Conserving marine archaeological silver coinage

Artifacts recovered from archaeological sites like shipwrecks hold
clues to our past. Historically, archaeologists have focused on
piecing these clues together almost exclusively from terrestrial
sites. With the advent of modern technology archaeologists are now
capable of excavating deep water sites and shipwrecks. With recent
changes in recovery techniques, technological advances in scientific
approaches to conservation must also change. Objects recovered from
marine sites are often more intact compared to contemporary
terrestrial finds but due to the chemical makeup of the host aqueous
solution maritime artifacts are highly unstable when brought into
our oxygen rich atmosphere. Their presence in situ provides an
archaeological foundation to understand the past civilizations, the
events in which they were involved and the customs of the peoples
who used them. Quite often it is these objects and their sites that
provide the basis for further study by archaeologists,
anthropologists and historians. To ensure further research which may
lead to possible new discoveries and fulfill educational and
cultural purposes, utmost care and protection is recommended in the
collecting, recording and preservation of maritime finds and sites.

The practice of marine archaeology is a double edged sword.
Archaeological excavation is needed to save threatened cultural
sites from destruction or environmental concerns, but is also a
destructive process since man is disturbing the in situ site.
Justification for excavation is typically derived from natural
results of environmental and geological processes--floods, droughts,
hurricanes, earthquakes, changes in river courses, changes in
coastlines and volcanism. Threats to our maritime heritage can also
come from human means: Urbanization, agriculture, war, and vandalism
are processes of destruction directly provoked by humans which
adversely affect all cultural property. The very act of
archaeological excavation itself can irreversibly damage these
sites; especially when the excavation is carried out without proper
conservation. Archaeological conservation, therefore, is concerned
with both site and object. Conservation of these artifacts and sites
are the responsibility of the archaeologist, crew, and conservator.

In terms of shipwrecks and silver artifacts, coins are numerically
the highest in quantity found in the site assemblage. When silver
coinage is presented to the conservator it is typically in the form
of concreted clumps. These cement-like concretions are formed of
layers of shells, sand and the associated matrix surrounding the
artifact. High grade silver coins usually have two layers of
corrosion and concretion that must be removed. The outside layer is
composed of silver and copper corrosion products, marine organisms
and seabed material. The inner layer is predominately AgCl, Ag2ClBr
and Ag2S (Pearson 1987, 238). Fortunately high grade silver is
relatively stable after removal from it aqueous host. Although some
conservators store the marine silver in a dry environment the author
prefers to keep the metal in its original solution and slowly dilute
with de-ionized water until a 100% de-ionized solution is acquired.
It is in this media that the author stores marine silver. Depending
on the conservator, marine silver is usually put through a series of
chemical and mechanical treatments which will be discussed briefly
in this article.

The first stage in conserving marine silver is the removal of the
concretion layer. Historically the outer concretion layer was
removed by immersing the coins in a 10% hydrochloric or formic acid
solution. This solution dissolves the CaCo3 and copper corrosion
products in the concretion layer. Copper corrosion products can
cause irreversible harm to the silver by forming a hard, glue-like
copper film which is difficult to remove in later steps. The
immersion process can take anywhere from 24 to 168 hours. With the
harmful potential of hydrochloric acid the author has experimented
successfully with a 5% acetic acid immersion over a period of 168
hours refreshing the solution every 24 hours. You can check the
acidity of your solution by dropping a particle of shell or any
carbonate into the solution. If bubbles appear your solution is
still active.

The use of acid is a highly controversial conservation issue due to
the fact that if not watched properly or mixed correctly acids can
easily take off important surface features and cause pitting. In a
trade where conservation is measured in nanometers it pays to watch
the solution carefully and to start with a higher dilution rate and
increase only if needed.

Once the immersion period is over the coins are then transferred to
an ultrasonic cleaner filled with deionized water. The ultrasonic
cleaner takes the place of the traditional cleaning of the outer
corrosion coating by hand thus minimizing handling and potential
damage. The coins can be run through a series of intervals flushing
the solution after every period. Care must be taken not to overload
the cleaner so that coins do not bounce or vibrate against each
other. For stubborn deposits a water jet has been used successfully
to remove these clumped layers. With the use of water jets one must
experiment in a trial and error period before actual use. The water
pressure and distance from nozzle to coin must be exact or the
artifact can be damaged beyond repair.

After the above mentioned steps the coins should now look somewhat
mottled with a few small clumped layers. This is normal. To remove
the dullness and leftover products a vibratory tumbler can be used.
Normal tumbling media is by far too aggressive so a mixture of 70%
ultra fine sodium bicarbonate and 30% fine sodium bicarbonate is
employed. The best polishing media by far is a mixture of 80% ultra
fine sodium bicarbonate and 20% fine fiberglass powder which the
author uses. This process should be undertaken outside or in a well
ventilated area using a high quality particle mask and eye
protection. The mixture only needs a few drops of water as an
agitator; you will want to avoid clumping. If clumping occurs you
must add a little more dry material. Close attention must be paid to
the vibratory process checking every 10-20 minutes. The objective is
to remove as little material as possible to preserve the original
surface sheen. If time permits, the individual coins can be hand
polished using the above mentioned mix and ones fingers. By doing
this you can continually monitor the artifacts progress.

When the coin has been removed from its cycle in the vibratory
cleaner it should be put back into the ultrasonic for a five minute
interval to clean any micro dust in the surface pores. The coin can
be left to air dry in a low humidity/temperature area or a hot air
gun can be employed. Giving the metal a five minute soak in
isopropyl alcohol or methylated spirits can be extremely beneficial
before the final drying occurs. At this point conservation has
finished, but some choose to further "dip" their coin. Thiourea
based dips have proven to be harmful when used improperly. Prolonged
exposure and too strong of solution have wreaked havoc upon coins
for the last 40 years. If a dip is wanted it should be highly
diluted and the coin should not be immersed for more than a few
seconds. A diluted dip can be made from 5% thiourea and a 1%
non-ionic wetting agent in distilled water.

By far the best results in conserving marine encrusted coins is
through the process called electrolytic reduction or more commonly
known as electrolysis. Electrolysis is a method of separating the
corrosion products from the solid silver coin surface by passing an
electric current through the metal. Like a magnet, corrosion layers
are attracted and pulled away from the coin and attach to a
sacrificial anode that is usually made of a mixture of platinum and
titanium. The electric current attraction is helped by the use of an
electrolyte; in this case a 5% formic acid or 5% sodium carbonate
solution. After thoroughly testing both electrolytes each have their
own attributes. Sodium carbonate is preferred because it is
relatively low in cost, safer, and environmentally easy to dispose.
The down side to electrolysis is that it takes a longer conservation
time period than the chemical method. Electrolysis takes anywhere
from 200-300 hours compared to the 24-48 hour chemical method. It is
for this reason that conservators and numismatics prefer chemical
cleaning especially with the task of conserving a bulk load of
silver in upwards of 30,000 coins.

When the coins have finished their cycle in electrolysis they can
usually be wiped clean and put into the ultrasonic for an additional
deep clean. If a higher sheen is desired they can then be put
through the vibratory tumbler process mentioned above. Please note:
serious damage can occur to the coin once it undergoes polishing. I
have found that in almost all instances the original sheen of the
coin will be lost no matter how diligent the conservator is in
monitoring the process. Electrolysis has proved time and time again
more effective at preserving the original luster compared to
chemical methods.

Numismatic conservation has always been a controversial topic. The
ideal place for the use of acids, chemicals and mechanical means to
clean coins will always be debated. Justification for the
conservator's choice of conserving these artifacts is based on the
knowledge that if not treated immediately these marine encrusted
artifacts might not survive for the future generation's academic
study and enjoyment. The conservation methods in this essay are
solely meant for coins needing emergency conservation and should
never be used on mirror like or cameo type monies. Furthermore,
because of the danger to oneself and the artifact, conservation
should never be undertaken by the novice.

In presenting my methods I am hoping to start a dialogue with others
that also have worked with conserving metal monies. This is one
method that has worked well for me in the past, I would really enjoy
speaking to others about critiquing this.

Wyatt Yeager


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                  Conservation DistList Instance 22:44
                 Distributed: Sunday, February 8, 2009
                       Message Id: cdl-22-44-003
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