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Subject: Mold on stone

Mold on stone

From: Stefanie Scheerer <stefscheerer<-a>
Date: Friday, November 11, 2005
Shelley Reisman Paine <shelley [at] srpaine__com> writes

>I am working on a set of oversize outdoor sculpture made from
>Carrara marble.  The sculpture stood outdoors for sixty years. After
>it was vandalized in 2000 the owner stored them for five years in 4
>crates (9.5 feet x 4 feet x 4 feet) filled with straw.  While in
>storage large dark gray areas developed.  Mary Lou Florian
>identified the material as alternaria or cladosproium.  The fungal
>structures are not normal hyphae but sclerotic--heavily pigmented
>(melanin) hyphae.

As you reported, the cause for the microbially induced damage has
already been identified by a microbiologist as two different fungi
(alternaria and cladosporium) with heavily melanised reproductive
structures. I would therefore rather concentrate on these
microorganisms than exploring if the staining could be due to the
presence of pigment-producing bacteria.

However, this does not mean that these bacteria may not be present,
as they have frequently been isolated from outdoor stone monuments.
In fact, the microbial community of stone surfaces is always an
interaction of many different microorganisms. Alternaria and
Cladosporium are very common fungi and their reproductive structures
are nearly everywhere. Therefore, it is not surprising to find that
they grew heavily when the statute was packed with straw, which
provides better growing conditions for these fungi than dry stone
surfaces that are exposed to sunlight.

The question is whether you really need to kill the fungi, as the
change in climate provided by unpacking the sculpture, will most
likely be enough to stop further growth (in fact the sculpture
survived for 60 years without significant fungal growth before being
packed). Further, a lot of conservation procedures will have
fungicidal action (e.g. treatment with 70% ethanol or other organic

However, you need to keep in mind that even when the fungal
structures are dead, they will still be a health hazard. Dead fungal
spores, when getting into your lungs, will still be recognized as a
foreign substance and may cause an allergic reaction. Hence, when
removing the fungal structures, appropriate protective clothing and
removal of the contaminated air is necessary. Some attempts have
been made with brushing and vacuuming (don't forget HEPA filters)
with micro-nozzles under a stereo microscope. You will most likely
not be able to remove the fungal structures entirely, as some of the
hyphae may penetrate fairly deep into the stone (up to a few
centimeters, depending on the condition and porosity of the stone).

The other question is how to remove the dark stains, which is rather
an aesthetical question than a matter of biodeterioration. I have
not read Delgado Rodrigues paper (as that particular issue of
Studies in Conservation is missing from our library). However, I am
sure that you will get useful information from this article.
Further, I am aware of a project at the Metropolitan Museum, on
removing melanised fungi from paper. The aim of the project was
"identifying the species present and developing 'designer' enzymes
that can effectively and safely remove the disfiguring stains
associated with them." as reported in Met Objectives in Spring 2002,
which can be found at


    **** Moderator's comments: The above URL has been wrapped for
    email. There should be no newline.

Further, dark stains on white marble sounds like the ideal situation
for laser cleaning. Potentially, the dark biological structures will
strongly absorb the laser radiation, while the white marble will
reflect it. I did some laser tests on biological growth on stone
samples from temples of Angkor in Cambodia (different stone and
microorganisms), and I had relatively good results. In cases where
the biological growth could not be removed, it was bleached, giving
at least a more satisfying aesthetical appearance. I hope this
helps. And please report on your method if you have any success!

Stefanie Scheerer
Microbiology Dept.
Cardiff University
PO Box 915
Cardiff CF10 3TL
Wales UK

                  Conservation DistList Instance 19:26
                Distributed: Saturday, November 19, 2005
                       Message Id: cdl-19-26-007
Received on Friday, 11 November, 2005

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