(^24) Feature
From THE DEEP
Precious metals from a 17th century
shipwreck yield their secrets to XRF
and SEM
By Inke Joosten, Conservation Scientist, Cultural Heritage
Laboratory, Ministry of Education, Culture and Science,
Amsterdam, the Netherlands.
My main expertise lies in the study of archaeological metal and
textiles. Currently, I am working together with researchers
Janneke van der Stok (Metals, Inc), Tonny Beentjes and
Maarten van Bommel (University of Amsterdam) to examine
precious metals after excavation (the AMOR project). We
selected several historically valuable gilded objects from
a 17th century shipwreck,
BZN17, which sunk near
Texel, the Netherlands. One
of the objects, a gilded brass
oval powder box with an image
of Venus and Cupid (pictured)
is of special interest because
we believe it may still have its
original beeswax coating. In
historic collections these type
of objects have been polished
and cleaned over and over again
to make them shine, removing
all original coatings. This box,
buried for centuries underwater,
may still preserve this important
information on its original
appearance. Our wider aim
is to establish a practical
and optimized research and
conservation strategy for marine
precious metals, to preserve this
important surface information.
Based on the analytical
results, we plan to develop a flowchart and workshops for
archaeologists and conservators to aid decision-making after
excavation of precious metal.
One of the difficulties in studying these objects is the
heterogeneity of the material and the fact that the objects
contain layers. For instance, if we want to analyze the
composition of the powder box with X-ray fluorescence
(XRF), several layers of material obscure the inside,
whether intentionally added (such as gilding) or corrosion
products. Another problem is that an archaeological object
may have been subjected to a conservation treatment that
has removed relevant information on manufacture, use and
burial conditions.
We prefer to study cultural heritage objects in a noninvasive
manner, extracting as much information as possible from
the object without taking samples, or using the smallest
samples possible. For the AMOR project we used X-rays
to study the construction of different objects, followed by
careful analysis with XRF. Where sampling was possible,
samples were embedded in epoxy and polished to study them
with optical microscopy and SEM-EDX. SEM-EDX is my
favorite instrument since it allows you to examine materials
in high detail and analyze the chemical composition at the
same time. Corrosion products give spectacular images!
Corrosion products also hold a lot of information about
the making of the object
and the burial conditions it
has been subjected to for all
those years. Organic materials
like our suspected beeswax
coatings can be analyzed by
GC-MS. Collaborations with
the Technical University of
Delft and the University of
Amsterdam are providing us
with access to promising new
techniques like neutron and
micro-CT imaging.
I am also involved in the
development of an “Irradiation
Passport for Art”. In the desire
to extract as much information
as possible from cultural heritage
objects, we are increasingly
exposing them to ionizing
radiation. Modern analytical
techniques use interactions
with photons, electrons and
ions to identify the materials in
the object. But those interactions could induce visible or
invisible alterations, the long-term effects of which are not
fully known. Exposure to radiation is cumulative, which
means that previous exposure may change the sensitivity
of objects or research samples. Therefore, we believe art
objects and research samples in the field of cultural heritage
need an irradiation passport, recording the location,
total exposure and circumstances of radiation exposure
to date.
“One of the difficulties
in studying these
objects is the
heterogeneity of the
material and the
fact that the objects
contain layers.”
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