I'v seen some of the experiments back in 1997. The silicone-treated sail
canvas from the Santo Antonio de Tanna (1697) was truly an impressive
sight..

"Beating the Ocean: Archaeologists Turn to Silicone to Preserve Waterlogged
Artifacts

By C. Wayne Smith

The dark leather is soft and supple, bending and twisting
easily. The old shoe got plenty of hard use, yet it still looks
as though it could be some small-footed owner’s prized
footwear. But the last time anyone slipped a foot into this
shoe was in 1686. For more than 300 years, it has been
soaking in saltwater and buried deep in the sediments at
the bottom of the Gulf of Mexico.

By rights, this shoe that went down with the French ship La Belle off the
coast of Texas should be stiff, discolored, and far too fragile to handle —
or simply disintegrated into a pile of dust and disfigured pieces. That it
looks almost as if it was only just taken off and placed at the foot of the
bed is testament to a remarkable new technique for preserving ancient,
waterlogged artifacts.

Traditional conservation methods are very slow — preserving the shoe would
take several months and require a great deal of labor. First, salts from the
sea must be removed by soaking the artifact in a series of freshwater rinses
for weeks on end. Next, the water would be replaced with polyethylene
glycol, known as PEG. PEG mixes easily with water, so gradually increasing
its ratio in the water baths eventually fills pores of the artifact with the
chemical and preserves the leather.

Then the shoe would probably be freeze-dried over a period of weeks and
finally displayed (or stored) in a carefully controlled environment that
limits exposure to ultraviolet light and maintains constant temperature and
humidity. Otherwise the PEG could interact with atmospheric moisture,
partially liquefy, and perhaps become unstable.

Our new silicone-polymer conservation process takes only days — and the
treated organic artifact, like the La Belle shoe, retains its original
shape, color, and texture — even its pliability.

After preservation with silicone oils, the diagnostic attributes of the
artifact and its microscopic structure are both preserved, which permits
detailed use-wear and chemical analysis even after conservation. Scientists
could, for example, determine the type of tree from which an ax handle or
ship’s timber was cut or identify the plant used to make rope or cloth. Such
details may be crucial in investigating a shipwreck, since they offer clues
to the origin of the vessel and the trade routes it plied.

Based on a series of tests and computer models, a properly treated artifact
might last more than 200 years before it needs another treatment. There
seems very little risk to most artifacts, even though the process is
technically irreversible.

The process (see “Chains Conserve Artifacts,” Page 64) uses long, chain-like
silicon molecules called polymers to permeate the interior structure of the
artifact. A second chemical, called a cross-linker, binds the polymers, and
these linked chains hold the object tightly together and protect it.

The genesis of silicone conservation was the pirate lair of Port Royal,
Jamaica, much of which slipped into the sea during a 1692 earthquake.
Archaeologist Donny Hamilton of Texas A&M University has been excavating the
underwater site for years. There, preserved under the shallow Caribbean
waters, was an archaeologist’s dream — an entire city frozen in time. But
the dream came with a nightmare: The city’s material culture might last for
centuries on the seafloor, but would quickly perish if brought to the
surface for study.

Seawater is a wonderful place to store organic artifacts. In the atmosphere,
the highly reactive oxygen of the air combines with elements in organic
material, breaking the original chemical bonds and changing the material’s
structure. This typically leads to disintegration and destruction. But items
that are buried under the silt and mud of the seafloor are often preserved
in an almost oxygen-free environment.

And seawater is typically cool to cold, which slows down chemical
reactivity. In the ocean depths, seawater is nearly freezing. Combined with
lack of light and diminished oxygen, deterioration is extremely slow.
Relatively good preservation of items as small as pollen grains and as large
as ships’ hulls helps explain the growing interest in deepwater archaeology.

Unfortunately, once such waterlogged artifacts are exposed to the air, their
lifetime may be very short. Structurally weakened as water dissolves the
internal matrix of the artifact and further deteriorated due to the
crystallization of minerals and seawater salts, artifacts made of wood,
glass, and even pottery may break or distort, eventually destroying the
item. Over the years, salts impregnate even the smallest pores, so they
cannot simply be rinsed out.

At Port Royal, Hamilton’s team faced conservation challenges not only with
organic artifacts but also with a wealth of glass jars and bottles. Three
centuries under water had altered the chemical composition of the glass,
leaving the surface in thin layers. If the glass is allowed to air-dry, or
if soaked-in salt crystallizes, the layers will pop off and destroy the
artifact. Standard consolidation treatments for many of the Port Royal
bottle assemblage have not worked well. Over time, many of the bottles have
required additional treatment to stop surface flaking and exfoliation of
large sections of glass. Even after treatment, the bottles are structurally
weakened and unnatural in appearance.

We turned for help to Dow Corning Corp., a major developer and marketer of
silicone products. Chemist Jerome Klosowski has spent his career seeking the
perfect sealant — a material that reacts with and chemically bonds to every
molecule of the surfaces to which it is applied. Klosowski, Hamilton, and I
recognized the potential of such a material for archaeology: If a polymer
could be introduced into the deteriorated matrix of an artifact, chemical
bonding would make it more stable, and oxidation and ambient moisture would
not be as great a threat. Preservation techniques based on silicone polymers
were the result of our collaboration.

We have used the technique on wood, leather shoes, old rope, sails, and
glass, all recovered from beneath the sea. When properly treated, the softer
materials can regain much of their original pliancy, so that leather flexes
appropriately and pieces of sail can even be folded. The glass, with its
layered surface bound together, looks, feels, and even sounds like glass.
This natural feel has a great appeal to archaeologists, conservators, and
the touching (when appropriate) public.

In a recent experiment, we subjected small fragments of Port Royal wood to
both air drying and treatment with polymers. The untreated wood naturally
fell into warped, discolored fragments, while the treated wood emerged
stable and with a natural appearance and surface texture. Even the color
looked appropriate, apart from a slight grayness due to the 400 years of
waterlogging.

Artifact preservation using silicone oils has proven its value on materials
recovered from the wreckage of La Belle, which was part of a failed colony
founded by the French explorer La Salle. These included wooden knife
handles, onion bottles, wicker baskets, and thousands of glass beads La
Salle brought to America as trade goods.

The process worked extremely well for preserving a wicker basket that had
been crushed under its load  of cannonballs. Spot treatments and the ability
to cast silicone support structures for the delicate basketry made it
possible to save more of the artifact than would have probably been possible
using traditional methods.

But among the most fascinating items we have preserved from La Belle are
buttons with pieces of thread still in them. After the silicone treatment,
the threads remain pliable and not matted together.  It would be very
difficult to obtain the same results using more traditional treatment
strategies. After four centuries, the treated thread feels almost as soft
and pliable as when it was new.

Artifact conservation using silicone oils is in its infancy.  The process is
simply not a panacea for the preservation of all artifacts. At best, we see
these technologies as additional tools in the conservation toolkit. With
silicone conservation methods, however, archaeologists and museum curators
have some viable new options for the preservation of badly deteriorated
artifacts and objects that are traditionally difficult to conserve and
maintain in museums and special collections. Better still, silicone oil
research will ensure that artifacts will still be available for generations
to explore with technologies not yet dreamed of.

C. Wayne Smith is an Assistant Professor and Director of the Archaeological
Preservation Research Laboratory at Texas A&M University in College Station,
Texas.

Chains Conserve Artifacts
By Nicholas Pingitore, Jr.

Silicone conservation, developed over the past five years, links long
chemical chains throughout and around an artifact, holding together and
protecting its surface. The secret ingredient is long molecules called
polymers formed around the element silicon, which was made famous by its use
in microchips.

Developed and patented by C. Wayne Smith of Texas A&M’s Institute of
Nautical Archaeology and Jerome Klosowski of Dow Corning, silicone
conservation begins by replacing seawater in the artifact with acetone (a
solvent). An acetone bath drives water from the pores of the material and
fills the spaces with acetone. The volatile chemical is then evaporated
away.

Next, the artifact is submerged, in a slight vacuum, in a mixture of the
long silicone polymers and a chemical called a cross-linker. The silicone
permeates the material, and the cross-linker creates new links that join the
silicone polymers together. The molecules of the artifact are encased in the
silcone chains — effectively coated and protected from attack by oxygen or
moisture.

In contrast to traditional preservation techniques using polyethylene glycol
(PEG), which can undermine the structural integrity of the artifact, the
silicone treatment causes no cellular distortion. Specific silicone oils and
cross-linkers can be chosen to produce the desired texture and strength —
leaving ancient ropes flexible, for example, while wood remains rigid.

Similar cross-linking is an essential feature of plastics, which use
carbon-based polymers. Plastic can be chemically engineered to a desired
flexibility or rigidity through the appropriate addition of either
cross-linkers or compounds that prevent such links.

Silicone conservation and other new strategies are clearly a reality. They
are not panaceas for all conservation needs, but they can put additional
tools in the conservator’s toolkit."

http://www.discoveringarchaeology.com/articles/032301-beatingtheocean.htm



Paulo Alexandre Monteiro

Centro Nacional de Arqueologia Náutica e Subaquática
Avenida da Índia, 136 - Belém
1300 Lisboa

351 - 96 - 24 13 815

http://w3.to/azores.wrecks
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