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Standards, Measurements and Testing
Nr. 19 - June-July '98

The secrets of alloys

Archaeologists too are interested in reference materials. By analysing thousands of objects from the Bronze Age, Iron Age and Roman times we can determine the composition of the alloys used by these civilisations. The samples developed on the basis of this information can then make it possible to increase our knowledge of these ancient objects and improve the methods used to conserve them.



How, in the major civilisations of the past which practised metal-working techniques, did the craftsmen prepare their alloys? This technical question yields historical answers which tell us a great deal about the know-how of lost civilisations and the transfer of skills within and between these societies. "The proportions of the various components vary widely, according to the production methods used - depending on whether the metal was simply cast or subsequently forged - and the desired aesthetic or mechanical result. Archaeologists are trying to determine the empirical know-how of these antique metal-workers," explains Philippe Brunella, a researcher at the European Archaeological Park of Bliesbruck-Reinheim (France/Germany).
The experts have long been fascinated by this question. Starting in the 1960s, spectrometry and microscopic observations provided them with the beginnings of an answer. But these analyses are incomplete and not very systematic. "The methods used are not clear, the nature of the samples - such as whether or not it is a modified material - is not specified, and the archaeological context is not given. This is why scientists felt the need to obtain certified reference materials," explains Michel Wuttmann, a researcher at the Institut Français d'Archéologie Orientale du Caire (IFAO).

Analysing the analyses


The IFAO is coordinating the IMMACO (Improvement of Means of Measurement on Archaeological Copper-Alloys for Characterization and Conservation) project, supported by the Standards, Measurements and Testing programme, on which seven partners from four European countries are working (1). Their aim is to establish reference standards which are representative of all the objects handed down from history's major metal-working civilisations.

But what must be certified? "It was necessary to determine both the number of materials and their composition in order to cover all the alloys produced between the time when metal-working first appeared and the end of Roman times." (Michel Wuttmann). This first phase of the project (1996-1999) is being conducted by the IFAO and researchers from the EDF-Valectra laboratory (2). "A vast bibliographical study allowed us to examine all the analyses made on copper-based metal objects from three periods - the Bronze Age, the Iron Age, and Roman times - and four countries - France, Germany, Greece and Italy. The research led to the selection of 3596 results of recent chemical or physio-chemical analyses. This statistical study allowed us to draw up, for each of the countries, tables giving the compositions of alloys for the purposes of standardisation," explains No‘l Lacoudre, manager of the EDF-Valectra laboratory. It was this laboratory which carried out a "physical" analysis of 15 objects from the Gallo-Roman site of Bliesbruck-Reinheim. "These were items which it was acceptable to sacrifice. There were cut up in order analyse the metal composition and corrosion products" (Philippe Brunella).

From Europe to Egypt


The IFAO also carried out other analyses of Egyptian objects from the Old and New Kingdoms. These analyses increased the geographical and historical range covered, and thus the potential users of the certified materials. "Metal-working techniques spread widely during the Bronze Age, in particular in the eastern Mediterranean, and a great many Egyptian objects are to be found in European museums. Archaeological research into these civilisations is also a field in which European institutions have been involved since the very beginning," (Michel Wuttman). The analysis of these objects supported the choice of five copper alloys made by the Valectra laboratory and proposed for certification.

On the basis of this information, a team from the Institute of Reference Measures and Materials (IRMM) at the Joint Research Centre in Geel (Belgium) was charged with manufacturing samples which would subsequently serve as certified materials. Chris Ingelbrecht, project manager at the IRMM, explains that the "principal difficulty lay in the fact that these alloys no longer exist today. For example, arsenic, a dangerous substance, is no longer used, so we developed an innovative system to reintroduce elements and reconstitute these materials."

Measuring, conserving and restoring

In addition to certifying reference materials, the IMMACO project also compiled a genuine guide to "good analytical practices". This tool will permit systematic composition analyses which can also be carried out at archaeological sites using portable instruments. "Archaeologists will thus be able to gather a very large number of more reliable measurements. This will give us correct statistics on alloy composition and at that point we will be speaking the same language," (Philippe Brunella).

Another far from insignificant result of the painstaking work of standardisation will be to provide conservation and restoration experts with new precision tools. Certain chemical or electrochemical methods used to repair the effects of corrosion are disputed due to the lack of control over the reactions produced on the surface of the object.

"Part of the project is devoted to the production of patina on reference materials, based on the study of the patina found on archaeological objects. Different methods can be tested which would be impossible to carry out on the original items," explains No‘l Lacoudre. "This processing will answer a number of questions concerning conservation, in particular the problem of the migration of various elements from the metal to the periphery. The IMMACO project not only permits an increased knowledge in the field of archaeology, but also aims to improve the conservation of an important heritage."

(1) Laboratoire de restauration de l'Institut Français d'Archéologie Orientale - Le Caire (France), Laboratoire EDF-VALECTRA (France), Laboratoire d'Etudes de la Corrosion de l'Ecole Nationale Supérieure de Chimie de Paris (France), Universitaire Instelling Antwerpen - MITAC (Belgium), Technische UniversitŠt Wien (Austria), Institute for Reference Materials and Measurements of the Joint Research Centre of the European Community (Geel/Belgium) and, as an associated partner, the SEK - EuropŠischer Kulturpark Bliesbruck-Reinheim (Germany).

(2) Belonging to Electricité de France (EDF), this laboratory is specialised in the conservation and preservation of ancient metal objects and is a frequent patron of scientific and technical project



The metal "road"

Copper was worked a long time before gold. The earliest objects, produced by cold-hammering techniques, were found in Anatolia, in western Iran, and date back to the 9th millennium BC, but it is another 2,000 years before we see the beginning of metallurgical techniques which require ovens reaching a temperature of over 1000°C. In around 6,500 BC, centres appeared in Anatolia and Mesopotamia. In around 4,500 BC, copper objects were produced in the Balkans, the technique later spreading west through the Mediterranean and along the Danube valley.
The progress in metal-working techniques came from the development of alloys. When another component is added to copper, the metal becomes more resistant and is also easier to shape or to melt at a lower temperature. Arsenic was the first ingredient to be added. This was later replaced by tin, to produce bronze. The Bronze Age thus began in France in the year 2000 BC. Bronze allowed high quality weapons to be produced and the craftsmen were employed in the service of warriors. In the first millennium BC, when it seemed to be difficult to obtain copper and tin, lead was used to produce alloys. This led to the Iron Age, which appeared in Europe around 1 200 BC and reached its apogee during the Hallstatt civilisation (Austria) in around 700 BC before spreading to Central and Western Europe.
It is this iron civilisation which was conquered by the Romans. Iron became the metal used in making weapons and tools, while bronze continued to be used for more refined objects and jewellery.

 


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