The metallurgical analysis in archaeology draws from the entire range of materials science methods of analysis, but several approaches have been particularly successful and therefore more widely adopted. These include methods which do not alter the physical integrity of the metal objects, such as neutron activation analysis (NAA), X-ray fluorescence analysis (XRF), electron Probe micro-analysis (EPMA) and proton-induced X-ray emission or gamma emission (PIXE or PIGE); those which require a sample to be removed and dissolved, such as atomic absorption analysis (AAS) and inductively coupled plasma either with optical emission spectrometry (ICP-OES) or mass spectrometry (ICP-MS); and those which remove a minute amount of material through evaporation or ablation, such as laser ablation ICP-MS, or secondary ion mass spectrometry (SIMS).
Ideally, the choice of analytical method and instrument is governed primarily by the particular research questions, moderated only by curatorial constraints. In reality, costs of analysis and ease of access to or availability of instruments often play a decisive role. For all methods it is imperative to monitor and report data quality (accuracy and precision) through publishing results for analysis of certified reference materials along with the unknown samples, in order to be able to compare data from different laboratories.
Main practical considerations are sample size and location. Archaeological metals are often heterogeneous with grain sizes up to several millimeters; intentional surface treatments such as gilding or patination, or unintentional alterations through corrosion or conservation treatments, can result in major differences in composition between the surface and the body of an object. Sampling and analytical method have to take account of this, allowing, for instance, for invasive sampling to characterize both the surface and the body, and to ensure sufficiently large volumes of material to be analyzed, either in situ or after removal of a sample from the object. These volumes can be much larger than normally offered by LA-ICP-MS or SIMS, which typically analyze volumes of a few tens of cubic micrometer only. A balance between the curatorial desire to minimize the sampling impact and the analytical need for a representative sample is sometimes difficult to achieve.
Identifying Composition and Condition
The analytical emphasis in identifying composition of nonferrous metals is on the main constituents down to about one half of one percent by weight. Components below that level have only a limited effect on the properties of the main metal, and their presence or absence will not have resulted in perceptible changes in properties that could be linked to archaeologically relevant alloy selection for particular types of objects or specific purposes. Correct alloy identification for categorization purposes or conservation treatment also does not require more detailed analyses. This type of information is easily available from a range of analytical techniques, including scanning electron microscopes with energy-dispersive spectrometry (SEM-EDS) and portable XRF instruments, and often without the need for invasive sampling. Surface analysis only requires a small area to be cleaned to expose fresh metal, and can be done in conjunction with normal conservation treatment. The nature and degree of corrosion is best studied by optical microscopy, ideally of a polished cross section, and in combination with X-ray diffraction and SEM-EDS analysis.
Analyzing iron and steel requires accurate determination of the carbon content at levels between 0.01% and 1% by weight. Few of the analytical instruments used in nonferrous metal analysis are capable of doing this, and methods established in industry normally require much larger and better preserved samples than typically available in archaeology. Here, optical metallography often is the most appropriate method.
Establishing Compositional Groups
Within the broader metal and alloy types, much information regarding distribution and circulation of metals between societies can be gained from a more detailed analysis of minor and trace elements. This enables forming chemically defined groups, informing on metal supply in a given society and adding an independent dimension to artifact groups defined by stylistic criteria. It requires comparing data from different laboratories and thus needs both precise and accurate data, typically using the more advanced methods of chemical analysis. Establishing compositional groups now often includes isotopic characterization and provenancing (see ahead), but remains an important line of enquiry on its own.
Reconstructing Metallurgical Practice
Chemical analysis of metal artifacts informs about alloying and refining practice, heterogeneity of complex objects (soldering, surface treatments, repairs, etc.), and possible corrosion effects. Analytical practice depends on the metal or alloy in question, and considers main components as well as trace elements and nonmetallic inclusions. The best analytical approach combines chemical and microscopic analysis to understand the spatial distribution of chemical components within an object. EPMA offers this combination in one instrument, but optical metallography can be combined with almost any chemical method. A major field in its own right is the analysis of metallurgical waste (slags, crucibles, furnace remains, etc.) to reconstruct smelting techniques (see Metals: Primary Production Studies of). The approach here is mainly based on geological methods.
Locating Geological Origin
Provenancing follows on from the formation of compositional groups and compares their chemical and, in particular, isotopic signature with ore deposits or production sites. The most widely used method is determining the isotope ratio of the four stable lead isotopes in a series of artifacts, and comparing it with the isotope ratios of known ore fields or smelting sites. The minute lead content present in most metals is sufficient to determine these ratios, enabling lead isotope analysis (LIA) for almost all metals and alloys. A mismatch between a potential source field and an object group demonstrates that that object does not come from that source, provided the source characterization is complete and the metallurgical history of the metal object does not include addition of alien lead for alloying or refining purposes. A match between source field and artifact is not on its own proof of origin, but requires further supporting evidence such as contemporary mining activity at and cultural contact with the source region. The lead isotope ratio of a source is determined by its geological history; thus, any source with the same geological history will have the same isotope signature, resulting in widespread overlap between contemporary ore sources. Provenancing by LIA relies equally on high-precision and high-accuracy measurement of isotopic ratios, using thermal ionization mass spectrometry (TIMS) or multi-collector ICP-MS with appropriate analytical protocols, and the availability of substantial comparative geological data even for remote and very small ore sources. The chemical composition of smelted metals does differ significantly from the composition of the ore, and enables only a broad discussion of potential relationship between source regions and metal artifacts; this can contribute significantly to the discussion in the case of overlap in isotope ratios between various possible source fields.
Interpretation of Data
Any data interpretation needs discussion of analytical error, accuracy, and precision. In addition, the interpretation of the chemical composition of archaeological metal artifacts requires an understanding of the effects of the different metallurgical processes involved in their production. Smelting, refining, alloying, and recycling or corrosion all influence the chemical composition of the metal; identifying their respective signatures and effects as the result of the production history of an object is at the core of metallurgical analysis. Understanding which signatures and effects are determined by natural processes and governed by the laws of physics eventually enables identification of those which are due to human activity and choice. Bringing these into the open is the ultimate purpose of metallurgical analysis in archaeology.
See also: Metals: Primary Production Studies of; Neutron Activation Analysis.
World History
