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Cleaning
Copper and its Alloys
see also:
Coin Cleaning
Cleaning
bronze and Romana
Cleaning
Copper
Conserving
Advanced Cleaning
Copper is used for coins and artifacts in either its natural form,
or more commonly alloyed with other metals. With tin added it is
called bronze, and if the tin content is very high the bronze may
be called potin'. When zinc is added to copper the alloy is called
brass. Lead and other metals may also be added to give slightly
different properties or appearances. Some specific mixtures include
gun metal, latten, Britannia metal, and Pinchbeck, whichimitates
gold.
Cleaning coins and artifacts that have been buried are usually
found with a green surface coloration, this primarily consists of
copper carbonate, but the corrosion products may be complex and
contain many other chemicals which give a different appearance,
and which may cause instability.
An ancient coin or artifact with a smooth green patina is more
collectable than a brightly cleaned object. Some ancient objects
found in river silts may have a brassy tone, but few collectors
are interested in ancient material that has been polished. As far
as coins are concerned, although ancient ones are more desirable
with a patina, recent ones are not. A rare Victorian penny with
either mint luster or circulation toning would be highly collectable,
but an excavated example with a green patina would be almost worthless,
such criteria mean that in most cases cleaning is unnecessary, except
where a coin or object is heavily encrusted, and thus the design
is obscured, or if it is unstable arid requires conservation.
In general, all copper or copper-alloy objects are best cleaned
mechanically. Stripping corrosion products using either chemical
or electrolytic means can cause unsightly finishes or induce instability
leading to further corrosion. It will be necessary to use these
methods in some cases, but only as a last resort.
Coins and artifacts with generally even patinas may have odd deposits
of further corrosion which can he removed by picking them off with
a dental tool, or by grinding them off with an abrasive drill bit.
Coins can often be considerably improved by rubbing with an ink
eraser, and sometimes this is the only means of identifying worn,
specimens. This method is particularlyeffective with copper coins,
jettons and tokens. Excavated examples often have an overall matt
copper tone with very little detail evident. These can first he
brushed to remove any loose powdery material, and then firmly rubbed
with a hard grade ink eraser, this works in a similar way to the
traditional method of taking a rubbing from a coin using a pencil
and piece of paper. What you achieve is the highlighting of the
raised parts of the design. The first visual change is a darkening
of the copper colour, followed by the appearance of bare metal on
the high points, which is often sufficient to allow a coin to be
identified.
Where more substantial corrosion is present, mechanical methods
can he acombination of picking with a dental tool, grinding with
an abrasive bit, or using an engraving tool with a specially adapted
bit to chip away at deposits of corrosion, Great care must be taken
not to pit the surface of an object by applying too much downwards
pressure, and consideration must be given to possible inlays or
enamels that might be present. Copper-alloy objects may also be
plated with gold or silver, and this too can be damaged if sufficient
care is not taken, with particularly stubborn spots of corrosion,
a drop of weak acid can he applied. After being left a little while
this is washed off and the now softened or reduced corrosion area
can be cleared by continued mechanical means. Areas of iron corrosion
depositedon copper-alloys can be dissolved using a localized application
of EDTA.
In certain circumstances, a fine deposit may overlie a good surface
that could be harmed by mechanical cleaning methods. Such situations
particularly apply to plated objects, such as some coins or buttons,
the use of acids would generally he too harsh, and could either
strip the plating, or leave any exposed base metal looking unsightly.
In such circumstances immersion in a solution of sodiumhexametaphosphate
will usually prove effective. This is a non-corrosive chemical but
which has the effect of slowly dissolving calcareous deposits which
commonly occur on copper-alloy objects. An object being cleaned
should be removed periodically and lightly brushed in fresh water
until enough of the deposit has been removed. Sodium hexametaphosphate
is particularly useful when deposits overlie a patina, as usually
it is possible to achieve a satisfactory result before the patina
is affected to any degree. The solution should be at between 5-15%,
and gentle warming will speed the reaction. Prolonged immersion
in sodium hexametaphosphate however will eventually strip an object
down to bare metal.
Where corrosion products form a solid crust that cannot be adequately
tackled by mechanical cleaning, electrolytic or electrochemical
methods may
be used. It must be noted however that these can remove all traces
of plating, enamels or inlay, and so should not be used if any of
these are suspected. Electrolytic cleaning can be effected by making
a 5% solution of citric acid to which is added a pinch of table
salt to improve conductivity. The object to be cleaned is then placed
in the appropriate holder, the anode inserted, and the current switched
on. The object will emit small bubbles and should be periodically
checked, washed and brushed to monitor its progress. It may be found
preferable to remove the object once some of the deposit has detached
and to finish cleaning by mechanical methods.
Electrochemical cleaning is carried out by placing the object in
Pyrex dish and covering it with coarse zinc powder. A 10% solution
of caustic soda is then pored over, a stainless steel utensil being
used to replace any zinc granules that have been displaced so as
the object is completely covered. The reaction may he speeded up
by warming. Using this process it is necessary to replace the zinc
powder periodically. The object being cleaned should be rinsed and
lightly brushed from time to time. The surface deposit will detach
progressively. It is recommended that when using this method, the
object should be stripped to bare metal as remaining corrosion can
harbor chloride traces which may lead to further decay. In both
electrolytic and electrochemical reduction methods copper may plate
back onto the article being cleaned, giving it a pinkish colour.
Ideally this should be removed as it can harbor harmful chlorides.
It can however also provide a preferable finish to a bare and pitted
surface resulting from total stripping. If retained it must be conserved
as described below, but might also be toned togive a more attractive
appearance.
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