They may protect the underlying metal or they may contain salts that will cause further corrosion after the object has been excavated (for more information on corrosion, see CCI Notes 9/1 Recognizing Active Corrosion). Corrosion products can form as thin, coherent layers or thick, disfiguring crusts which obscure the details of the object. white metal on a copper cooking pan is most likely tin. Identifying the object will provide clues to the type of plating, e.g.
White metal is particularly difficult to identify in the field tin, zinc, silver, chromium, and nickel have all been used as plating at different times in history. For example, a highly corroded, broken coin from a 16th-century site in Canada was not positively identified as a coin until after it was identified as silver. In some instances the situation is reversed, and identifying the metal helps to identify the object. mortar bombs, grenades) that are intended to shatter, or those for which brittleness is not a problem (e.g. Very tough, resilient objects such as sword blades and axe heads are forged from steel and wrought iron objects such as hollow shot (e.g. For example, cast and wrought iron can be difficult to distinguish, but knowing the purpose of the artifact can assist in identifying which type of iron is present. Therefore, identifying an artifact and knowing its function can assist in identifying the metal(s) present. The properties of a metal, such as hardness, resiliency, ease of working, weight, chemical reactivity, melting point, and appearance, determine how it is used. Simple tools such as a magnet and a source of low-power magnification will help in conducting these examinations. Detailed elemental analysis can then be done in a laboratory. Although it is not likely that specific alloys can be distinguished in a field situation, it is usually possible to sort artifacts according to type of alloy. Most metals can be identified to some degree using the following observations. a silvery metal found in a well-sealed deposit on a 16th-century site should not be aluminum (and if analysis proved that it was aluminum, this would be an indication that the site had been disturbed).
The probability of finding a particular metal or alloy on a particular site depends on the cultural context of that site, e.g. Precious metals such as silver and gold may be found as plating on a base metal, or as coins and jewellery. The most common metals found on archaeology sites are cast and wrought iron copper and its alloys (brass, bronze) lead, tin, and their alloys (pewter, Britannia metal) and zinc (either in the form of plating or alloyed with copper or lead).
#Green patina copper series
This Note describes a series of characteristics that can be evaluated to distinguish different metals. Knowledge of the characteristics of various metals, when they were produced, and how they were used will help in identifying them (consult "Bibliography" for good sources of information). Complicating the problem is the fact that most metal objects are composites of more than one type of metal, each type contributing its unique character to the whole. The bits that remain are the materials that have best survived in the unique chemistry of a particular site. Not only are most of them fragmentary, but burial alters their composition. Unfortunately it can be very difficult to determine the composition of archaeological artifacts. Identification of the component materials is also the first step in proposing a conservation treatment or reventive conservation measures. As composition of an artifact is always related to its function, this information is fundamental to archaeological research. "What is this made of?" is one of the first and most frequent questions when an artifact is found.
#Green patina copper pdf
( PDF Version, 161 KB) CCI Note 4/1 is part of CCI Notes Series 4 (Archaeological and Field Conservation) Introduction
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