Below is an article about the methods of cleaning old coins written by O.V. Vasilyeva, an employee of the State Hermitage Museum.

Translated from Russian and formatted for online presentation by Bein Numismatics.

O. V. Vasilieva, CLEANING ANCIENT COINS FROM NUMISMATIC COLLECTIONS


Published in: "The past of our Motherland in the monuments of numismatics", Avrora, Leningrad, 1977

The purpose of this message is to acquaint interested readers with the simplest and safest methods of cleaning coins, which the author can recommend based on his many years of experience in the restoration of archaeological metal in the State Hermitage. Clearing coins is one of the most important types of restoration, it is essential for their study, dating, identification and storage. Ancient coins made of silver and copper alloys found in hoards or in other archaeological excavations most often come to the restoration workshop highly corroded, and sometimes completely mineralized. Under the influence of water and salts, copper is destroyed in the soil with the formation of green carbonates and chlorides, red copper oxide and black oxide. Tin and lead compounds are found in the corrosive layers of bronze coins. Silver-copper coins are often coated with copper corrosion products. Sometimes silver under the influence of chloride salts turns into a gray-violet chloride - horny silver. In air, copper alloys darken from oxidation, and silver - as a result of the formation of sulfide. The restorer must be very careful about the restoration of numismatic monuments. In particular, it is necessary to make the right choice of the reagent required for cleaning from corrosion, since a reagent successfully used for some coins may be unsuitable for others, that is, it will lead to their destruction or to serious distortions of their appearance. It is also desirable to strive for the preservation of the "noble patina" - a kind of certificate of antiquity of objects.

When starting to clean coins, one should first of all find out: what metal they are made of, whether they have a metal core, to what extent it has survived, what salts the coins are covered with. It is important, for example, in corrosion crusts on copper coins to determine the presence of copper chloride, which in humid atmospheric conditions causes an active corrosive process - “bronze disease.” To detect copper chloride, a humidifying chamber of a simple device is used: a glass vessel is filled with water to about 1/3 of its volume a perforated (perforated) plate of plastic, plexiglass or porcelain is placed above the water, on which coins with a corrosion layer are placed. The vessel is tightly closed with a lid. In this state it is left for 1-3 days. Humidifying chamber accelerates the chemical processes caused by air humidity, which helps to identify the foci of active corrosion and makes it possible to choose more successful cleaning methods. In addition, the presence of coins in the chamber helps to loosen the surface layer of oxides, facilitating further cleaning operations. If the corroded layers contain copper chloride, small droplets of liquid, either colorless or slightly greenish, will start to appear on the coin surface. Otherwise, the droplets do not form: copper carbonate, nitrous oxide and copper oxide are stable in humid conditions (note that horn silver (Chlorargyrite) formations on silver coins are also stable). If copper chloride is found, it must be removed from the deepest pores very thoroughly. Often in archaeological excavations, coins are found "boiled", that is, they form a shapeless mass due to the oxides connecting them. We do not recommend separating such coins by heating and immersing them in cold water. To avoid any losses, such a “lump” should be placed in a 5-10% solution of caustic soda and heated to 30-50°. After this, the coins can be separated from each other, the condition of each of them can be determined and further cleaning can begin.

The main methods of cleaning coins are:

  • mechanical,
  • chemical, and
  • electrochemical.

When restoring, a combination of the mechanical method with others is most often practiced. Mechanical treatment consists of removing surface deposits: earth, sand and individual bumps of corrosion. It is performed with a variety of brushes (bristle, brass and glass fiber), scalpels and dental burs of various sizes. However, these tools must be used with great care. With inept and improper use, they can easily cause scratches, which to some extent may distort the legends, images, etc.

The chemical method, in combination with the mechanical method, can help clean the coins covered with a thick layer of corrosion products, but retaining the metal base, as well as coins made of gold and silver. To remove the cherry-red coating of copper oxide from copper coins, the coins should be immersed in a 5-15% ammonia solution, but so that they are completely immersed in the solution, because in contact with air, the effect of ammonia on the metal is destructive. Instead of ammonia, you can successfully use a 5-10% solution of ammonium carbonate, which is less aggressive towards copper and bronze. If copper coins are covered with salts of copper carbonate (dark green), these layers should be removed with a 5-10% citric acid solution. It slowly dissolves copper salts and oxides and does not dissolve metallic copper.

There are also copper coins covered with a yellowish-pale coating of lead carbonate. This suggests that either there is a lot of lead in the alloy of the coins themselves, or they remained in the ground together with other lead objects. To remove such plaque, a 10% solution of acetic acid should be used, which easily dissolves lead carbonate. Sodium hexametaphosphate (SHMP) can serve as a "softener" of corrosion deposits on copper coins. This is a glassy, ​​highly soluble salt. Dissolution should be carried out with continuous stirring, since the glassy mass adheres to the bottom of the vessel. SHMP is used in the form of a 5-20% solution. In a cold state it acts slowly. To speed up the process, it is necessary to heat the solution to 60-80°. SHMP is absolutely safe for metal and thanks to its slow action, it makes it easy to trace the cleaning progress and to use machining in a timely manner.

The restorer often has to deal with coins that are completely mineralized, which are not always possible not only to open, but also to preserve. The success of the treatment depends to a large extent on which copper compounds have replaced the metal as a result of complex mineralization processes. First, you should carefully, without physical stress, remove surface layers. If in this way it is possible to reveal this or that ornament, inscription, etc., then this should be limited. If mechanical processing has not "opened" the coin, a 5% solution of caustic soda or ammonium carbonate is used. removing the coins from the solution, the softened layers should be gradually removed mechanically, thereby contributing to the acceleration of the cleaning process.

Cleaning of silver coins is carried out mainly by chemical methods. Such cleaning is reduced to the removal from the surface of oxides and salts of other metals, mainly copper compounds. Copper connections on silver coins are recognized by their green color. To remove them, put a coin in a glass vessel or in a porcelain evaporating cup and pour 5% sulfuric acid solution. To speed up the process, you can clean the solution with a heated solution, from time to time taking out the coin, rinsing with water (preferably running water), then brushing off the mechanically softened salts with bristle or glass fiber brushes. With repeated operations of this kind, the layers gradually decrease and completely disappear. You can also use a 5-10% solution of formic acid (preferably heated to 50-70°). It softens well and removes oxides and carbon dioxide compounds of copper on silver. The hotter the solution, the stronger its effect. Formations of violet-gray chlorargyrite (mineral form of silver chloride, AgCl) on silver coins are exceptionally soft. Such coins often lack a metal core, and during cleaning, the possibility of their complete loss is not excluded. But if the crust of chlorargyrite is not too thick, the coin can be immersed in a 5-10% solution of ammonia or ammonium chloride. In these solutions, the chlorargyrite is softened and then carefully removed mechanically.

The method of electrochemical reduction can be applied to coins that have retained a metal core, but have very dense, hard, "thick" oxides, as well as to fragile coins. This method does not require special equipment. For electrochemical processes, zinc and aluminum are usually used. Practice has shown that for cleaning coins, it is best to use plates made of sheets of zinc or aluminum, with holes punched in them (similar to a grater.) As an electrolyte, sodium hydroxide or formic acid of 5-10% concentration is used. The zinc or aluminum plate is placed at the bottom of the glass container so that the progress of cleaning is clearly visible. On the “grated” surface of the plate, several coins are laid one to the other, which are covered with the “grated” surface of another similar plate. After that, the container is filled with the prepared solution so that the coins with the plates are completely hidden. This method of cleaning should be carried out especially carefully with frequent monitoring of the progress. The softened corrosion products must be mechanically removed from time to time during the process and the coin should be examined with a magnifying glass to determine the degree of its cleaning. After inspection, if a clearer view of the surface elements and patterns is needed to identify the coin, it is again subjected to electrochemical processing.

Note that gold and "new" coins that have not been in the ground, but only have been in circulation for a long time, can be cleaned quickly and easily. Fatty substances must be removed from them with acetone, gasoline or alcohol and immersed in a 5-10% solution of sulfuric, citric or formic acid. After cleaning the coins in one way or another, they should be thoroughly washed ("boiled") in several changes of boiling distilled water until the water is neutral according to the universal indicator and the soluble chlorides are completely removed: the cooled sample of the last wash water should not become cloudy from the introduction of a drop of 1.7% solution of silver nitrate. Following the washing, the coins are dried in acetone, then in alcohol for 30-60 minutes. For this purpose, you can also use a thermostat (special drying cabinet), keeping coins in it for no more than two hours at a temperature of no higher than 100° C.

Copper coins that showed signs of bronze disease, after cleaning, are again placed in a humidification chamber. If drops of copper chloride appear in some places, these pockets are thoroughly cleaned mechanically, and the coins are "boiled" again. If the copper coins after cleaning have an unpleasant shine, it is possible to artificially create a patina from light to dark brown. To do this, you need to take 50 g of copper sulfate and 5 g of potassium permanganate per liter of distilled water. The coins should be immersed in the prepared solution previously heated to 70-80° and kept in it until the desired shade is obtained. After drying, the coins must be preserved with a protective coating - paraffin or synthetic resins. For this, it is recommended to use 5% solutions of polybutyl methacrylate in acetone, toluene or xylene and polyvinyl butyral in alcohol or a mixture of alcohol and benzene (1: 1). After preservation, coins should not be stored in "piles", but in separate boxes or trays with compartments. In this form, they will not touch and preserve the protective coating better. In addition, when examining restored coins, it will be easier to identify those ones on which corrosion pockets have reappeared.

Additional information about the restoration of metal can be found in the following books:

  • M. V. Farmakovsky "Conservation and restoration of museum collections" (M., 1947), ISBN: 978-5-4458-7394-5
  • H. D. Plenderlis "Conservation of antiquities and works of art" ("Messages of the All-Russian Central Scientific Research Center of Scientific Research, Moscow, 1964 , No. 10-11).

In addition to supplementary readings proposed by O.V. Vasilyeva, Bein Numismatics recommends the following publications on the subject of cleaning, restoration and preservation of historic artifacts:

  • Gerasimova, N. G., Panchenko, N. A. 2017. In Relikviia. Restavratsiia, konservatsiia, muzei (Relics. Restoration, Conservation, Museums) 34, 5–12 (in Russian).
  • Gettiens. R. J., Organ, R. M. 1968. In Soobshcheniia VCNIILKR (Proceedings of VTsNILKR) 3 (in Russian).
  • Kurganov, N. S., Gorlov, K. V.. 2017. In Stratum plus. Archaeology and Cultural Anthropology (6), 361–370 (in Russian).
  • Minzhulin, A. I. 1992. Vvedenie v restavratsiiu metalla (Introduction to Metal Restoration). Kiev: “KVIUS” Publ. (in Russian).
  • 1935. Ocherki po metodike tekhnologicheskogo issledovaniia, restavratsii i konservatsii drevnih metallicheskih izdelii (Essays on the Methodology of Technological Research, Restoration and Conservation of Ancient Metal Objects). Series: Izvestiia Gosudarstvennoi Akademii istorii material'noi kul'tury (Bulletin of the State Academy for the History of Material Culture) 130. Leningrad: “OGIZ” Publ. (in Russian).
  • Skott, A. 1935. Ochistka i restavratsiia muzenykh eksponatov (Cleaning and Restoration of Museum Exhibits). Series: Izvestiia Gosudarstvennoi Akademii istorii material'noi kul'tury (Bulletin of the State Academy for the History of Material Culture) 114. Leningrad: “OGIZ” Publ. (in Russian).
  • Shemakhanskaya, M. S. 2015. Metally i veshhi. Istoriya. Svoystva. Razrushenie. Restavratsiya (Metals and Articles. History. Properties. Destruction. Restoration). Moscow: “Indrik” Publ. (in Russian).
  • Aslund A., Bergsten C. J., Chiavari C., Martini C., Niklasson A., Prandstraller D., Svensson J-E. 2007. In Metal 07: Interim Meeting of the ICOM-CC Metal WG Amsterdam, 17-21 September 2007. Book 5. Protection of metal artefacts. Amsterdam: Rijksmuseum, 16–22.
  • Bellegem M., Wang Q., Fletcher P. A 2011. In Paul Mardikian, Claudia Chemello, Christopher Watters and Peter Hull (eds.). Metal 2010, Proceedings of the Interim Meeting of the ICOM-CC Metal Working Group, Charleston, South Carolina, USA,11-15 October 2010. Clemson: Clemson University, 348–356.
  • Organ, R.M. The current status of the treatment of corroded metal artifacts. // Corrosion and Metal Artefacts. Special publication 179 / Brown, B.F., Burnett, H.C., Chase, W.T., Goodway, M., Kruger, J., and Poubaix, M. (eds). Washington, D.C.: National Bureau of Standards, 1977, Р. 107–142.

    Link to the research paper in the Smithsonian Libraries and Archives.