Cleaning and Treatment of Ancient Metal Artifacts and Coins
Part - 1 - One
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 verdi-gris. (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, verdi-gris, 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.
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.
Following are enumerated the different types of copper patina you are likely to encounter:
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 judgement 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.
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.
Redish 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.
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 it's environment, and the third, brittleness, is the result of the copper which 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 stabily 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 is 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 jig-saw grain pattern on the surface of the artifact. Higher copper content silver alloys are resistant to this phenomenon.
End Part - 1 - continued in Part - 2 -
Archeology Pty Ltd, does not accept any liability or responsibility for any outcome or end result achieved or attained by using or following any or all directions contained in this eBay guide. This information is provided on the basis of good faith and use of any or all information is completely at the readers/users risk. Remember, this is a general guideline and should not be applied without testing and adequate safety precautions.
Restoration of Bronze & Silver Antiquities Part (1) One
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20 December 2005
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