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 preservatives are
continuously transported from the wood 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 finishes prior to the actual treatment.”
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