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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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