The Cleaning, Restoration, and Preservation of Ancient Bronze Coins and Artifacts With Some Comments On the Patination of Ancient Silver and Bronze.
by Robert Beauford
What happens to ancient metal as it ages?
Why does ancient metal deteriorate as it ages, and what is the green and black stuff that covers artifacts? The metals from which most artifacts are made are not naturally occurring in the ground. Instead they are alloys, or combinations of several different refined metals melted together and mixed up to create a new metal with certain useful properties such as low melting temperature, resistance to corrosion, or flexibility. For example: Bronze is generally something like 85% copper and 15% tin melted together. This combination of tin and copper yields a metal alloy with a lower melting temperature than either of its component metals, as well as excellent flow characteristics for making cast objects. The advantages, over copper, that are provided by creating the bronze alloy were so valuable to humanity that their discovery represents one of the fundamental technological changes in human history.
So, the metals from which artifacts have been made have been altered in two ways. First by refining, or using heat to make pure metal from metal ores, and second by alloying, or combining refined metals, again with heat, to create new metals with improved useful properties.
Metals, in nature, the way they are found in the ground, are generally fairly stable. The gemstone, malachite, for instance, is a stable form of copper found in nature. It has naturally combined with things in the environment to create a substance that looks almost nothing like the metal copper, yet it is made of more than 70% copper and can be refined to create metallic copper. When the metal we know as copper, which is not a stable material, is returned to the earth, it will unrefine itself. This means that it will slowly recombine with elements in the soil, and the result, within a few hundred years will be a layer of malachite and other related minerals on the surface of the metal. This is the type of deterioration known as verdigris. Another example of this is the black or gray tarnish that you see on silver items. The tarnish is silver metal combining with sulfur in the environment, and copper which has been alloyed into the silver, combining with oxygen, both returning to a stable natural condition, and at the same time, becoming less attractive and useful.
So, to clarify, metal ores are, through heat, refined and purified into pure metals that must eventually, at normal temperatures, combine with elements in their environment and return to their more stable natural states. This process can take hundreds or even thousands of years, and is what we know as patination, verdigris, corrosion, rust, tarnish and the various other sorts of crud that gradually cover aged metal.
The second important thing that happens as metals age is that alloys may separate. In other words, those metals that are alloyed, or made of a combination of two or more metals, may separate slowly into their original components. An example of this is ancient silver coins, which become brittle. Silver used in coins is almost always a combination of silver with about 1.5 to 15% copper. Adding a little bit of copper to silver makes the normally soft silver harder, and more resistant to wearing down. Silver and copper don't really mix all that well, however, and over time (300-500 years or more), at normal temperatures, the copper will sometimes begin to again separate itself from the silver. The technical name for this process is low-temperature migration or precipitation of copper at crystal grain boundaries. The 'grain boundaries' part refers tot he fact that copper coming out of the alloy tends to migrate to and accumulate at the edges of the natural crystals within the metal. This is known, to coin collectors, as crystallization of the metal, although it is really just the crystals of the metal becoming increasingly visible over time as the copper and corrosion products accumulate. The effect of all of this is that the metals becomes weak or brittle.
To restate and clarify this point: Some alloys are not stable. Over hundreds or thousands of years, they will begin to separate back into their original metals before combining with chemicals, such as oxygen, to form the more stable natural materials that we know as minerals or metal ores.
Two major things affect how a coin deteriorates: the composition of the coin, and the chemistry of the environment in which it is preserved. Even so, there are a limited number of major chemical changes that typically happen to bronze as it ages. The environment affects this process through soil chemistry, fluid movement through the soil surrounding a bronze, and through atmospheric conditions that affect soil temperature, moisture, and so on. These factors are collectively referred to as 'the conditions of preservation.' The visible results of the changes in the bronze are collectively referred to as the patination, patina, encrustation, or verdigris. I will use the terms patina, patination, chemical decay, encrustation, and so on interchangeably.
Most of the changes that occur in bronze over time are the result of interactions of the copper in the alloy with the environment. The tin is relatively inert and is stable in alloy with copper. That is, it won't separate like silver and copper. The chemistry of patination and encrustation for ancient Roman bronze coins can also be made a little more complex due to the replacement of tin in Roman coin bronzes with lead. Many 1st to 4th century Roman bronzes were made with lead bronzes composed of a percent or two of tin, 5 to 15% or more of lead, and copper. Even in these coins, copper chemistry dominates patination, though small quantities of lead oxide are also found in the material that forms on coin surfaces.
Image: Patinas are seldom, if ever, composed of a single mineral. They are typically layered and intergrown mixtures of several different chemical compounds. Here, a somewhat unattractively patinated antoninianus of Claudius Gothicus, minted between 268 and 270 A.D., shows a complex patina formed of red-brown copper oxides overlain by green to black copper sulfates and carbonates, topped off with a mass of very firmly cemented sandy soil on the figures neck. A closer look would reveal additional minor constituents.
So, what is this crusty stuff on my coin or artifact and can it be cleaned?
Let's start with an important note: Most patinas are desirable, valuable, and attractive, and should not be removed. Some however are not. You will need to use your own judgment in this regard, paying attention to esthetic, and the potential destructiveness of the particular elements present. Bear in mind that most artifacts and coins have multiple of the following reactions occurring simultaneously on their surfaces, and that, cumulatively, they are the irreplaceable signature of the ages.
Following are listed a few of the different types of copper patina you are likely to encounter. No natural bronze patina is composed of a single mineral. Patinas are complex surfaces typically composed of a mix of the minerals described below. Though the entire surface is composed of microscopic intergrown mineral grains, it gets even more complex when large surfaces are considered... various portions of a coin will often be dominated by different types of patina.
Dirt: Looks and acts pretty much like dirt, but may be far more difficult to remove than ordinary dirt. The word 'induration' means the hardening of sediments into stone. It occurs due to processed that are not unlike the growth of lime, rust or other crud in sinks. Dissolved material, carried in rainwater or groundwater, is carried into uncemented sediments when they become wet. The water dries, and the dissolved material is left behind, like lime in pipes, slowly cementing the soil particles together and closing microscopic voids in the sediment. The result can be tough, stone-like soil, partially indurated (hardened) by deposited calcite, rust, or by dissolved material from the coin itself.
Image: The typical dark, reddish-brown appearance of an attractive and protective copper oxide patina on an ancient coin of Theodosius I (379-395 A.D.). Lighter areas are indurated (hardened) dirt - it appears to be a clay-rich soil cemented by calite, or lime.
Pretty and protective:
Copper Oxides - Cuprite (Cu2O), Tenorite (CuO), etc. - Oxygen is common in the environment (about 20% of the atmosphere!) and is an extremely reactive element. As a result, copper-oxide mineralization is by far the first forming and most frequently present form of patination. The specific molecules (minerals) that are formed by the reaction of copper with atmospheric oxygen can vary, and become more complexly varied with the passage of time. The most common of these copper oxides are cupric oxide, and cuprite, or cuprous oxide. Cuprous oxide is generally reddish in color and tends to form first. It quickly converts to cupric oxide, which is dark brown or black in color. Virtually all ancient bronze coins have at least a thin layer of brown copper oxide directly on the metal surface. In some cases, this layer can become moderately thick
Copper Sulfide and Sulfate patinas represent the 2nd most common group of patinas on modern copper and bronze objects as well as a significant portion of the black to green patinas on archeological bronzes. Sulfur is available as a reactive ion both in the atmosphere and in the soil. It originates from the decay of sulfur bearing minerals such as iron pyrite as well as from natural processes such as volcanism. Today, most atmospheric sulfur originates as pollution, though significant natural sources contribute to local abundances in soils and surface water in some regions. Copper sulfide is typically dark grey to black, and is difficult to distinguish from typically intergrown copper oxide. Copper sulfate is generally green to very dark green, and is slower to form. Once well developed, it can be very stable, and acts as a durable protective coating. Some of the associated minerals may be Brochantite - Cu4(OH)6(SO4) and Ponsjakite Cu4(OH)6(SO4)*H2O.
Copper carbonate minerals also account for a significant portion of the green patina found on ancient bronzes. The reactive carbonate ion is pervasive in the environment due, among other reasons, to the chemical dissolution of limestone and dolostone, two of the worlds most common surface rock types. It is present in virtually all groundwater and in most soils. The copper carbonate minerals, essentially malachite and azurite, can range from green to blue. Copper carbonate grows due to an environmental reaction with copper oxide, not elemental copper, so it will only form on top of the brown or red copper oxides. Because copper oxide is more stable than copper carbonate,irregular carbonate deposits can sometimes be selectively removed leaving the red or brown beneath, though this should not be attempted lightly. Failure can ruin a coin.
Copper acetate - Just as copper carbonate minerals form from copper oxide minerals instead of copper metal, the copper acetate and hydrated copper acetate minerals form from already reacted copper. In this case, the precursor is the copper carbonate minerals. Copper acetate is green, and was historically used as a pigment. It typically does not completely replace, but rather occurs intermixed with copper carbonate. This mineral results, more or less, from the exposure of copper carbonate that has already formed on a coin or artifact to acetic acid (vinegar) in the environment. Since acetic acid, which is a common result of the decay of organic materials, is common in soils, it is a readily available in many preservation environments.
Cuprous Chloride and Cupric Chloride - (pale green powdery spots on the surface of a coin or artifact). While there are generally a number of reactions occurring on the surface of a coin at any given time, the presence of chloride ions is the most destructive, since they produce hydrochloric acids which eat your coin, thus producing more Cuprous Chlorides to... etc. until there is no artifact. (see Bronze Disease - below.)
Reddish Warts - The nature and cause of these varies significantly, but they are typically bad, reflecting underlying pitting of the metal. Put simply - bumps in the patina typically mean pits in the metal. Metallic copper is far more compact than the minerals that are formed as it combines with ions from the air and soil to form patina. If an unusually agressive reaction is attacking a point on the coin's surface, the volume of the accumulating stack of copper minerals can be visible as an outlying high point. Investigation will reveal that the warts are typically layered. Depending upon whether the original reaction has ceased or is still running its course below the surface, this may mean concentric, hard red layers of oxides filling a remnant pit, or it could mean a thin cap of reddish, black, and green oxides, carbonates, and sulfates that have formed over a still active copper pitting (probably chloride) reaction. Sometimes, the active reaction can be seen as light green powdery minerals revealed if the pit is cleaned out. (see section on chlorides, above, and discussion of 'bronze disease' below). The scaly bumps that are formed usually rise about 2-7mm above the surface of the metal. It can be removed but is extremely destructive, leaving large pits and destroying most details. Bronze that shows this type of degradation is frequently unstable and soft even in the areas not directly affected by the warty encrustations themselves. (I avoid coins with this symptom.)
There are really only 3 major chemical changes that you will face dealing with ancient silver. Silver combines with sulfur, and chlorine and, less readily, with nitrate ions and oxygen, mostly resulting in gray to black patinas.
The first two important chemical reactions are of the silver itself with its environment, and the third, brittleness, is the result of the copper that is almost always present in silver alloys.
Silver Sulfide (gray to black) This is the commonly known silver tarnish. It occurs rapidly and consistently to nearly all silver artifacts.
Silver Chloride (horn silver) Horn silver is present on many ancient silver coins. It is relatively soft, and can be difficult to remove due to its tendency to smear and obscure features of the coin. Horn Silver is the combination of part of the silver in the coin or artifact with chlorine to form silver chloride. It has a somewhat purple-ish to silvery-yellow in appearance, and projects slightly from the surface of the artifact or coin, affecting its appearance.
Image: an SEM image of the fractured edge of a severely embrittled Roman Republican denarius from the 2nd century BC. This coin fractured during normal handling, revealing a combination of large crystal grains and coroded boundaries, a recipe for structural weakness. The purity of the metal was in excess of 96%. the image was prepared on a Nova Nanolab FEG Scanning Electron Microscope with attached energy dispersive x-ray spectrometer at the Arkansas Nano-Bio Materials Characterization Facility at the University of Arkansas, Fayetteville.
Embrittlement - Silver can only stably contain about 1% copper at room temperature. This amount increases as the silver is heated thus the use of heat to combine silver and copper in alloy. Because silver alloys cannot be stored or used at the high temperatures at which they are formed, degradation of the alloys at room temperature is an ongoing constant for most silver artifacts. Unstable alloys that contain more of a metal than is sustainable at normal temperatures are referred to as super saturated. Such alloys are created because they provide superior working properties int he metal. Pure silver is very soft, has a high melting temperature, and flows poorly when poured for casting. The addition of copper to the silver hardens the metal, increases durability, drops the melting temperature, and provides better flow characteristics during casting. In practical terms, this is accomplished by melting the metals together. When the silver is cooled rapidly from high temperature the dissolved copper is trapped in solution and then precipitates out over time. Embrittlement typically takes longer than the expected working life of an object, but becomes a very significant factor over hundreds to thousands of years, until an artifact may literally crumble with rough handling.
Embrittlement, as stated previously, is the condition commonly know to collectors as crystallization.
Embrittlement is primarily the result of inter-crystalline corrosion at the microscopic level due to the selective precipitation of copper from the silver alloy at the crystal grain boundaries. Discontinuous precipitation of copper may soon provide a new method for the dating and authentication of silver artifacts. Discontinuous precipitation occurs primarily in silver alloys with a 1.5% - 10% copper content, and shows up as a crystalline or jigsaw grain pattern on the surface of the artifact. Higher copper content silver alloys are resistant to this phenomenon. I am unaware of any method for successfully treating embrittlement, but awareness and understanding of the process and results may help the owner of an artifact to more safely handle and better protect it.
The cleaning of antiquities and coins in particular is a subject of many debates in the collecting and scientific community. Some believe that ancients should never be cleaned or altered, but should be left entirely for future generations with better technologies of restoration, while others don't hesitate to throw whole piles of ancient coinage in the harshest acid baths and rock tumblers. A wise coarse, it seems, is to pursue a middle ground, evaluating each coin or object on its own merits and needs, and using cleaning technologies appropriate to the specific conditions found on the surface of the artifact.
Further, one should note that, because the deterioration of a coin's surface over time is rarely uniform, the best cleaning techniques will apply different techniques and skills to different parts of the coin or artifacts surface as appropriate.
I recommend doing your cleaning experiments on cheap coins and artifacts that are already damaged or are in extremely poor condition. You are likely to damage or destroy some pieces in learning. Hopefully the percentage of items that you damage will decrease as you gain experience. Always do a little experimenting with lesser pieces before trying to clean an important piece. When in doubt, hire a professional.
Keep in mind that the layers of patina on your coin or artifact are, unless they are of a destructive type, the best protection that your item has from the environment. The stable boundary provided by a good patina provides a chemical and mechanical buffer that can protect a coin or artifact for thousands of years. Once an item is cleaned, the rate of deterioration increases several hundred times over until a new patina or tarnish layer is formed.
Whatever the method you choose in cleaning an artifact, the ideal objective is to uncover a protective, attractive patinated surface if such exists. In some cases the encrustation will have no such layer underlying it, and the objective will be to thin the existing encrustation to show more detail, or to remove it altogether. The only reason for cleaning an item, and thus a prime consideration in the process, is maximizing attractiveness.
There will not necessarily be any easy way to clean a given artifact, but here are a few guidelines for general approach:
1. Choose simple washing and olive oil first, over mechanical means, and choose mechanical means over chemical techniques.
2. For best professional results, Choose localized techniques over global ones (alter the areas of the artifact that need alteration and not the whole coin).
3. It is better to stop to soon than to late.
Always restabilze any object exposed to chemicals in the cleaning process.
Again, let me repeat for clarity's sake. If you try cleaning ancients you will ruin some of them. The information following should give you a good start. I will list the techniques from safest to least safe for the artifact - You are responsible for your own knowledge in handling chemicals, which you choose to use, in a safe manner. Good Luck!
Repeated soaking and washing in plain or distilled water is generally safe and appropriate for the removal of simple dirt and clay accumulations. A toothbrush may be used for greater effectiveness.
Soap and Water:
Household soap, detergents, and shampoos are effective in cleaning many items that are lightly encrusted. Effectiveness is increased by the use of a toothbrush or other stiff bristle, no metal brush. Soap and water is minimally effective on patina type encrustations.
Soaking bronzes in Olive Oil is the oldest and most reliable and least destructive cleaning technique that I know of. You cannot leave an object in olive oil too long. This technique has been used for hundreds of years, and it is the only cleaning technique of which I have never heard a condemnation.
It is, however, also the slowest technique I know of. You may not see any results for a month or more and complete cleaning of an article can take many months. Some encrustations will not respond at all to this technique. It is, however, the only technique other than microscopic cleaning with hand tools that I personally will use on expensive or fine coins.
To speed up the cleaning process the temperature can be raised, but you need a thermostatically controlled cabinet to regulate it. The oil should not be heated to above handleable temperature. Don't use gas burners or ordinary electric hotplates to heat the oil! Olive oil is very flammable.
To use olive oil, place the object to be cleaned in olive oil in a closed container. To hasten the process; use a toothbrush to occasionally gently scrub the coin. Change the oil monthly or bi-monthly.
High Pressure Water and Mechanical Washing:
Techniques such as water picks, dishwashers, and placing items in a sock in the washing machine are potentially effective in removing dirt type encrustations only. Beyond this you will need to use your judgement as to the potential effects on fragile artifacts and expensive appliances.
Ultrasonic cleaning devices are useful for removing loose layers of dirt, and they are to some extent effective on more permanent encrustations. With a good ultrasonic device, the crud will just fall off, but over cleaning is possible. This process is very gentle, but many ancient items are structurally fragile or brittle. They can shatter, break, chip, or develop shiny spots if left in too long or if they were weak to start with. Note, the $10 ultrasonic jewelry cleaners you can get at the jewelry store or department store will have no effect. You will need a professional model for it to be of any use.
The brass brush is a fast and usually safe approach for preliminary removal of dirt and clay deposits on bronze items. Some people hate this technique, some love it. I have tried it with great results in some cases, and damage to the artifact in others. The real trick lies in knowing when to stop. Some brushes are too hard and will scratch your coins; some are to soft and will rub off a layer of brass on your items. I like the brass tire and suede brushes manufactured by Kiwi (as in the shoe polish). You can find them at Wal-Mart in the states. Never use this technique on a high quality coin, or on any item made of gold or silver.
Using a rotary rock tumbler to clean coins is generally a bad idea.
Vibrating tumblers of sort used in jewelry trade are somewhat better. There are many different kinds of media for use with this type of tumbler, from walnut husks, to wire clippings, to steel shot, and fast cutting ceramic and plastic abrasives. The results, therefore, can vary from the equivalent of a gentle scrubbing with a toothbrush in soapy water, to the equivalent of sand blasting away half of the mass of the article. A remarkable degree of control is possible with a lot of information and experience, and efficient destruction is possible without. The challenge here is that a tumbler of this type is a fairly high tech piece of equipment that must be used correctly with the correct media, time, soap, and judgement about the condition of the article. Experience and knowledge is the key.
Dremels, Foredoms, and related tools:
While they are wonderful tools, there is absolutely nothing that I have discovered to date or have ever heard of that you can do with a Foredom or Dremmel that is in any way beneficial to any ancient coin ever. High RPM's is the opposite of control. So, put down the power tool, step away from the defenseless coin and take ten deep slow breaths.
Hand Cleaning or tool cleaning is one of the best ways to clean ancient artifacts, particularly coins. Hand cleaning is the process of cleaning a coin or object by sharpened instruments with the aid of a magnifying tool. The ideal setup is a binocular microscope and light source and an assortment of steel, plastic, glass, and wooden tools fashioned for cutting picking, and probing. The reason this technique is desirable, is that it gives the maximum control in applying appropriate techniques to appropriate areas on the surface of the artifact, according to the conditions of the surface. This is always desirable over global techniques, which apply uniform indiscriminate processes to the entire surface of the object.
The reason I have placed this technique near the bottom of the list with the potentially destructive ideas is that without lots of practice, you can do a whole lot of damage to a valuable coin with a sharpened blade with just a slip. Geta just doesn't look nearly as handsome without his nose.
If you want to try manually cleaning coins or artifacts, and I recommend it whole heartedly, start with cheap ones.
Get yourself a good microscope or table magnifier, and a few dental picks. Sharpen the ends of them into small blades and probes and such of different shapes. Add a bamboo chopstick with a nice sharpened end, a few tooth picks, and whatever else you can think of. Your tool kit will rapidly refine itself according to your tastes, preferences, and dexterity.
You can try adding a lubricant to the equation to protect the object's surface and loosen the encrustations if you feel comfortable with the idea.
Work on small areas of the coin or artifact, keeping the edge of your tool at a close angle to the surface you are working on. If you are lucky or skilled, and you have patience, you can clean down to a fine hard patina with remarkably beautiful results.
The most important elements in this technique are patience, and knowing when to stop. As with all cleaning methods, when you first start, you have a high probability of doing damage.
Electrolytic Cleaning is the technique of reducing the patina and encrustations on the surface of an object back to metal, by suspending the object as a pole in a electrolyte solution and running a charge through it. It is a remarkable technique, which I have never attempted. The same process can be accomplished on a tabletop by various chemical methods involving metals and chemistry that together generate an electrical flow. I understand the process to be particularly effective in the treatment of horn silver. Following is one technique:
For table electrolytic reduction of horn silver, wrap the artifact in aluminum foil so that the foil is fully in contact with the surfaces, but slightly open at the edges. Put a small amount of MALT vinegar in a shallow dish and immerse the coin in it for a short time (30 to 60 seconds). You will probably see small bubbles coming out as the silver chloride is reduced and chlorine is released. This must sometimes be repeated a number of times. The silver will form a pure silver deposit on the surface that must then be removed. If it is a very light coating it will probably rub off easily with your fingers, but often it is heavier (forming a bumpy surface) and more difficult to remove. A short soak in diluted ammonia will help loosen it, but in the end it often must be removed with pins and needles under a microscope.
If you want to get more involved, you can build an electrolysis machine with a battery or plug and rectifier, some wire, a tank and some chemicals. I am sure instructions can be found on the net. Note this involves liquids, electricity, and some harsh chemicals. Don't screw with it unless you are serious about cleaning artifacts, and do your research first.
If you choose to use chemicals to clean your coins or artifacts, read the following section on Restabilization to fully halt the chemical processes that you begin.
Always use the weakest chemistry appropriate to the job. Read all the instructions, wear appropriate safety equipment and clothing, and work in a well-ventilated area or outside. Practice on cheap coins or artifacts first, and don't use chemicals on important or expensive coins.
I am only going to share a few items, which give reasonable results. There are a large number of kitchen, household, and bathroom cleaning agents which people have used for cleaning antiquities.
Soaking in Calgon Water Softener works rather nicely for cleaning many bronze and silver objects. The article is soaked in dilute or full strength solution for a time ranging from 1 day to 2 weeks, with daily scrubbing with a toothbrush. The results are adequate, but overcleaning is possible. Left long enough, Calgon will strip all patina so check the progress frequently.
Many collectors praise the days when you could get these now prohibited trichlorotrifluoroethane based products. They apparently worked wonders in removing encrustations and left a nice patina. The chemicals were commonly used (by the thousands of tons) as refrigerants, and were outlawed because they destroyed the ozone. If you have some lying around, the few grams necessary for cleaning coins probably aren't going to do much environmental damage. The chances of finding one of these solvents are remote.
Acids are the fastest removers of artifact encrustations. They are also the most potentially destructive to the artifacts. I would never use them on good coins. Vinegar and Lemon juice are fairly weak starting points. Muriatic swimming pool acid will remove dirt and encrustation. So will jewelers pickle, dilute hydrochloric, and any number of others. The result is frequently an ugly red artifact that must be brushed and artificially retoned. Another readily available chemical group which strip coins and artifacts quickly are commercial bathroom cleaners such as Lime-A-Way. I have even heard of Naval Jelly being used also. Chemicals should be considered only as a last resort. They should be used safely in controlled surroundings, and the article to be cleaned should be emersed for no more than the minimum amount of time necessary to do the job. With experience and knowledge, acceptable results can be achieved in some cases. This is a matter of experience, trial, and error only, however, as the initial condition of each coin or artifact defines the amount of exposure necessary to achieve a desirable result with that item. Despite all of these warnings, acid treatment is, for some artifacts (seldom if ever coins), and for some end results, the appropriate and desirable alternative, second only to electrolysis. All acid treated items must be stabilized.
Darkening Harshly Cleaned Metals
The ethics of repatination is a fiercely debated topic. Few will argue, however that harshly cleaned or stripped coins benefit greatly from some darkening or toning. As with cleaning, however, take into account that changes you create are permanent. Always ask yourself if this object might really be better off just the way it is. To accomplish toning or darkening of a coin or artifact, you have to initiate one of several different chemical reactions. As with cleaning techniques, I will tell you that you will screw some of them up so don't experiment with anything that is important to you.
By far the easiest and safest way to tone a bronze or silver object is to bake it in an oven at 350 or so Fahrenheit. This will stimulate an oxidation reaction in either metal and darken it somewhat. Getting the item to darken evenly may be a challenge. You may be able to get similar results through boiling in distilled water for several hours, but I have never tried it.
The second technique for darkening these metals, and by far the most common, is application of sulfur compounds. This causes copper and/or silver sulfides and sulfates to form. The result in either metal is a darkening. The products used for this purpose are generally called flour of sulfur, liver of sulfur or something of the sort. Many of the commercial preparations prepared for darkening or toning metals are also sulfur compounds. The result of mixing liver of Sulfur with water is a caustic solution, and caution is called for. The best results I have achieved in experimenting with these materials came from mixing a very weak solution (a couple of grams or less to a cup or two of water), and allowing the solution to sit for several hours until it is mostly inactive. Then, in this super-weakened solution, I soak the item for 30 minutes or so until toned to the desired extent, remove it, and rinse it. Any time you tone or darken an item with a sulfur compound or any of the many commercial compounds on the market, you need to fully restabilize the item or the reaction may continue slowly on, possibly leading to severe damage to the artifact or coin.
Restabilization and Storage of Artifacts
This section is more important than its length would seem to indicate. Restabilization is the all important process of removing chemicals and halting the various chemical reactions that you might choose to start in the process of cleaning or repatinating (darkening) an artifact. Any time you treat an artifact with any chemical, it is advisable to go through a process of restabilizing it.
Restabilization is accomplished by simply placing your coins or artifacts in distilled water and allowing them to soak for a week or so. Two weeks is preferable. Change the water each day. Boiling will accelerate the process somewhat and make it more thorough.
Following soaking, dry your item thoroughly in a heated place. You need to get all of the water out of it. Alcohol can be used to help remove water. Just wet the object in alcohol before allowing it to dry, and it will dry quicker and more thoroughly.
You can stop there, or you might choose to use archival, inert microcrystalline Renaissance Wax to coat your artifact to protect it from further degradation from environmental exposure. Don't coat an uncleaned or inadequately dried item. You can as easily trap harmful compounds in, as keep them out. Renaissance wax can be gotten from Rio Grande Jeweler's Supply. http://www.riogrande.com/
Bronze Disease: It's Cause and It's Cures
Bronze disease is a destructive reaction, which occurs on the surface of coins, and artifacts fashioned from copper alloys. Bronze disease will appear a green or blue-green color and will be soft and powdery. It can be rubbed off with a finger, but it will re-appear as more powdery growth. Anything too hard to gently rub off is probably not bronze disease. It is relatively rare but very destructive. Those living in dry climates will probably never have a problem with it, because the reaction requires a relative humidity of 39% or higher to become active.
The reactive chemicals involved are cuprous chloride, which combines with oxygen and water in the air to form destructive hydrochloric acid, which eats the metal, forming cuprous chloride again and so on... Of all of the various reactions occurring on the surface of an artifact at any given time, cuprous and cupric chloride are the most rapidly destructive.
The most effective cure seems to be a three-step process:
1) Light brushing to remove as much of the powdery growth as possible while not harming the surface of the coin or artifact.
2) Then let the coins soak in distilled water, changing at least every day, for two weeks to a month. This serves to remove the chlorides. They are carried away in the water slowly. The length of the soaking process and the frequent changes of water are both critical.
3) Then bake the coins in a normal kitchen oven at 300 degrees F (500 C) for two to three hours to drive out all of the moisture present.
At the end of this process, if all has gone well, the green powdery bronze disease will have vanished. The coins may be slightly darkened due to oxidation while baking, and there will be slight scarring where the bronze disease was present.
The artifact may be lightly polished with a soft cloth if necessary.
Archival Storage of Ancient Coins and Artifacts
'Archival' curation and storage refers to the handling and storage of important materials, like ancient coins, in a way that prevents ongoing deterioration and does not introduce new causes of deterioration. The archival storage of metal coins and artifacts is easy. Just store them in an a sealed inert gas chamber filled with nitrogen or argon, at a uniform temperature, and never touch them. In other words, nobody actually achieves real archival storage or handling. It is an ideal to be striven for, but requires an enormous initial and ongoing budget in order to do well at any large scale. Even most museum and university collections manage, at best, a relatively cool, dry room and a minimum of handling. Individual collectors can do at least this well or even better with only a small investment. A reasonable effort is within even the most modest collector's reach. The basics are: Keep coins in a cool, dry place, handle as little as necessary, wear clean cotton or powder free latex gloves for handling (or at least wash and dry hands thoroughly before handling), and invest in an acid free box and some acid free envelopes to store coins in. These kinds of supplies used to be difficult to find and expensive to buy, but today, there are many online sources for archival storage materials for coins and other important collectibles. Even a brief search will turn up a range of retailers. Acid free paper envelopes are better than plastic or wood, though there are some reasonable solutions that allow semi-archival storage at the same time as allowing visibility.
Knowing what not to do is probably just as important as knowing what to do. The opposite of archival handling is touching coins with salty, oily, sweaty hands and then putting them into the horrible little cheap plastic sleeves that are so pervasive among coin collectors, and then storing these in a garage in Florida or another hot, humid region. All coins slowly decay, but this kind of storage can accelerate the rate of decay by thousands of times.
Coating coins with protectants is not generally considered a good idea, as these trap corrosives against the surface of a coin and often create microenvironments that accelerate decay. If you are confident that you have a relatively clean, stable coin and that the article is totally dry, you may wish to coat a coin with Renaissance Wax to protect it during handling. The source for renaissance wax is listed at the end of the preceding section on Restabilization.
Web links constantly change, but a few good current semi-archival storage tips and products can be found at:
Copper and Bronze in Art - Corrosion, Colorants, Conservation, by David A. Scott, is the best book on bronze conservation that I have seen.
Amateur cleaning of coins and artifacts by dealers and collectors has been going on for years, and will continue into the indefinite future. In the past, the processes used were a mix of family recipes and techniques, rumors and bad ideas, and a smattering of concepts borrowed indirectly from the world of professional artifact conservation, which is, itself, a relatively new profession. The world and language of professional, scientific artifact curation is largely a mystery to the average collector or dealer, but through the internet, it is not inaccessible. A few good places to start include:
This article is meant to provide information only. I do not provide public or private artifact cleaning, conservation, appraisal, or any related services. The article provides only a general overview of a very complex subject. Please remember that we are only temporary stewards of the artifacts we may posses or for which we are responsible. Use any of the techniques mentioned at your own risk and with careful consideration.
Thanks and All the best to you!