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5. Cleaning Coins |
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Many say coins should not be cleaned at all. Patinas validate the authenticity of an ancient coin as this process can only happen in nature, thus are the irreplaceable signatures of the ages. Because of this most patinas are desirable, valuable, and attractive, and should not be removed. But some are not. Most coins have several of the above reactions occurring simultaneously on their surfaces. Concerning cast coins, they are more porous than struck coins and their metal is even more vulnerable to attack by cleaning chemicals than a struck coin.
You will need to weight this out whether to keep the present esthetic of a coin or if the potential destructiveness of the particular elements present need to be removed. Some of how far to clean a coin is determined by how far its surface image may need to be better revealed.
The best site from my past surfing I knew of on cleaning of coins was: Cleaning Page by Robert Beauford (http://www.enchanted-treasures.com/cleaning/CleaningPage.htm) which is no longer posted as such. I cannot find him or his site any longer but had archived his page. Should anyone connect the two of us I would extend my apology to him for pasting his archived page into my site yet commend him for his written work I will reproduce here, in its original entirety.
For your Safety!!! An important Note on Chemical Cleaning I have not found this anywhere else but on one site related to cleaning of commercial or industrial copper sheets. This was in reference to the use of diluting acids to clean copper from their verdigris or patination.
”A stronger (faster and deeper working) solution can be made of 1:2 parts concentration sulfuric acid and distilled water. Be sure to pour the acid into the water NOT the other way round. Use as above, except you need gloves. To neutralize the acid, mix up some sodium hydroxide (lye) with water. A solution of 1:100 is good enough. Add the lye slowly to the water, as this is also an exothermal process (generates heat). Don’t leave the solution on the copper too long, as it will turn your fresh copper into CuOH. Rinse with water.”
Sulfuric acid is not recommended in the use of cleaning coins. The important note here is to pour the acid into the water so you don’t get a spitting acidic solution as it boils!
The following is Robert Beauford’s original Cleaning Page
Cleaning Page by Robert Beauford http://www.enchanted-treasures.com/cleaning/CleaningPage.htm (no longer a viable link)
The Cleaning, Restoration, and Preservation of Ancient Bronze Coins and Artifacts With Some Comments On the Patination of Ancient Silver and Bronze.
Contents: What Happens To Ancient Metal as It Ages? Cleaning Techniques Darkening Harshly Cleaned Coins Restabilization Bronze Disease and Its Cures Further References and Resources.
What happens to ancient metal as it ages?
Introduction:
Why does ancient metal deteriorate and what is that 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 and thus combined.
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. Malachite, the gemstone, is for instance, 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 copper, which is not stable, is returned to the earth, it will unrefine itself, slowly, recombining naturally 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. This is silver combining with sulfur in the environment, and copper 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, and the other properties of aged metal.
The second important thing that happens as metals age is that, those which are alloyed, or made of a combination of two or more metals may separate slowly into their 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 is precipitation of copper at the grain boundaries, which means copper coming out of the alloy at the edges of the natural crystals of the metal. This is known as crystallization of the metal, to coin collectors, all though it is really just the crystals of the metal becoming visible as the copper comes out of the alloy and begins to corrode, thus weakening the metal.
To clarify this point, some alloys are not stable, and, over hundreds or thousands of years, they will begin to separate back into more stable natural states.
Bronze
There are a limited number of major chemical changes that happen to bronze as it deteriorates, depending on age, soil conditions and a few other things that 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 and patination and encrustation interchangeably.
Pretty much all 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.
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 enumerated the different types of copper patina you are likely to encounter:
Dirt: Looks and acts pretty much like dirt.
Pretty:
Copper Oxide, Cupric Oxide, Cuprous Oxide: (red, brown or black) - 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.
Copper Sulfate or Sulfide (green to black)
Copper Carbonate (accounts for most green patina and occasional blue) Copper carbonate is a reaction to copper oxide, not to copper, so it will only form on top of the brown or red copper oxides. Because copper oxide is more stable than copper carbonate, the green can sometimes be selectively removed leaving the red or brown.
Copper Acetate (green, occurs frequently with copper carbonate)
Destructive:
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.
Reddish Warts - I'm not sure what this is, but its bad. This is scaly bumps usually 3-7mm high that form on 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 personally will not buy pieces with this symptom, as they tend to look bad and do not respond well to cleaning.
Silver
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 back 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 it's appearance.
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, and the degradation of this alloy at room temperature. Most silver alloys are what are called super saturated, meaning they contain more copper than is stable at room temperature. When the silver is cooled rapidly from high temperature the copper is trapped in solution and then precipitates out over time.
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 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.
Cleaning Techniques
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. You will 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.
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!
Water:
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. |
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