archaeometallurgy

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Bastian Asmus 2012:

Medieval Copper Smelting in the Harz Mountains, Germany (= Montanregion Harz, Bd 10 [Hg. Christoph Bartels, Karl Heinrich Kaufhold und Rainer Slotta]), Bochum (Veröffentlichungen aus dem Deutschen Bergbau-Museum Bochum Nr 191), 2012, ISBN 13: 978-3-937203-63-8

395 pages, english, colour, Appendix for microscopic identification (reflected polarising lihjt microscopy) of slag phases, metal phases, ore minerals and their analysis by SEM-EDS, WDX and EPMA-WDS as well as XRF for bulk chemical characterisation.
44,- Euro

Medieval copper from the Harz

The work deals with the evaluation of a high medieval smelting site in the Harz, near the UNESCO World Heritage sites Rammelsberg and Goslar. With more than 1000 m² excavated it is to date the most extensive archaeometallurgical investigation of a smelting site in medieval central Europe. It is a site where the polymetallic Rammelsberg ore was smelted to produce copper, lead and silver. The site can be considered as a typical example of a high medieval smelting site in the Harz. Particular emphasis was placed on an interdisciplinary approach which drew upon historical, archaeological and material scientific sources.

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Abstract

The Rammelsberg deposit in the Harz mountains in Germany is among the largest metal deposits in the world and has been in continuous use for more than a millennium. There is much controversy as to the nature of the metals produced and the processes involved from the ores of this deposit. This thesis deals with the largest and most accurately excavated smelting site of the high medieval period in the Harz mountains called Huneberg and may be regarded as typical for region and period.
Traditionally historians connect the Rammelsberg with silver production, the mining historians, however point out that the deposit is too poor in silver and that copper was produced in the high medieval period. Modern economical literature classifies the Ram- melsberg as a lead-zinc deposit.
To contribute towards the understanding of these questions an archaeometric study of archaeometallurgic waste- and byproducts, such as slag, furnace lining, furnace wall, litharge and spilled metal drops was undertaken. It builds the base of the interpretation of the metallurgical activities that have taken place at the Huneberg and is contrasted with previous studies. It is suggested that copper, lead and silver in form of argentiferous lead were produced on site, using a complex multi-step process, taking full advantage of the numerous structural features of the site, e.g. the three furnaces present on the site. Successive smelting episodes produced black copper of increasing purity as well as a rather rich argentiferous lead. Because the site is similar to may other sites it is further suggested the mode of metal production at the Huneberg followed a more or less stringent set of recipes and traditions. The mass of 1600 kg slag recovered from the site suggest a copper production of some 600 kg or less, depending on the ore quality. Lead is thought to have been produced in similar quantities, which in turn would mean that the site produced also 1.4 kg of silver during its operation.

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Slag samples of a medieval (12th century) copper smelting furnace.
Top left: #21, XPL. Under crossed polarisers the zinc sulphide can be seen as tiny dendrites within the glassy matrix. The other phases are willemite (wlm) and copper (Cu), which is usually asso ciated with the (Zn,Fe,Cu)S phase, an accompanying phase of copper matte at the Huneberg, from which it probably segregates upon solidification. Free iron oxides are seen as dark and much larger dendrites.
Top right: Another section of #21, under plain polarised light. It shows the abundant spinels, the iron oxides and copper inclusions. Most notably it shows the free baryte (brt) inclusion in the slag matrix.
Bottom left: #22, shows more free baryte (brt), together with a lead antimony matte. The long needles are a mixed iron and zinc oxides and are believed to be franklinite (frk?). #104, is rather more affected by corrosion than the other two samples. It shows mainly spinel (sp), free iron oxides and some willemite (wlm). Spinels are in the same order of magnitude than in the other samples.

The colloquial term Schlackologie is used by several German colleagues in the archaeometric sciences when it comes to the analysis and interpretation of mostly metallurgical slags. Literally it would translate to slagology. But why are slags investigated at all and what exactly is slag?

What are slags and what do they consist of?

Slags are by-products of metallurgical or pyro-technological processes and usually remain at the production site as waste product. For example metallurgical smelting slag is usually composed of five separate components:

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