Conservation of ancient copper coinage

Since the beginning of human tool usage and the development of
metallurgy very few metals have been utilized. The metals of history
(iron, tin, copper, lead, silver, and gold) are those which were
recognized and intentionally used with consistent regularity to
manufacture monies, tools, weapons, ornaments, and other
paraphernalia. These metals were used individually, but also in
combination with the others (including zinc and tin) to form more
serviceable alloys, such as bronze, brass, and pewter (Hamilton).

Fresh from manufacture the various metals and their alloys,
excepting gold, react with their environment and begin a corrosion
process that converts them to more stable compounds. Before
competent conservation techniques can be applied to a metal
artifact, it is essential that the conservator be aware of the
corrosion products that results from exposure to different
environments. The nature of the corrosion products has a determining
factor in the technique and procedures that can be used effectively.

Cupreous metals are made from the various mixes of copper and copper
alloys. Readily identifiable metals such as bronze which is a mix of
tin and copper, and brass which is a mix of copper, lead, and zinc
are the most notable of the copper alloys used for coin production
in antiquity. Due to their high position in the EMF series cupreous
coins are usually the most prevalent and most desired objects left
from archaeological sites. Although cupreous metals are considered a
noble metal this does not stop the oxidation process in coinage. In
adverse environments cupreous metals form oxidizing products such as
cuprous chloride (CuCl), cuprous oxide (Cu20), cupric chloride
(CuC12), and the familiar greenish blue colored cupric carbonates
[Cu2(OH)2CO3] and [Cu3(OH)2(CO3)2].

Coins recovered from a terrestrial environment are usually oxidized
by cuprous sulfide and cuprous chloride. Most recovered coins can
either be covered with a black powder like substance or with
pitting. Although both alterations can be unpleasant to the eye
neither does serious harm to the object. Fortunately, these surface
contaminations can be treated with various weak acid solutions such
as formic or citric acid. Unfortunately for highly corroded coins
more in depth treatments are needed. These advanced techniques are
hampered by the fact that there is no ideal treatment for one
singular cupreous coin available today. Whether ancient copper coins
are recovered from the earth, encrusted from the sea or general
circulation treatments remain the same.

Fortunately, not all patinas are corrosive (inert) and can add a
beautiful chocolate-brown, golden-red or shades of green to the
coin. These inert, non obscuring patinas actually form a protective
barrier against corrosive compounds. The conservator must be able to
differentiate between these eye pleasing patinas and corrosion
products. Coins that have been assaulted by corrosion products will
inevitably crumble, pit and disintegrate. When starting the
conservation planning of the object one must look at four principle
questions before conservation begins. Mac Dowell states these four
points clearly:

First, is the corrosion active--i.e. is the corrosion process still
continuing? If the corrosion is active, bright green spots of loose
and powdery corrosion products will be present on the surface of the
coin. If these are brushed off they will grow again quickly,
especially during periods of damp weather. This condition, known as
bronze disease, is best treated by complete removal of the corrosion
products, provided that the surface detail of the coin will not
thereby be lost.

Second, does the corrosion layer enhance the appearance of the coin
by giving it an attractive patina? Where the corrosion is not
active, the patina should normally be retained, and any cleaning
should be limited to removing soil, dirt, etc., from the surface of
the patina.

Third, is the coin clearly legible? If the corrosion products
obscure details of the legend or type, the first step is will be to
remove any surface accretions of soil, dirt, grease, etc. If the
legend or type is still not clearly legible, it may be necessary to
remove completely the corrosion products, even though the corrosion
is not active-provided that the coin has a good solid core of
uncorroded metal.

Fourth, is there a sound metal core, which is likely to preserve all
the details of type and legend? If there is a serious danger that an
attempt to remove the corrosion products would destroy the coin, a
much more complex conservation process must be followed.

Cupreous coins are usually in need of at least three steps of
conservation to be considered stabilized. The primary step a
conservator must take is that of stabilizing the coin. In the past
it was the standard to let ancient coins stand dry in packaging. We
know now that this step was the primary factor in the loss of many
early bronze coins coming from archaeological investigations
throughout Euro- Asia. Successful pre-treatment solutions such as
sodium sesquicarbonate and sodium carbonate can be used as
cleaning/storage mediums for the coins. Secondary and third steps
concerning the conservation of the coins entails the use of
benzotriazole (BTA), acids and an ultrasonic cleaner. Lastly, proper
storage of the objects in a low humidity environment is essential.

I have found that soaking ancient copper in sodium sesquicarbonate
to be extremely beneficial to the over all health of the coin. In
fact, the sesquicarbonate soak can sometimes be the only step in
conservation that is needed. Sodium sesquicarbonate acts a mild
alkaline stabilizer and cleaner for the coin. Because of the mild
cleaning action of the sesqui corrosion products will dissolve
especially with a light brushing of the coin everyday with the
fingertips or through the use of an ultrasonic cleaner. Sodium
sesquicarbonate is completely safe to humans and the environment
being an inert carbonate. Another inert cleaner that has been used
successfully for pre-treatment cleaning is sodium hexametaphosphate
(Calgon) although it only acts a cleaner and not a stabilizer. These
solutions must be carefully monitored daily to maintain the correct
pH. From my own research my ideal range for stabilization is pH
8-10. In metals conservation when using and mixing solutions only
use de- ionized, distilled or reverse osmosis water as your base.

After the sesquicarbonate soak it is essential to leach out all
chemicals that have impregnated the surface pores of the coin.
Although not corrosive, sesquicarbonate will form a white build up
on the coin if not leached out. Leaching can be done by removing the
coin from the stabilization bath and placing it into a vat with a
purified water base mentioned above. I like to put the coin in an
ultrasonic cleaner containing successive baths of hot and cold
de-ionized water. After every five minute period I flush the
solution exchanging it for a new one leaving the coin to soak
overnight. I repeat this process for a minimum of three dollars to
ensure cleanliness. An ultrasonic can only be used for coins that
have a solid metal core.

When the coin has finished its leaching process it is time to
thoroughly dry the coin using methylated spirits. For this step the
coin can simply be placed in a container with alcohol and left
overnight. I change my alcohol solution once the next day leaving
the coin immersed for a total period of 48 hours. As a personal
choice when drying copper coins I add a 1% by weight chemical named
Benzotriazole (BTA). BTA acts as a completely undetectable corrosion
barrier against bronze disease. I also use a lacquer called Incralac
which contains trace amounts of BTA. The Incralac dissolves
completely in the methylated spirits (use a reduced 3% by weight
solution) and gives the coins just a slight sheen. If you choose not
to use the reduced lacquer, unnoticeable microcrystalline wax can be
rubbed into the coin creating an excellent barrier against
corrosion.

Although not recommended, if a coin needs to be entirely stripped of
layers of corrosion citric and formic acid can be used as a mild but
effective stripping agent for copper alloys. The stripping solution
is made from 5-10% citric or formic acid by weight with 1-2%
thiourea by weight to deionized water. The pH of the solution should
be close to 2 while rising to pH 3-4 when the coins are added to the
solution.  Even though the solution is already weak the thiourea
acts as a barrier against the organic acid attacking the pure copper
layer. After the initial pre-treatment of the object it is necessary
to remove the remaining solution and chlorides from the metal. This
can be achieved by soaking or washing the object in deionized water
for a period of 3-6 months. My own experimentation in chloride
removal has shown that placing the artifact in a solution of heated
deionized water can decrease the overall time of soaking. After the
object has been thoroughly washed the water must now be removed by
soaking in a solution of methylated spirits such as acetone, ethanol
or 90% isopropyl alcohol. A small size object such as a coin should
soak overnight.

After the water removal the copper coin should be treated similarly
to the sesquicarbonate method soaking in a solution of 3%
benzotriazole and 1% water by weight in methylated spirits. It is
beneficial to let the object soak for 48 hours or under vacuum
impregnation.  After the BTA soak the object can be air dried in a
cool room, rubbed with microcrystalline wax or coated with reduced
Incralac.

Copper coins recovered from marine environments contain high levels
of chlorides and other soluble salts as a result of long term
exposure to seawater and need intensive conservation procedures. To
prevent further corrosion and degradation of the coin it must be
leached of all salts.  This leaching process is called desalination
and can be accomplished using various methods and chemicals. Unlike
non-metal artifacts that undergo desalination in baths of filtered
water, metal desalination is a process that requires more elaborate
conservation measures.

The best method for removing chlorides from metals is called
electrolytic reduction or more commonly electrolysis. Electrolysis
is a method of separating the corrosion and chloride products from
the metal core by passing an electric current through the metal
itself. Like a magnet, corrosion layers are attracted and pulled
away from the artifact (cathode) and attach to a sacrificial anode
that is usually made of a mixture of platinum, titanium or stainless
steel. The electric current attraction is helped by the use of an
electrolyte in the solution. In the case of ferrous metals a 5%
sodium hydroxide or 5% sodium carbonate solution makes an effective
electrolyte. After thoroughly testing, both electrolytes each have
their own attributes and should be researched accordingly. 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
washing methods mentioned earlier.

An electrolytic cell consists of a vat or other liquid holding
container with two electrodes, the cathode (artifact), and the anode
(platinum, titanium, steel) with an electrically conductive additive
called the electrolyte (sodium hydroxide, sodium carbonate). The
electric current comes from a DC power supply which causes the
oxidation reduction at the cathode. While the cathode is connected
to the negative side of the power supply, the anode is connected to
the positive side. The advantage of electrolysis is that many coins
can be hooked up at once with the adjustable current density from
the power supply. A good rule for current density is to run less
stable metals at 0.5 amps while more stable artifacts can receive as
much as 3.0 amps. Time of electrolysis depends on such factors as
metal type, metal density and chloride exposure. Typically, well
intact coins can take up 100-150 hours in electrolysis. To finish
the coins use the same chemical leach, drying and coating methods as
the chemical washing and acid stripping procedures mentioned above.

Copper alloyed coins require a dry environment with a; relative
humidity of 30% or less. A perforated bag of self-indicating silica
gel should be placed in the storage container with a relative
humidity indicator card. The relative indicator humidity cards
indicate a wide range of humidity and should be checked at regular
intervals. If the humidity levels rise above 40% the silica gel
should be replaced or more should be added to the container (Hodges,
1987). The silica gel should never be sprinkled loose into a
container nor should it come into direct contact with the surface of
the object. Instead, place the silica gel inside a cotton bag before
placing in the container.

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 22:52
                 Distributed: Thursday, March 12, 2009
                       Message Id: cdl-22-52-005
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