The Restoration and Conservation
of Ancient Copper Coins
The Restoration and Conservation
of Ancient Copper Coins
Dr. Bruce C. Cresson
Dr. Alton Hassell
Doyle W. Lynch
May 1, 1991
This paper describes the results of my research to determine the best available methods for cleaning (or restoring) ancient copper coins from excavation sites so that they can be properly identified.
Some Photographs of Restored Coins
Before and After photographs of 2 coins.
This paper describes the results of my research to determine the best methods available for cleaning ancient copper coins from excavation sites so that they can be properly identified. My research has been limited to deal primarily with the cleaning (or restoration) of copper coins with a solid metal core for several reasons. Coins were chosen over other metallic artifacts because they are, in addition to being small and fairly common, often valuable keys in providing absolute dates. Copper coins were chosen because they are more commonly found in excavation sites than silver or gold coins and are considerably more apt to serious corrosion. A solid metal core was also found to be necessary since little can be done to restore those without such a core even though further corrosion can be prevented (conservation). Though this paper is not conclusive, it does provide a good foundation for further, more detailed research.
I would also like to note that I am aware that cleaning coins is a controversial topic. Many numismatists have at least a slightly negative opinion about coin cleaning. They feel coins should not be cleaned unless absolutely necessary (if at all) since the available methods can be harsh and even destructive to the coins if not carefully followed. However, the electrolytic reduction process as described here does not seem to harm the coins in any way (as long as the directions are followed) and the procedures are simple and straightforward. Furthermore, some type of cleaning must often be done to allow examination of the coin's stamp and to prevent the further corrosion of the coin.
A Brief Introduction to the Corrosion Problem
After minting, a coin begins to corrode as soon as it comes into contact with air. A thin layer of copper on the coin oxidizes to form a consistent and nonconcealing layer of copper (I) oxide (cuprite) called a patina. Numismatists sometimes refer to this process as "toning." This "dull-looking" patina usually protects the coin from further corrosion though impurities from the air, ground, and sea can cause further corrosion. A good patina should not be removed. If it is removed, the bright, newly-exposed surface will once again oxidize, causing a slight loss of the coin metal.
Unfortunately, the corrosion process often does not stop with toning, especially once the coin has been buried. Acids and salts present in the ground and sea attack the metal. Plenderleith and Werner note, "Contamination is the rule rather than the exception because soluble salts are widely distributed in nature, in the soil as well as in the sea, and for this reason they are usually present in excavated material" (246). The buried coin's patina then thickens and the its cuprous oxide compacts into purplish-red cuprite. Basic green (due to malachite) or blue (due to azurite) carbonates can then become imbedded in the cuprite (246). Other chemicals can also cause further corrosion. According to MacDowall, sulfide turns the coin's patina brown or black, oxide turns it red, and sulfates turn it green (22). However, these processes should stabilize eventually with no further loss of metal though the coin may no longer "look" very metallic.
It is important to note that the single most destructive force in causing corrosion is moisture; most of these reactions would not occur without its presence. Moisture also causes soluble salts in the soil to dissolve and form an electrolyte that conducts electricity, resulting in electrolytic corrosion. If metals such as gold or silver are located nearby, the problem becomes more serious. A natural electric cell is formed which saves the more noble metal (gold or silver) by corroding the less noble one (the copper). However, the presence of other less noble metals will save the copper at their expense. Of course, this process can cause particular problems when the coin's copper is alloyed with other metals.
The most damaging corrosion occurs when chlorides and moisture come into contact with a coin, and chlorides are unfortunately quite common in the ground. The copper and chlorides react to form cuprous chloride, which causes progressive corrosion in the presence of moisture. It leaves the surface with patches of pale green, powdery material. This is commonly referred to as bronze disease and must be taken care of by removing the chlorides before the whole coin eventually disintegrates.
In conclusion, excavated coins almost always need to be thoroughly washed and dried to prevent further corrosion. Even though an equilibrium is often achieved in the ground, this equilibrium is destroyed when the coins are excavated and exposed to a new environment. Furthermore, this corrosion layer often obstructs the image stamped on the coin. To examine the coin's stamp, the corrosion must be removed.
However, a few notes must be remembered when cleaning a coin. First, as little cleaning as possible (with as few different chemicals as possible) should be done. Since coin cleaning processes are not completely understood, the less that has been done to the coin means the less chance of later problems caused by the cleaning. Second, cleaning should be done carefully to prevent disastrous results, such as damaging the coin. Thirdly, a record should be kept of each coin's treatment for later review.
Prior to electrolytic reduction, as much of the encrustation as possible should be removed with water and a toothbrush. If necessary, a bamboo stick (or skewer stick) sharpened to a point can be used to remove stubborn bits of encrustation before and after electrolytic reduction (Beaubien). These sticks are not hard enough to scratch the surface of the coin. Most other means of mechanical cleaning can scratch the coin since, by definition, mechanical cleaning is a type of abrasion.
Many sources on ancient copper coin cleaning list various preliminary processes to use. Most utilize acids or other chemicals that are potentially hazardous to the coins. Among these are soaking in an alkaline Rochelle salt solution, dilute sulfuric acid, or dilute formic acid. When tested, these chemicals seemed to do nothing for the coins that the electrolytic reduction did not accomplish without the hazards.
The first preliminary step tested was soaking the coin in a Rochelle Salt solution. The solution was made by mixing 150 grams of Rochelle Salt (sodium potassium tartrate) and 50 grams of sodium hydroxide in 1 liter of distilled water. This solution succeeded in dissolving most of the green material (cupric carbonates). The coin was left with mostly black and dark red material (cuprous oxide) on it, which was not noticeable before treatment. However, this process took up to twenty hours (or longer), depending on the amount of corrosion. The solution also turned blue, which indicates that some of the copper is probably going into solution.
The second preliminary step was soaking the coin in a 10% sulfuric acid solution (3 mL acid to 27 mL distilled water) for just a few minutes. When tested, this took off the black and red cuprous oxide, but it also turned the solution blue, again indicating the loss of copper. A 30% formic acid solution can be used instead of sulfuric acid, but it too can damage the coin. In conclusion, these two steps do take most of the dirt and corrosion off, but they have two weaknesses: the process is long, and it seems to be removing some of the coin metal (as well as pitting the coin) in addition to removing the corrosion.
The most valuable preliminary process discovered was soaking each coin in a 15% solution of sodium hexametaphosphate (Calgon) made by mixing 75 grams of sodium polyphosphate to enough distilled water to make 500 milliliters of solution. This removes most of the surface dirt (characterized by a gray dirt color) that refuses to come off by brushing the coin under running water or by electrolysis alone. Unfortunately, this process can take more than a day, even when used in combination with an ultrasonic bath for brief periods of time. However, it was later discovered that this process is not necessary in cleaning coins with a solid metal core if a minor adjustment is made to the electrolytic reduction process.
Real progress in restoring the coins was noticed with electrolytic reduction. Following Plenderleith and Werner's lead, a 5% sodium carbonate bath was chosen over a sodium hydroxide bath (198). Though a sodium hydroxide bath gives a slightly faster reduction rate, it is harder on the coin metal than sodium carbonate, especially if certain trace metals are present with the copper. Increasing the solution from 5% to 15% sodium carbonate seemed to increase the rate of reduction slightly, but this was not tested thoroughly enough to determine with certainty.
A 6/12 volt 6 amp car battery charger was used as the power source. Twelve volts was preferred, giving faster results than six volts. As recommended by Ostertag, the coin was connected to the current by an alligator clip that was not allowed to touch the surface of the solution. This forced the reduction to occur primarily on the coin instead of on the clip (55). Copper alligator clips were used to prevent any possible side reaction with the clip. The clips were also kept clean to prevent contamination of the solution and replaced when they become corroded.
A small piece of platinum was used as the anode for experimental purposes since platinum is inert. However, a stainless steel container or rod seems to work fine. Though Plenderleith and Werner recommend using two anodes, one on each side of the coin (198), only one was used in these experiments. However, it was observed that increasing the size of the anode can significantly increase the rate of reduction.
The coin was used as the cathode and only one coin was cleaned at a time. If absolutely necessary, multiple coins can probably be done if they are very similar in metal content and amount and type of corrosion. However, it is best to only do one coin at a time since cleaning multiple coins could allow foreign elements from one coin to go into solution and redeposit on another coin. This is most undesirable when working with coins containing corrosive salts.
Following Dr. Franklin's suggestion, it was discovered that adding a very small amount of lab detergent to the sodium carbonate solution eliminates the necessity of presoaking the coin in Calgon. This helped remove the dirt encrustation during electrolysis, saving the many hours needed to soak the coin in Calgon and simplified the cleaning process by removing a step. Alconox (an anionic powder detergent) and Alcoject (a non-ionic powder) were tested. Both seemed to give approximately the same results. A cationic detergent was not available to test, but it is suspected that it would not work as well as the anionic powder. The only disadvantage for using Alconox was that it sudsed profusely. As a result, more solution had to be repeatedly added. It would be very advantageous to find a non-sudsing anionic detergent.
The coins were removed from the solution before the current was turned off. This prevented removed chlorides and salts from redepositing on the coin. Plenderleith and Werner recommend keeping the current density at approximately 10 amps/square decimeter of cathode area (198). However, no deleterious effects were observed when the amperage was raised above this limit, and it seemed to speed up the process. However, lowering it below the recommended 2 amps/square decimeter limit could result in the removed ions being redeposited on the coin (198).
Each coin was left in the solution as long as necessary to remove all corrosion obstructing the design and the solution was changed when it become dirty. Frequently changing the solution was especially necessary for coins containing much lead as Caley notes (qtd. in Plenderleith 199). The process took anywhere from 10 minutes to a little over an hour, depending on the condition of the coin. Leaving the coin in the solution for too long did seem to make the coin more brittle though no other deleterious effects were noticed.
This process seemed to effectively reduce the incrustation back to copper from copper oxide, eliminate saline matter, and transfer chlorides from the cathode to the anode as Plenderleith and Werner note (197). However, it is important to note that the reduced copper is not put back on the coin exactly as it was before corrosion. It is finer and will reoxidize slightly more rapidly. However, this is acceptable since it will reoxidize uniformly as long as all the salts and other chemicals are removed from the coin. Any traces of salts or other chemicals left on the coin after electrolysis can be removed by thoroughly washing and drying the coin.
A very important note is that chemists speak of positive and negative in different terms than commonly found on car battery chargers and many other AC to DC power converters. This normally means that the black clip should be the anode and the red clip the cathode. It is very important that the coin is the cathode. Reversing the polarity can cause pitting in addition to further oxidation. An everyday copper item can be placed in the solution to test whether the positive and negative clips are correctly hooked up. Testing it both ways will not leave any question as to which way is desired.
Due to time restraints, washing and drying methods were not tested. However, it is important to thoroughly was and dry the coins to prevent further corrosion. MacDowall and Plenderleith and Werner both describe the process summarized here. The coins are first soaked for fifteen minutes (2 hours if corrosion has been extensive) in distilled water that has been brought to a gentle boil. The coins are then allowed to cool. This is then repeated at least four more times. The water is then changed and the whole process repeated until the water does not test positive on a conductivity meter for chlorides or other salts and the pH remains that of the water (MacDowall 29). To speed up the process, the coins can be periodically cooled in an ultrasonic bath for five minute intervals (30).
According to Plenderleith, the metal underlying the corrosion layer is micropourous and acts "as if it was a mass of capillary tubes" retaining residues of chlorides. Heating the coin causes any air and/or liquid in the capillaries to expand and leave the metal. Cooling the coin then sucks in fresh water to dissolve remaining chloride salts (191-192).
Plenderleith and Werner recommend drying the coins in a vacuum desiccator filled with silica gel after the coins are briefly immersed in acetone (202). If a vacuum desiccator is not available, the coins can be heated in an electric oven at 105 degrees Celsius for 24 hours (MacDowall 30).
Further corrosion can then be prevented by applying a protective coating to the coin, preferably one that can be easily removed. Renaissance Wax and Polyvinyl Acetate (PVA) Lacquer (sometimes called Gelva V-7) are two recommended protective coatings that can be used. Proper storage is also important in preventing further corrosion. The coins should be stored in an area with as low a humidity as possible and packed in an airtight container with silica gel. Remember that most corrosive reactions occur due to the presence of moisture. Handling should also be kept to a minimum and preferably done with cotton gloves.
Rae Beaubien recommended x-raying the coins. This would reveal the stamp hidden under the incrustation and is cost-effective, non-destructive, and time saving. Some experimenting might be needed to determine the correct milliamperage and kilovoltage to use. However, the equipment was not available to test this.
If a coin is found without a solid metal core, Plenderleith and Werner recommend soaking in Calgon to remove chloride salts (255). This procedure is described in the preliminary treatment section, earlier in this paper. To prevent further corrosion caused by bronze disease, a coin could be soaked in 3% benzotriazole in ethanol for 24 hours with the temperature slightly increased. Benzotriazole is a corrosion inhibitor that chemically "seals off" the chloride salts and prevents them from causing further corrosion (Beaubin). Cobratec 99 is another good corrosion inhibitor.
Notes on Photographing Coins
Photographing coins is an art that is not as simple as it might seen. It was discovered that if certain steps are not taken, the photographs and slides will be blurred and detail will be difficult (if not impossible) to discern. Though photographing coins is not necessary for cleaning them, coins will often be cleaned for photographic purposes. Therefore, a few tips for taking good photographs of coins have been included here.
- A sturdy copy stand should be used to keep the camera steady with (daylight balanced) light from only one side. The light should hit the coin at an angle to cause a slight shadow, which is necessary to see detail. Lights from two angles will not give a shadow.
- macro lense will be needed. It should allow the camera to be close enough to the coin that the coin covers most of the viewfinder.
- Kodak Ektachrome 50 film for slides or Kodak Ektar 25 film for photographs should be used, or their equivalent. This allows the most detail when the picture is enlarged.
- The f-stop should be set to 16 to give the greatest depth of field.
- A "gray card" should be used to determine the shutter speed. If a light background is used, automatic meters on most cameras will read the light off the background, and the shutter will not be open long enough for the coin. The coin will be dark as a result. This is solved by using a gray card to select the proper shutter speed.
- A shutter release cable should be used to release the shutter. This will prevent any movement from blurring the picture.
Sources for Chemicals
Most of the chemicals and supplies discussed in this paper can be obtained from any chemistry lab or ordered from Fisher Scientific. Cobratec 99 can be ordered from Mauemee Chemical or Sherwin Williams. Consult a chemistry professor for help in obtaining the chemicals. Two exceptions are:
Alconox and Alcojet detergents:
215 Park Ave., So.
New York, NY 10003
1447 Peach Tree St.
Atlanta, GA 30309
Beaubien, Rae (Objects Conservator at the Conservation Analytical Lab for the Smithsonian Institute). Telephone Interviews. Fall 1989.
Franklin, Thomas C. Personal Interviews. Fall 1989 through Spring 1991.
MacDowall, David William. Coin Collections: Their Preservation, Classification, and Presentation. Paris: UNESCO, 1978.
Ostertag, Frank R. "Cleaning Bronze Artifacts." Curator 20.1 (1977); 53-57.
Plenderleith, H. J., and A. E. A. Werner. The Conservation of Antiquities and Works of Art; Treatment, Repair, and Restoration. 2nd ed. London: Oxford UP, 1971.
Also consulted were:
Aldaz, A., T. Espano, V. Montiel, and M. Lopez-Segura. "A Simple Tool for the Electrolytic Restoration of Archaeological Metallic Objects with Localized Corrosion." Studies in Conservation 31 (1986); 175-176. (Helpful for applying methods to larger artifacts)
Frank, Charles. Coin Preservation Handbook. US: Coingard Industries, 1964. (Deals with modern coins)
MacLeod, Ian Donald. "Conservation of Corroded Copper Alloys: A Comparison of New and Traditional Methods for Removing Chloride Ions." Studies in Conservation 32 (1987); 25-40. (More technical than the other sources)
Organ, Robert M. "The Corrosion of Tin, Copper, Iron and Steel and Lead." Preservation and Conservation: Principles and Practices. Ed. Sharon Timmons. Washington: Preservation P, 1976. 243-256.
Sease, Catherine. A Conservation Manual for the Field Archaeologist. Archaeological Research Tools 1-4. Los Angeles: Institute of Archaeology, U of CA, 1987.
Welter, Gerhard, and James J. Curto. Cleaning and Preservation of Coins and Medals, including Paper Money Restoration and Preservation. 1976. Long Island City: Sanford Durst, 1987. (Many of his methods can be destructive to the coin!)