|
|
CLEANING AND RESTORATION Before
attempting any major cleaning of your Bronze Roman Coins you should First
of all equip yourself with the following. 1.
A Soft, Medium and Stiff Toothbrush. 2.
Washing up liquid. 3.
Clean Olive Oil 4.
A Glass or Plastic container big enough to hold 1 pint. 5.
Plastic or wooden toothpicks 6.
A roll of kitchen towels. First
of all make a solution of washing up liquid with warm water and soak the
coins in it for 30 minutes. Remove the coins one at a time and brush Them
with one of the toothbrushes (which one depends on the amount of dirt On
the coins) Now
repeat the process. You should find that some of the coins are now Cleaned
and can be cleaned in fresh water and dried with the kitchen towel. The
remaining coins should now be immersed in the olive oil and left for 4/5 Days
to soak. Remove the coins one at a time and brush with the stiff Toothbrush.
On any of the coins that still have dirt attached rub them with The
point of the wooden toothpicks, this will sometimes remove lumps of Dirt.
(I actually use the blunt end of an artist paintbrush made from Hardwood) If
you still have dirty coins try soaking for a further 4-5 days in olive Oil
and brush again. For
more serious cleaning please read the details below. Have
fun cleaning the coins and good luck with what you find. Cleaning
Copper and its Alloys 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, which Imitates
gold. Cleaning Coins
and artifacts that have been buried are usually found with a green Surface
coloration, this primarily consists of copper carbonate, hut 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 won, specimens. This method is particularly Effective
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 a Combination
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 He
cleared by continued mechanical means. Areas of iron corrosion deposited On
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 hut 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 he 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 to Give
a more attractive appearance. Restoring With
bent objects, thin section copper-alloy items calm be reshaped by Annealing,
but care must be taken not to overheat, as extensive oxidization Will
occur and reduce the attractiveness of the object. More substantial Copper-alloy
objects are best left as found. Some
copper-alloy items may he improved by enhancing the patina. A patina Might
for example be powdery or rough-textured. This can often be improved By
brushing with a typeface brush. If the colour of the patina is thought to He
too light, or perhaps having a patina of different shades, then the Addition
of a colored wax can he useful. Because chemical additives in Commercial
products can potentially cause long-term deterioration, they Should
not he use on important items. Where their use is appropriate, Green,
brown or black boot polish sparingly applied and then brushed in will Give
a dark glossy tone. Plain beeswax may also ho used with less drastic Coloration.
Conservation lacquers can also be applied for appearance Purposes. When
cleaning has left a copper-alloy object looking bright, it may he Thought
desirable to restore some form of patina or toning to it. It is Possible
to produce convincing green patinas, but this is a difficult and Chemically
hazardous process. Other safer methods rarely produce Satisfactory
results. The best option if it is felt necessary to tone down Bright
metal is to use Tourmaline. As described in the section on silver Objects.
With copper-alloys, degreasing with acid may render objects liable To
further deterioration and so conservation will have to follow. Conserving Many
copper-alloy objects will remain stable when excavated, and will Require
no particular conservation treatment. The application of waxes or Lacquers
as mentioned previously may however aid protection as well as Improving
the appearance of an object. One
of the most destructive threats to copper-alloy objects is a condition Known
as bronze disease'. This can he recognized by small areas of powdery Green
corrosion, which erupts from below the surface of an object. This is Caused
by the presence of chlorides which were sealed within the corrosion Products.
And which can become active if the environment of an object is Altered.
This may also occur when an object has been subjected to chemical Or
electrolytic cleaning. Once it takes hold, bronze disease is a Progressive
form of corrosion, which, if left unchecked, can totally destroy An
affected object. The
only solution is the removal of the affected areas. This can he achieved Either
by totally stripping an object hack to hare metal, with all its Attendant
disadvantages. Or by picking out any small spots with a dental Tool
or adapted engraving tool. If any trace of bronze disease is left, it Will
re-occur. To
conserve an object, either following the removal of bronze disease or After
any form of cleaning and/or toning, treatment with benzotriazole (BTA For
short) is now generally recommended. Before using BTA, an object must First
be degreased by Immersion in alcohol or acetone, this should then be Followed
by a prolonged soaking in a solution of caustic soda, which will Reduce
subsurface chlorides and better prepare an item for treatment. The
next step is to remove the object and allow it to dry, after which it Should
be placed in a 5% solution of BTA, dissolved in alcohol, It should be Left
for several days. Then removed and once again allowed to dry Thoroughly.
The final stage is to brush off any loose precipitate that may Have
formed, and to then apply several coats of a protective lacquer, Preferably
Incralac or another suitable proprietary conservation lacquer. Domestic
varnishes have additives, which may be harmful in the long term. The BTA
solution can be used repeatedly, and only needs disposing of once it Becomes
discolored. THIS ADVICE WAS KINDLY GIVEN TO US AS GUIDE NOT A GOSPEL METHOD OF CLEANING, YOU DO SO AT YOU'RE OWN RISK!!!! |