preventive rather than remedial, and most of the preservative was confined near the surface. Later, the safe handling of DDT-treated objects became a matter of concern, as did the potential for adverse effects when DDT bloomed on the wood surface.
While DDT can be extracted using conventional solvents, these solvents can be damaging to the artifacts or the pigments used to color or decorate them. Many wooden artifacts were coated with a variety of pigments and sealants in specific sequence to create certain hues. The coatings consist of both organic and inorganic compounds. Extraction with organic solvents can selectively remove pigment components, producing deleterious changes in both color and coating adhesion.
One alternative to conventional solvent extraction could be the use of supercritical fluids (SCF). SCFs have the ability to penetrate materials like a gas, thus offering the potential for easily penetrating a material or the film coating on the surface to strip away residual pesticide. The other advantage of using SCFs is that solubility is infinitely adjustable by varying either temperature or pressure within the critical region (Clifford 1998). These properties have encouraged the use of SCFs to recover natural products from a variety of wood-based materials (Fremont 1981; Larsen et al. 1992; Terauchi et al. 1993; Ohira et al. 1994). SCFs solubilize materials like a liquid solvent, but it may be possible to adjust conditions so that DDT is solubilized without affecting pigment components.
DDT can be readily extracted from contaminated soils using supercritical (SC) carbon dioxide (Snyder et al. 1992, 1993). There is relatively little literature, however, on either the ability of SCF to extract pesticides from wood or the effects of extraction on the integrity of the artifact or surface coatings. Sahle-Demessie et al. (1997) successfully extracted pentachlorophenol from wood in work that led to two patents (Levien et al. 1994; Ruddick and Cui 1995). More recent studies have explored extraction of other pesticides, but DDT has been infrequently used for wood protection. SCF extraction of DDT from soil using carbon dioxide with or without a co-solvent appears to be relatively straightforward, suggesting that similar processes might be useful for remediating pesticide-contaminated wood artifacts (Snyder et al. 1993). While DDT may be easily extracted using SCFs, the process may have deleterious effects on the artifacts. We explored the potential effects of SCF extraction on the color of various simulated artifacts and its ability to remove DDT.