Proceedings e report - Firenze University Press

ORIENTATED INVESTIGATION TO KILL THE MYCELIA OF THE DRY ROT
FUNGUS, SERPULA LACRYMANS, WITH MICROWAVES
Merle Strätling 1 *, Wibke Unger 2 , Karin Petersen 3
1 National Museum of Art, Architecture and Design, Oslo, Norway
2 University of Applied Sciences, Eberswalde, Germany
3 University of Applied Sciences and Arts, Hildesheim, Germany
Abstract
The Serpula lacrymans is one of the most important wood decay fungi in buildings. Chemical and physical
methods of treatment have been used in the past. In a Masters thesis at the University of Hildesheim the
possibility of a thermal treatment using microwaves was investigated. Infected wet and dry pinewood samples
were exposed in an open system to microwave fields (at a frequency of 2.45 GHz). Here different kinds of
radiations were used for various lengths of time up to temperatures of 50°C. It was concluded that the tested
methods are not suitable for the treatment of sensitive wooden artefacts.
1. Introduction
The dry rot fungus Serpula lacrymans (Wulfen: Fr.) Schroeter holds a special notoriety in the built
environment due to its vitality, its very destructive potential and the huge costs involved in rectifying
the damage caused.
The German Industrial Standards, under DIN 68800-4, recommend infected buildings are treated by
eliminating the source of moisture, generously removing all visibly affected areas, and applying a
chemical protecting agent. In the case of historic buildings with valuable works of art, like painted
wooden beams, polychrome panelling or carvings, and furnishings, such a form of intervention would
be very harmful and certainly contradict the ethical demand to preserve as much as possible of the
original substance. For infected timber that has to remain in the installation position, or can not be
removed due to its heritage value, the German DIN 68800-4 gives the option to treat the fungi with
special physical technology, provided that you can guarantee that you kill all the wood-destroying
organisms.
The existing physical possibilities of killing the dry rot fungus were all been based on heat,
particularly treatment with warm air. Treatment with electromagnetic waves has been established so
far only in Denmark. The reasons for the comparatively limited application in Germany are due to the
little knowledge about usage, physical effects, advantages and also disadvantages. Furthermore the
little scientific research that exists does not provide a sufficient basis to make an assessment
From a technical viewpoint the treatment of brickwork, cement, concrete or clay with microwaves is
unproblematic, as long as this does not involve walls that are several metres thick and which can not
be heated up due to the limited depth of microwave penetration. Usually it is possible, depending on
the dielectric permittivity, to heat the materials up to 60°C relatively quickly. However, with wood
there is not much experience and most companies do not even treat it. To great is the danger of
damage in the form of cracks, leaking resin, or damage to the transparent coating or paint.
Furthermore, the heating with microwaves is due to the electromagnetic field distribution very
inhomogeneous and that is why you have to anticipate high differences in temperatures.
Concerning disinfection, the literature indicates various lethal temperatures for the mycelia of Serpula
lacrymans, depending on the experimental set-ups [1]. Within the scope of experiments with heat
coming from outside, the temperature and duration time goes from 17 hours at 35 - 37°C until 15
minutes at 40 - 60°C. According to the latest insights by HUCKFELDT [2] at least 65 - 70°C are
needed to kill the mycelia effectively. Conspicuously lower temperatures were reported from
experiments with microwave treatments. BECH-ANDERSEN & ANDERSEN [3] needed only 10 min
at 37°C to kill surface mycelia in small diameter glass tubes and KJERULF-JENSEN & KOCH [4]
reported that 40 - 50°C was appropriate for infected sawdust. In both cases the temperature had to be
* E-mail: wunger@fh-eberswalde.de
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press