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Sainte-Marie Among the Hurons,
near the present-day town of Midland, Ontario, was established as a Jesuit
mission in 1639. For 10 years it served as a retreat for Jesuit missionaries
working in small villages in Huronia and as a centre of French culture in the
area. The Hurons were allies and trading partners of the French and had firm
control over this area, which was valuable for its strategic location and rich
fur trade. Ongoing hostilities between the Huron and the Iroquois reached a
peak in 1648-49, resulting in decimation of the once large Huron nation and
withdrawal of the French from Huronia. In March 1649 two Jesuit Fathers, Jean
de Brébeuf and Gabriel Lalemant, were tortured and put to death by the
Iroquois. Several days later their remains were recovered by the Jesuits and
returned to Sainte-Marie, where they were interred together. Later the same
year, the Jesuits themselves destroyed Sainte-Marie to prevent it from falling
into the hands of the Iroquois.
In 1954, during excavations at Sainte-Marie, a small lead plaque was
discovered. The plaque inscribed
P. Jean de Brebeuf
bruslé par les Iroquois
le 17 de mars l'an
1649
identified this as the burial place of the Jesuit martyrs. The plaque has
been on display since 1971 at the interpretation centre of the reconstructed
mission of Sainte-Marie Among the Hurons. Several years ago the plaque began to
actively corrode, as white corrosion products started to form on the grey
plaque. When the extent of corrosion noticeably increased, the plaque was sent
to CCI for treatment.
We were unable to find a reason for the plaque beginning to corrode or for
the increase in the rate of deterioration. There had been no change in the
local environment or the display case in recent years.
The plaque was very heavy for its size, indicating the presence of a good
deal of metallic lead. It was dark grey in colour, with an uneven, patchy
surface and areas of dark brown discoloration. Black material appeared to
highlight some of the incised letters and white fluffy powder covered areas of
active corrosion. The corrosion products were identified as basic lead
carbonate and possibly lead formate. Basic lead carbonate was also identified
in a sample of the black material by Analytical Research Services staff Jane
Sirois, Marie-Claude Corbeil and Elizabeth Moffatt. An earlier analysis by Jane
Sirois and Judi Miller detected lead acetate. These are the forms of corrosion
that normally occur on lead in the presence of organic acid vapours. Basic lead
carbonate can protect the lead underneath if it has formed in a homogeneous
layer that is strongly adherent. However, in the presence of small amounts of
organic acid vapours, basic lead carbonate forms as a loose, non-adherent
powder. The organic acid vapours stimulate corrosion and act as a catalyst.
Once active corrosion has started, it can continue for a prolonged period in
the presence of carbon dioxide alone, the organic acid being regenerated and
largely reused as the porous corrosion layer thickens.
We had two priorities in conserving the plaque: to halt the ongoing
corrosion and to document the plaque as fully as possible.
The plaque was examined by x-radiography. The incised lettering appeared to
be present to varying depths in the metallic lead, not just in the corrosion
layer. A number of corrosion pits or casting flaws also showed up on the
radiograph, suggesting that a completely stripped surface might be quite
pockmarked.
There are several different approaches which have been used for the
treatment of lead: stripping the corrosion product by chemical or electrolytic
methods; and reducing the corrosion products back to metallic lead by chemical
or electrolytic means. It was necessary to remove, decrease or convert the
corrosion to allow access to the lead acetates and formates, the source of the
ongoing corrosion. Though these are both readily soluble in water, they were
not accessible under the carbonate layer.
Before choosing the most appropriate treatment for the plaque, we decided to
do a series of experimental treatments on corroded lead samples. The first step
was to prepare the corroded test samples. Pieces of lead were prepared to
approximately the same size as the plaque, with incised lines or lettering.
They were put into a humidity chamber to corrode, in the presence of moisture,
carbon dioxide and oak shavings (a source of acetic acid). Needless to say, the
lead samples steadfastly refused to corrode for the first several months. After
six months, most of the lead samples had at last built up a layer of white lead
carbonate and were ready for the experimental treatments.
In the meantime we began to consider our other priority - documentation of
the plaque. CCI staff have already benefitted from research on a laser scanner
being done at the Photonics and Sensors Section, Laboratory for Intelligent
Systems, Division of Electrical Engineering, National Research Council of
Canada. Laser scanning seemed the best way to accurately and minutely record
the surface detail of the plaque, and to enable a replica to be made without
directly handling the surface. The laser scanner records information on
dimension and reflectivity. On the plaque, which is about 9.4 cm by 5.2 cm, the
NRC laser scanner was used to record data at 1500 points across the long side
and 1000 points across the short side — a total of 1,500,000 pieces of
information. The data were stored in a computer, where they could then be
manipulated a variety of ways or used to control a milling or engraving device
to produce a replica. In previous work, NRC had never tried to engrave detail
as fine as the corrosion layer of the plaque, so it was necessary to find a
suitable engraving material. Several materials were tested, including modern
resins, foams and lead. It turned out that there was a low-tech answer for this
high-tech situation — plaster. Cast blocks of moulding plaster were found
to take a high degree of detail. Though plaster is physically a soft and easily
damaged material, it is very stable chemically. Several copies of the plaque
were engraved, each copy taking about two days to cut. These plaster replicas
can now be coloured to closely resemble the original or can be used to make
further reproductions.
Once the lead test pieces were corroded we started testing several possible
treatments: electrolytic consolidative reduction in sodium hydroxide or
sulphuric acid electrolyte; consolidative reduction in sodium dithionite (a
strong reducing agent); and dissolution of the corrosion products in a
chelating agent (DTPA — diethylene triamine pentaacetic acid — or
EDTA — ethylene diamine tetraacetic acid). Consolidative reduction is
often used on objects in which the surface detail is present only in the
corrosion layers and not in the metallic lead. In this case, the only way to
save the information on the surface of the object is to convert the corrosion
into metal. The corrosion is more voluminous than the metal, so when the
corrosion is reduced to metal, the resulting surface has a less compact, lacy
appearance — not the original surface. In our trials we found that the
electrolytic methods produced unpredictable results. It is also not possible to
see what is happening during the process, since the object is sandwiched
between layers of sponge to keep the corrosion in place.
The sodium dithionite treatment also results in reduction of lead
corrosion to metallic lead. However the newly converted lead did not remain on
the object. A dark grey deposit on the bottom of the treatment container was
identified as lead by Jane Sirois. The surface of the test pieces after
treatment was uneven in colour, and some corrosion remained in the incised
lines.
The best results were produced by cleaning in DTPA or EDTA. We chose DTPA,
since it is already used to treat lead objects in the CCI Archaeology section.
The plaque was immersed in a bath of DTPA and almost immediately, it began to
react, with small bubbles forming on the surface as the lead carbonate became
soluble, releasing carbon dioxide. The uneven discoloration on the surface
began to disappear, being replaced by an overall lighter grey colouring. At the
same time, the black material highlighting the lettering became more apparent.
After about one and a half hours in DTPA, the appearance of the plaque had
greatly improved: the lettering was easier to read, and the colour was more
even. The lead carbonate layer was much thinner, and the metallic lead was
exposed at the corners and in several spots on the surface. We stopped the
treatment at this point, because the appearance was better with part of the
corrosion layer left in place, rather than completely stripped. Partial
cleaning may also have allowed the lead acetates or formates to dissolve.
The plaque was washed in running tap water and in changes of boiled
distilled water to remove chemical residues. It was rinsed through acetone,
dried and immediately placed in a desiccator containing dried silica gel and
activated charcoal.
Lyndsie Selwyn, a conservation scientist in the Conservation Processes
Research laboratory at CCI, conducted a literature survey and provided advice
and assistance at all stages of the treatment.
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