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Detox cure for art treasure

September 09, 2002

Many museums and churches own pieces of art which were treated with toxic pesticides in the past. Between the 1940’s and 80’s, it was quite usual to protect wood against insects and microorganisms with PCP (pentachlorophenol), DDT and lindane. Though the adverse effect was only recognized later: Lindane and PCP evaporate into the air, while DDT often builds up a white crystalline coating on the surface of the wood. Restorers may be able to clean the surface, but it is nearly impossible to penetrate deeper into the wood. Depending on how much DDT has been soaked up by the sculptures, the crystalline layer will reappear soon after the treatment.

Looking for ways to effectively detoxify wood sculptures even from the inside without damaging them, the German Environmental Foundation (Deutsche Bundesstiftung Umwelt) financially supported a project. Its leader is Dr. Achim Unger from the Rathgen research laboratory run by the Berlin museum authorities. Within this project, researchers from the Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT have been working on the development of a sophisticated extraction process over the past years. Their method is based on treating the objects with supercritical carbon dioxide, similar to established processes like the recovery of natural flavors or the decaffeination of coffee and tea.

Under pressures between 100 and 400 bar and at temperatures of 35 to 70 °C carbon dioxide is a supercritical fluid - a physical state between gas and fluid. One of the primary advantages of this solvent is its deep intrusion into the wood without swelling it. The supercritical carbon dioxide is pumped through a closed cycle inside a high-pressure set-up. In this way, the wood preservative are continiously transported from the wood piece into an activated carbon filter. Computer-controlled pressure and temperature are of primary importance to the process. If, for instance, the pressure would be released too quickly, the paint and adhesive layers or even the wood itself could be damaged by the expanding gas.

Currently, the scientists are able to remove up to 75 percent of DDT from the wood and up to 90 percent of lindane. “Our aim is to clear off more than 90 percent of all toxic preservatives”, says UMSICHT engineer Erich Jelen. “One of our greatest challenges is the nature of the valuable artifacts. We therefore have to test the behavior of various paints, glues and finshes prior to the actual treatment.”

Fraunhofer-Gesellschaft