Restoration of Bronze

Cleaning and Treatment of Ancient Metal Artifacts and CoinsPart - 1 - OneIntroduction:Why does ancient metal deteriorate and what is that green and blackstuff that covers artifacts? The metals from which most artifacts aremade are not naturally occurring in the ground. Instead they arealloys, or combinations of several different refined metals meltedtogether and mixed up to create a new metal with certain usefulproperties such as low melting temperature, resistance to corrosion, orflexibility. For Example: Bronze is generally something like 85% copperand 15% tin melted together and thus combined.So, the metals from which artifacts have been made have been altered intwo ways. First by refining, or using heat to make pure metal frommetal ores, and second by alloying, or combining refined metals, againwith heat, to create new metals with improved useful properties.Metals in nature, the way they are found in the ground, are generallyfairly stable. Malachite, the gemstone, is for instance, a stable formof copper found in nature. It has naturally combined with things in theenvironment to create a substance that looks almost nothing like themetal copper, yet it is made of more than 70% copper and can be refinedto create metallic copper. When the metal copper, which is not stable,is returned to the earth, it will unrefine itself, slowly, recombiningnaturally with elements in the soil, and the result, within a fewhundred years will be a layer of malachite and other related mineralson the surface of the metal. This is the type of deterioration known asverdi-gris. (Another example of this is the black or gray tarnish thatyou see on silver items. This is silver combining with sulfur in theenvironment, 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 intopure metals that must eventually, at normal temperatures, combine withelements in their environment and return to their more stable naturalstates. This process can take hundreds or even thousands of years, andis what we know as patination, verdi-gris, corrosion, and the otherproperties of aged metal.The second important thing that happens as metals age is that, thosewhich are alloyed, or made of a combination of two or more metals, mayseparate slowly into their components. An example of this is ancientsilver coins which become brittle. Silver used in coins is almostalways a combination of silver with about 1.5 to 15% copper. Adding alittle bit of copper to silver makes the normally soft silver harder,and more resistant to wearing down. Silver and copper don't really mixall that well, however, and over time (300-500 years or more), atnormal temperatures, the copper will sometimes begin to again separateitself from the silver. The technical name for this is precipitation ofcopper at the grain boundaries, which means copper coming out of thealloy at the edges of the natural crystals of the metal. This is knownas crystallization of the metal, to coin collectors, all though it isreally just the crystals of the metal becoming visible as the coppercomes out of the alloy and begins to corrode, thus weakening the metal.To clarify this point, some alloys are not stable, and, over hundredsor thousands of years, they will begin to separate back into morestable natural states.BronzeThere are a limited number of major chemical changes that happen tobronze as it deteriorates, depending on age, soil conditions and a fewother things that are collectively referred to as 'the conditions ofpreservation.' The visible results of the changes in the bronze arecollectively referred to as the patination, patina, encrustation, orverdigris. I will use the terms patina and patination and encrustationinterchangeably.Pretty much all of the changes that occur in bronze over time are theresult of interactions of the copper in the alloy with the environment.The tin is relatively inert and is stable in alloy with copper. Thatis, 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 arenot. You will need to use your own judgement in this regard, payingattention to esthetic, and the potential destructiveness of theparticular elements present. Bear in mind that most artifacts and coinshave multiple of the following reactions occurring simultaneously ontheir surfaces, and that, cumulatively, they are the irreplaceablesignature of the ages.Pretty: Copper Oxide, Cupric Oxide, Cuprous Oxide: (red, brown orblack) - Cuprous oxide is generally reddish in color and tends to formfirst. It quickly converts to cupric oxide which is dark brown or blackin color. Virtually all ancient bronze coins have at least a thin layerof brown copper oxide directly on the metal surface. CopperSulfate or Sulfide (green to black) Copper Carbonate (accountsfor most green patina and occasional blue) Copper carbonate is areaction to copper oxide, not to copper, so it will only form on top ofthe brown or red copper oxides. Because copper oxide is more stablethan copper carbonate, the green can sometimes be selectively removedleaving the red or brown. Copper Acetate (green, occurs frequently withcopper carbonate)Destructive: Cuprous Chloride and Cupric Chloride - (pale green powderyspots on the surface of a coin or artifact) While there are generally anumber of reactions occurring on the surface of a coin at any giventime, the presence of chloride ions is the most destructive, since theyproduce hydrochloric acids which eat your coin thus producing moreCuprous Chlorides to... etc. until there is no artifact.Redish Warts: - I'm not sure what this is, but its bad. This is scalybumps usually 3-7mm high that form on the metal. It can be removed butis extremely destructive, leaving large pits and destroying mostdetails. Bronze that shows this type of degradation is frequentlyunstable and soft even in the areas not directly affected by the wartyencrustations themselves. I personally will not buy pieces with thissymptom as they tend to look bad and do not respond well to cleaning.SilverThere are really only 3 major chemical changes that you will facedealing with ancient silver. Silver combines with sulfur, and chlorineand, less readily, with nitrate ions and oxygen, mostly resulting ingray to back patinas. The first two important chemical reactions are ofthe silver itself with it's environment, and the third, brittleness, isthe result of the copper which is almost always present in silveralloys.Silver Sulfide (gray to black) This is the commonly known silvertarnish. It occurs rapidly and consistently to nearly all silverartifacts.Silver Chloride (horn silver) Horn silver is present on many ancientsilver coins. It is relatively soft, and can be difficult to remove dueto its tendency to smear and obscure features of the coin. Horn Silveris the combination of part of the silver in the coin or artifact withchlorine to form silver chloride. It has a somewhat purple-ish tosilvery-yellow in appearance, and projects slightly from the surface ofthe artifact or coin, affecting it's appearance.Embrittlement - Silver can only stabily contain about 1% copper at roomtemperature. This amount increases as the silver is heated, thus theuse of heat to combine silver and copper in alloy, and the degradationof this alloy at room temperature. Most silver alloys are what iscalled super saturated, meaning they contain more copper than is stableat room temperature. When the silver is cooled rapidly from hightemperature the copper is trapped in solution and then precipitates outover time.Embrittlement, as stated previously, is the condition commonly know tocollectors as crystallization. Embrittlement is primarily the result ofinter-crystalline corrosion at the microscopic level due to theselective precipitation of copper from the silver alloy at the crystalboundaries. Discontinuous precipitation of copper may soon provide anew method for the dating and authentication of silver artifacts.Discontinuous precipitation occurs primarily in silver alloys with a1.5% - 10% copper content, and shows up as a crystalline or jig-sawgrain pattern on the surface of the artifact. Higher copper contentsilver alloys are resistant to this phenomenon.End Part - 1 - continued in Part - 2 -Archeology Pty Ltd, does not accept any liability orresponsibility for any outcome or end result achieved or attained byusing or following any or all directions contained in this eBay guide.This information is provided on the basis of good faith and use of anyor all information is completely at the readers/users risk. Remember,this is a general guideline and should not be applied without testingand adequate safety precautions.