A METALLOGRAPHIC INVESTIGATION OF ANCIENT COOKING UTENSILS

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1 A METALLOGRAPHIC INVESTIGATION OF ANCIENT COOKING UTENSILS D. Kountouras, S. Papanikolaou, P. Serdaris, S. Maropoulos Mechanical Engineering Department, Technical Educational Institute of Western Macedonia, Kozani, Greece Abstract A metallographic study of ancient metal finds dating from the 15 th to the 12 th century BC is presented in this investigation. The samples studied come from ancient cooking utensils found in Macedonia, Greece. Their preparation for the metallographic examination combined the classic methods of metallographic sample preparation with special methods of cleaning and protection used for archeological samples. The finds were made of a copper alloy of a composition close to the one used nowadays (Cu-11,3%Sn) but with a considerable percentage of arsenic (As), coming from the ore used, which with a dispersion mechanism during everyday use of cooking utensils contaminated the food and as a result the people consuming it. This may explain the high mortality rate observed in the area at the period discussed which resulted in the use of ceramic cooking utensils and the use of the specific copper alloy only for weapons and artifacts other than utensils. Key words: copper alloys, bronze, boiler, archeological corrosion Introduction The samples studied are metal fragments coming from archeological excavations in Macedonia, Greece. They are the remains from cooking utensils dating to the Late Bronze Age, 15 th - 12 th century BC. The material is a copper tin alloy. A comparison with other bronze items found in the excavation showed that the specific alloy was widely used. It was produced locally and various items for different uses were made from it such as basins, plates, shields, helmets, pots and boilers. The pieces of metal sheet studied were parts of a bronze boiler which was found buried underground. The soil due to its chemical composition and the intense presence of moisture acts as a corrosive towards metal and alloys buried in it [1]. More specifically the original boiler bronze sheet corroded and with time was split into very thin sheets. It was not possible to assemble these so as to construct the boiler to its original form. Experime ntal Cleaning Procedure-Optical Stereoscope The as received pieces of metal sheet are shown in Figure 1. They were all covered with three consecutive layers of material. The surface layer consisting of soil, clay, plant roots and small pebbles, the middle layer consisting of copper oxides that had turned into ore and the inner layer which was pure copper oxides. 207

2 Figure 1: The pieces of metal sheet as received The first procedure involved cleaning of the metal samples. With the use of an optical stereoscope (x25) the outer layer of large particles of soil-c lay-roots-stones was removed. The samples were further cleaned with an ultra sound cleaning apparatus using an ethyl alcohol distilled water solution (50% vol.). The solution temperature was 40 C and cleaning lasted 120 minutes. They were then dried and cleaned using tweezers and scalpels followed by a further clean in the ultra sound cleaning apparatus. It was observed that the material removed during cleaning had a vivid green colour which is characteristic of copper oxides. The samples were photographed using the stereoscope, their dimensions were measured, weighed and the atmospheric conditions were recorded. The measurements are given in Table 1. Table 1: Sample measurements sample length width thickness weight (gr) temperature ( C) humidity % Optical microscopy The thickest of the samples were moulded in Bakelite using a specimen moulding press. They were then ground with 1000 grid grinding paper at 250 rpm. This removed any remaining nonmetallic materials. They were then polished with alumina to 1μm surface finish. Etching was carried out using a special agent commonly used for ancient metal objects consisting of 120 ml ethyl alcohol, 30 ml HCl and 10 gr FeCl 3 for 3 sec. Several micrographs were taken using the digital camera of the optical microscope. Scanning electron microscopy (SEM) The preparation of samples for scanning electron microscopy was the same as that for optical microscopy but they were then covered with a thin film of graphite for better observation. Chemical analysis of the samples at different distances from the inner surface of the boiler metal sheet was carried out using the x-ray diffraction facility of the SEM. Results and Discussion 208

3 Optical examination with the stereoscope, Figure 2, showed (a) the soil matter attached to the surface, (b) the layer of oxides covering the entire surface of the metal sheet. The arrows in arrows in Figure 2 (b) show the oxides that have turned into ore. Figure 2 (c) shows a characteristic detail where the sheet has a folded form, (d) the metal core, (e) a crack extending across the entire sample and (f) the beginning of a crack. (a) (b) (c) (d) (e) Figure 2. Optical stereoscope photographs (f) It was evident that the second layer of copper oxides turned into ore and when this detaches itself drags with it, due to intense bonding, the surface layer of oxides thus creating micro-cracks leading to local micro-destruction of the surface. A detailed study of the microstructure under an optical microscope and a comparison with optical micrographs of archeological alloys of the same period and the specific region [1, 2] showed that the material is a typical copper-tin alloy. Figure 3 clearly shows the metal grains, grain boundaries and the typical grain size. Bands, characteristic of a tin containing microstructure, are also observed within some of the grains. Two alloy phases are present, phase α (dark zones) and phase ε (light zones). The light coloured zones contain grains of a fine grain size exhibiting increased hardness whereas the dark zones are microstructural areas where copper is dominant and exhibit a lower hardness [3]. In addition, there are numerous pores in the structure as well as dark spots 209

4 indicative of the oxides caused by corrosion [4]. A very interesting observation is the intergranular corrosion caused by the environment. (a) (b) (c) (d) Figure 3: Optical micrographs of the alloy structure (a), (b), (c) (x400) και (d) (x500). Scanning electron microscopy was used in order to further study the microstructure and determine the chemical composition of the metal sheet at various distances from the inner surface of the boiler sheet, Figure 4. The chemical analysis results are given in Table 2 Figure 4. SEM chemical analysis at different points. 210

5 Table 2. Chemical analysis results Sptm O Cu As Sn Total The different metal weight percentages in the copper alloy are given in Table 3. A considerable percentage of arsenic is present in the alloy. A closer look at the chemical analysis results showed that the amount of arsenic decreased toward the inner surface of the boiler indicating that through a dissemination mechanism some As was lost during cooking with resulting health hazards long proven. Table 3. Metal weight % in the alloy Conclusions metal weight % dominant value Cu ,5 Sn ,3 As ,2 1. The ancient boiler material was a Cu-11.3%Sn alloy widely used at the time for different purposes. 2. All samples were heavily corroded with a metal core remaining. Corrosion is uniform and more intense intergranularly. 3. The alloy contains a small amount of As the percentage of which appears to be decreasing toward the inner surface of the boiler with detrimental consequences to the health. References [1] ASM Handbook, Volume 9, Metallography and Microstructures [2] Metallography and Microstructure in Ancient and Historic Metals, DA VID A. SCOTT. [3] Cartechini L, R Arletti, R Rinaldi,WKockelmann, S Giovannini and A Cardarell (2005). Neutron scattering material analysis of Bronze Age metal artefacts, i, JOURNAL OF PHYSICS: CONDENSED MATTER. [4] Angelini E., A. Batmaz, A. Cilingiroglu, S. Grassini, G. M. Ingoc and C. Riccuccic, (2010). Tailored analytical strategies for the investigation of metallic artefacts from the Ayanis Fortress in Turkey, Wiley Interscience. 211

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