The general characteristics of metals are that:
They are ductile-they can be deformed easily
They transmit heat and carry an electrical current
They can be melted and cast into a shape
They can be melted, sprayed , electrically plated or rolled onto each other to create sandwich-like coatings, such as tinning, silver plating and gilding
They can be mechanically joined to each other by folding or riveting, and they can also be joined together by soldering.
Two or more metals can be blended together to alter the properties of a given metal creating an alloy. For example, copper is often found alloyed with silver to make the silver less ductile.
Individual metals have individual characteristics that are important in their identification. For example, iron is magnetic and lead is unusually dense.
The condition of archaeological metal is complex and varies widely, depending on the nature of the metal or alloy and the conditions in the burial environment. Metals are rarely found in a pure metallic state. They are most often encountered naturally in the form of ores (metal ions that are bonded with other materials such as oxygen, water, sulphur containing compounds or hydroxides). Metals are extracted from ores by a process known as smelting in which heat is used to break down chemical bonds. The newly produced metal is not very stable. It will begin reacting with oxygen, water and other environmental contaminants almost immediately. This process (the reversion of the metal form to the more stable ore form) is known as corrosion. Different metals corrode at different rates. Some metals, known as base metals, corrode more rapidly than others, known as noble metals. Corrosion products come in many forms and colors. They can occur as a thin, distinct layer or patina, which is usually stable. They may also occur as voluminous, hard, or powdery accretions and can be associated with pitting and surface loss. Rough and uneven corrosion layers on archaeological metals are not necessarily indicative of active corrosion but areas of flaking and cracking often are. Archaeological metals are often quite brittle as a result of corrosion. Oxygen, chlorides, sulphides and acidic conditions are all damaging to archaeological metal and its alloys. The degree of preservation is dependent on the amount and the combination of these variables in the burial environment. Changes that occur during the corrosion process often alter and obscure the original surfaces of the artifact. However, original surface details, applied materials (such as silvering or gilding) and organic remains (or pseudomorphs of organic materials) may be preserved within the corrosion layers. It is important to take this into consideration when developing a treatment approach so that important information within the artifact is preserved.
X-Radiograph and image by H. Wellman. Used by permission of the Maryland Archaeological Conservation Laboratory
Careful examination and/or analysis of the corrosion products can reveal important information about the burial environment, the nature of the artifact, and its state of preservation. X-Radiography can be a very useful tool in identifying technological information and the condition of metal. Careful investigative cleaning and microscopic examination are important tools used to aid in understanding the nature of the artifact and its state of deterioration. These techniques may also reveal the presence of surface detail and preserved organic remains within corrosion layers. Understanding the complex relationship of these factors will affect conservation treatment decisions.
Treatment methods are usually determined by the stability of the artifact and the end goal of treating the artifact. Decisions are based on the analytical value of the artifact and its relationship to the assemblage as a whole. Treatment decisions are also based on whether the artifact will be placed in storage and what the storage conditions will be. Other considerations to take into account are whether the artifact will be part of a study collection or whether it will go on display. All of these factors should be part of an ongoing collaboration between archaeologists, conservators and curators or other museum professionals.
The primary components of metals treatments are usually examination, corrosion removal, desalination, the application of corrosion inhibitors and the application of moisture barriers. Corrosion removal tends to be an aesthetic decision as much as it is a research tool. There are two approaches that are often taken. Each has pitfalls. One approach is to remove all the corrosion layers and attempt to stabilize the remaining metal. This can be problematic because valuable information is often preserved in the corrosion layers and is lost in this technique. Another is to maintain and stabilize the large part of the corrosion layers while using limited corrosion cleaning to enhance the appearance of the piece and reveal information trapped in the corrosion layers. With either approach, corrosion removal must be controlled. Corrosion removal is often done using mechanical (i.e. with hand tools), chemical or electrolytic methods. Chemical and electrolytic methods can be more intrusive and can potentially change the character of the metal and result in the loss of important archaeological and technological information. Mechanical methods may also be damaging if a metal is soft. Depending on the stability of the artifact, metal artifacts may be desalinated, treated with a corrosion inhibitor , and/or given a protective coating . Desalination is necessary for all iron excavated in the Mid-Atlantic area but is not always necessary for other metals, unless they come from marine environments. Metals may also be stabilized using appropriate preventive measures, such as storage in controlled environmental conditions with appropriate enclosures.
All metals should be stored using archival quality materials. It is important to avoid using acidic materials. Generally, archaeological metals should be stored in low RH conditions (see How do I store different types of excavated materials? and How do I create a desiccated microclimate for storage?). Actively corroding metal should be stored in RH below 35%. Iron artifacts should be stored in an RH of 20% or lower. Artifacts should be kept in closed containers or spaces with a desiccant to control the RH and to limit accumulation of dust on the surface of the metal.
How do you assess an iron artifact and decide on its treatment?
Archaeological iron is perhaps one of the most difficult materials to treat due to the size and number of artifacts made with this material, as well as to the peculiarities of the material itself. Iron corrodes (rusts) when it is exposed to water and oxygen. This process is catalyzed by the presence of salts. During burial, iron objects are changed partly or entirely to corrosion products, which can incorporate other materials and obscure the object’s original appearance. Significant traces of the original surface may lie within this corrosion crust, though they can be more difficult to reveal than with other metals. The purpose of treatment is to prevent further deterioration and to reveal technological information. Iron treatments are usually assessed based on their ability to remove agents causing deterioration (in the case of iron, these are mainly chloride salts), their ability to reveal and preserve the technology of the artifact and their ability to protect the iron from further corrosion.
Prior to treatment a number of factors must be considered in order to determine the condition of the artifact. These include: determining how the piece was constructed, assessing whether it was made of cast or wrought iron, determining whether other materials were also employed in the construction, and assessing the degree of corrosion that has taken place on the artifact. X-radiography is a powerful technique for addressing these questions and most conservators employ it as a first step. X-radiography will show whether cracks or casting flaws, are present, it will give a good indication of how much of the core metal remains in the artifact, and will reveal applied materials such as gilding, paint, enamel or tinning that may be obscured by the corrosion layers. Because of the rapidity with which iron corrodes there is a high probability that organic materials may have become incorporated into the corrosion products. These materials often provide valuable information about the way in which the artifact was used, the materials buried with it, and the burial environment itself. The presence or absence of mineral preserved organics can only be assessed by careful examination of the surface of the artifact under low magnification. Occasionally this assessment must be combined with small amounts of corrosion removal. Another important component of iron conservation is determining the free chloride content of the artifact as chlorides contribute to most of the forms of “run away” corrosion of iron.
All these condition factors will help to determine how the object needs to be treated in order to stabilize it and preserve the information it contains. If mineral preserved organics or applied materials are present on the surface of the piece or if the object is in delicate condition the treatment that is adopted will most likely take the least intrusive and most passive approach possible. If, on the other hand, there are no applied materials or mineral preserved organics on the surface and the piece is in good condition overall, it may be possible to adopt a more interventive and/or aggressive approach.
What is electrolytic reduction and what are its risks and benefits?
Electrolytic reduction is a conservation treatment that can be used on some metal artifacts. It is commonly used on iron excavated from marine environments, but it has occasionally been used on other metals, including lead and silver under special circumstances. It should not be used on all metal artifacts, particularly highly corroded ones, as it has the potential to be a very damaging treatment if used incorrectly or inappropriately.
The corrosion of metals in an archaeological environment occurs as an electrochemical process. When dissimilar metals come into electrical contact in the archaeological deposit, less noble metals corrode preferentially to more noble metals. (This process also occurs on a microscopic level within metal alloys). Serving as the anode, the less noble metal donates electrons to more noble metal, or the cathode. This exchange of electrons is similar to the flow of electricity through a battery and can result in a variety of complex corrosion products.
Electrolytic reduction (ER) is a process that reverses the flow of electrons in the galvanic cell, ultimately converting corrosion products to a more stable or easily removed form. In short, the electrolytic reduction treatment involves establishing a galvanic cell in which the archaeological metal serves as the cathode, and a more noble metal serves as the electron donor, or anode. A positive rectified current is induced in the anode and a negative rectified current in the cathode while both are immersed in a conductive medium or electrolyte. A control panel allows manipulation of the current flow that the anode and cathode receive. Variables include the condition of the artifact, the electrolyte used, the anode metal chosen, the configuration of the anode relative to the artifact surface, the method of establishing electrical contact between the artifact and the anode, the intensity of current applied and the regularity of monitoring. Control over these variables is vital as electrolytic reduction has the potential to damage artifacts irreversibly if carried out in an uncontrolled manner.
During electrolytic reduction, a number of different processes occur:
Corrosion products are converted, or reduced, to a more stable form (such as magnetite in iron) or to a metallic form (in the case of lead and silver). If the metal artifact is highly corroded this conversion may result in the loss of surface detail, an undesirable consequence. Metallic products that are formed on the surface of the artifact are sometimes beneficial and aesthetic, at other times they can obscure surface details or can be somewhat unnatural looking (such as those formed when silver is electrolytically reduced).
Negatively charged chloride ions are attracted from the object to the anode, thus removing them from the artifact. Chloride ions are a particularly aggressive form of ion that contaminate many buried metal artifacts, especially those recovered from marine environments. If left within the artifact, chlorides can react with moisture and oxygen to form acidic byproducts, which cause further cyclic corrosion. Removal of chloride ions is often the main aim of an electrolytic reduction treatment.
Hydrogen gas in the form of bubbles is released at the surface of the metal core of the artifact, beneath the corrosion layers. This reaction can help to mechanically loosen marine encrustations on artifacts from marine sites. However, it can be very damaging to heavily corroded artifacts that have only residual core metal. To preserve the corrosion layers in such cases, the hydrogen bubbling must be carefully limited through control of the applied current (or an alternative treatment may be chosen).
Electrolytic reduction can be a useful tool in cleaning and stabilizing metals recovered from marine archaeological sites. Iron artifacts from marine environments contain extremely high chloride levels and often have surfaces that are too fragile to clean by other mechanical methods.
Electrolytic reduction is economically feasible and can be accomplished with minimal health and safety risks. However, the risks to the artifact must not be taken lightly. Electrolytic reduction can be an aggressive treatment and is not recommended before taking many considerations into account and weighing the alternatives. Before beginning an ER treatment of iron, it is important to determine the extent of the corrosion, the degree of chloride contamination, and the reason for removing the corrosion layers. ER is not recommended for highly corroded artifacts or for general removal of surface rusting. If not implemented properly, electrolytic reduction can cause structural damage, such as loss of surface details, and ruptures along existing internal flaws, and can generally leave a pitted, stripped metal surface. Thus, it is not recommended that electrolytic reduction be undertaken without careful consideration of all the factors involved. Consultation with a conservator is strongly advised.
Should I polish archaeological metals?
It is unnecessary and often highly undesirable to polish archaeological metals. Even modern or antique metals such as a silver service should not be regularly polished, as this action eventually wears down the surface, removing engravings, imprints, tool marks, and other details.
Metals that have been in a burial environment have changed considerably from their original condition, both chemically and physically. They have lost some or all of their original metal to corrosion. They have acquired corrosion products that may protect the object or provide information about the composition of the object or its burial environment. The corroded surface may preserve traces of organic materials that were originally part of the object, such as a wooden handle for a tool, or the leather scabbard of a sword. Therefore, quite often, it is best to leave corrosion products in place.
A conservator may remove corrosion and other deposits from the surface of a metal object in order to stabilize the object, expose surface details, or gain more technological information. Often, however, only a portion of the corrosion is removed. If a metallic surface is exposed, it may be weaker, pitted, and of a different shape and composition from the original metal (for example, enriched or depleted in a certain element) due to corrosion. It may contain faintly preserved tool marks or other traces of manufacture or use. Polishing (or any kind of aggressive cleaning) changes the character of the surface, causes loss of metal surface and original detail, and may leave chemical residues on the metal. It is not expected that an archaeological metal will have the appearance of a modern, shiny metal. The corroded appearance of an archaeological metal is part of its history, and should not be altered by polishing.
What is a corrosion inhibitor?
Typically a corrosion inhibitor is a chemical that reacts with the surface of a metal to create a protective coating. This coating is unlike the one formed by a plastic or wax, as it is formed by a chemical reaction between the inhibitor and the metal ions at the surface of the object. In other words the protective coating becomes part of the surface of the object.
Two corrosion inhibitors used commonly in archaeological conservation are benzotriazole (BTA) and tannic acid. Benzotriazole (BTA) has proven to be very successful for stabilizing copper alloys. Although BTA has been in use since the 1960’s, research is ongoing to find out exactly how this inhibitor actually works. The one disadvantage of using BTA is that it is a suspected carcinogen, so safety precautions should be taken when treating the object and handling it after treatment. BTA is also useful for stabilizing silver objects that have been alloyed with copper. Tannic acid is used to inhibit iron corrosion. Brushed onto the surfaces of iron artifacts, it inhibits corrosion through the formation of an insoluble iron tannate complex on the surface of the iron.
Canadian Conservation Institute (1997) “Tannic Acid Treatment.” CCI Notes 9/5, Canadian Conservation Institute: Ottawa.
Madsen, H. (1985) “Benzotriazole: A Perspective.” UKIC Occasional Papers No. 4- Corrosion Inhibitors in Conservation: pp. 19-20. Merk, L. (1981) “The Effectiveness of Benzotriazole in the Inhibition of the Corrosive Behavior of Stripping Reagents on Bronzes.” Studies in Conservation 26:73-76.
Skerry, B. (1985) “How Corrosion Inhibitors Work.” UKIC Occasional Papers No. 4- Corrosion Inhibitors in Conservation: pp. 5-12. Turgoose, S (1985) “Corrosion Inhibitors for Conservation.” UKIC Occasional Papers No. 4-Corrosion Inhibitors in Conservation: pp. 13-17.
What is a moisture barrier?
Moisture barriers are coatings that are applied to some conserved metal objects to protect the surface from water vapor. Additionally, they usually offer some protection against the deleterious effects of dust and air pollution. Examples of moisture barriers are waxes (such as microcrystalline wax and carnuba wax) and acrylic resins (such as Acryloid B-44, the main resin component of Incralac lacquer, and Acryloid B-72). Waxes are less efficient at protecting an object from moisture vapor and some research indicates that in humid regions such as the Mid-Atlantic, waxes may fail (resulting in pin prick corrosion) up to four times more often than acrylic resins, when all other aspects of the treatment were identical. Use of a moisture barrier with archaeological metal does not negate the need to provide low humidity storage, since even the best coating is slightly permeable. When using a moisture barrier with archaeological iron in particular, one should always store the object in a low humidity environment in order to maximize the efficiency of the coating.
Almost all coatings fail over time and will require reapplication at some point. The failure rate will depend on a number of factors including the way in which the coating was originally applied, the amount of light the coating receives, the amount of moisture in the environment, and whether the coating is abraded by handling or other wear. Additionally, the efficacy of the moisture barrier depends in large part on the quality of its application-small pinpricks or gaps in the coating or areas of excessive pooling will cause the coatings to fail faster. A visibly failing coating should be removed, as it can cause further problems by setting up differential corrosion cells through the presence of areas of exposed surface next to areas of coated surface.
The application of a moisture barrier is not a substitute for necessary conservation treatment; a coating applied to an untreated metal artifact will almost certainly fail and thereby exacerbate corrosion problems.
Johnson, R. (1984) “The Removal of Microcrystalline Wax from Archaeological Ironwork.” Adhesives and Consolidants IIC: London. pp. 107-109.
Keene, S (1984) “The Performance of Coatings and Consolidants Used for Archaeological Iron.” Adhesives and Consolidants. IIC: London pp. 104-106.
Pascoe, M. (1982) “Organic Coatings for Iron: A Review in Methods.” Conservation of Iron: National Maritime Museum Monograph, No. 53:56-7.
Williams, E. (2002) “Conservation Assessment of the Archaeological Collection at Colonial Williamsburg.” Journal of Middle Atlantic Archaeology 18: p97-103
Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm