A LATE BRONZE AGE STONE MOULD FOR FLAT ...

A LATE BRONZE AGE STONE MOULD FOR FLAT AXES FROM CASARÃO DA MESQUITA 3 (SÃO MANÇOS, ÉVORA, PORTUGAL) António M. M. Soares – Instituto Tecnológico e Nuclear, Sacavém, Portugal Pedro Valério – Instituto Tecnológico e Nuclear, Sacavém, Portugal José C. Frade – Instituto Português de Conservação e Restauro, Lisboa, Portugal Maria J. Oliveira – Instituto Português de Conservação e Restauro, Lisboa, Portugal Diana Patoilo – Instituto Português de Conservação e Restauro, Lisboa, Portugal Isabel Ribeiro – Instituto Português de Conservação e Restauro, Lisboa, Portugal Luis Arez – ARQUEOHOJE, Viseu, Portugal Filipe J. C. Santos - ARQUEOHOJE, Viseu, Portugal M. Fátima Araújo – Instituto Tecnológico e Nuclear, Sacavém, Portugal

ABSTRACT A fragment of a stone mould for casting flat axes was found into one of the more than forty pits excavated at a Late Bronze Age settlement (Casarão da Mesquita 3) located at south-western Portugal. The fragment belongs to one of the two halves forming the mould and exhibits part of a carving that corresponds to an axe edge. An outstanding feature that can be seen in the mould is a black greasy powder sticking to the carving surface suggesting that this powder was certainly used as a dressing during the casting operation. Several analytical techniques, namely EDXRF, XRD, FTIR, and Py-GC/MS, were used in order to identify the mineral composition of the stone mould, the kind of cast alloy and the origin and composition of the black dressing. The mould is made of steatite. The bronze alloy cast in the mould would probably be a ternary alloy, since Cu, Sn and Pb were identified in the carving. P was also detected by wet chemistry and EDXRF in the mould dressing as well as remains of organic substances, which were identified by FTIR and Py-GC/MS. These results suggest that a smoky flame from burning bones was used for the mould dressing. A small bronze bead, a bronze blade fragment and two copper green minerals were also found at Casarão da Mesquita 3. All these metallurgical remains attest that bronze metallurgy was performed during Late Bronze Age at this archaeological site.

KEYWORDS Stone mould, dressing, burning bones, bronze metallurgy, Late Bronze Age, EDXRF, XRD, FTIR, PyGC/MS.

INTRODUTION During the Environmental Impact Assessment for the new Monte Novo Irrigation Block, which is related with the implementation of the Alqueva Project, a large Roman villa was identified at a place named Monte da Mesquita (São Manços, Évora). Their remains could be affected by putting the irrigation project into effect. The mitigation strategy concerning the eventually threatened archaeological heritage at Monte da Mesquita would be to undertake watching brief by archaeologists when a large infrastructure – a water pipeline – would be inserted crossing the area. In this manner, when the arable ground was removed by machinery near fifty pits dug in the soft rocky substrate were discovered at Casarão da Mesquita 3(see Fig. 1), not far from the Roman villa. The preservation by record of all those structures was approved by the authorities. The excavations were carried out last year by an archaeological company. The field works revealed that these pits were filled up with earth containing kitchen refuse, fragments of pottery, and other

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artefact remains, ascribing those negative structures to a Late Bronze Age settlement, certainly located nearby.

Fig. 1 – Aerial view of the pits

Among the recovered artefacts, a fragment of a metallurgical stone mould for flat axes (Fig. 2) and two bits of copper green minerals, one metallic bead and a fragment of a metallic blade (Fig. 3) were identified.

Fig. 2 – The fragment of the stone mould for flat axes

The mould fragment belongs to one of the two halves that should form the metallurgical artefact and it exhibits part of a carving corresponding to a flat axe edge. An outstanding and rare feature that could be seen in the mould fragment is a black greasy powder sticking to the carving suggesting that this powder was certainly used as a dressing during casting operations.

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In order to characterize all this metallurgical remains and to identify the related metallurgical processes carried out at the Late Bronze Age settlement, several analytical techniques were used. The results obtained are presented ahead.

Fig. 3 – Metallurgical remains found at Casarão da Mesquita 3

MATERIALS, ANALYTICAL TECHNIQUES AND RESULTS Stone Mould EDXRF EDXRF analyses were performed using an Energy Dispersive X-Ray Fluorescence spectrometer – Kevex 771. This system is equipped with a 200 W Rh X-ray tube as the primary excitation source. Appropriate secondary targets and filters (Ti, Fe, Ge, Zr, Ag and Gd) are also available to optimise the excitation conditions. The characteristic X-rays emitted by the elements present in the sample are collimated at 90º and measured in a cryogenically cooled Si(Li) detector with 30 mm2 of active area and a resolution of 165 eV at 6.4 keV. Two excitation conditions were used to promote the sample characteristic X-rays, i.e. silver secondary target and gadolinium secondary target. The first condition utilizes a tube voltage of 35 kV and a current intensity of 0.5 mA, while the latter utilizes 57 kV and 1.0 mA. Each spectrum was collected during 500 s of live time. The stone mould was analysed in the inner and outer surfaces, in order to identify any metal remains eventually present in the inner surface (see Fig. 4). The comparison of the chemical elements present in the two analysed surfaces evidences the enrichment of copper, tin and lead in the inner (carved) surface of the artefact. Therefore the stone mould was certainly used to cast a bronze axe using probably a ternary alloy, since Cu, Sn and Pb were identified in the inner surface.

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Fig. 4 – X-ray spectra of the inner (blue) and outer (red) surfaces of the stone mould

XRD Petrographic characterization of the stone mould through a mineralogical analysis was carried out in a Bruker D8 Discover diffractometer using Cu kα radiation. The identification of crystalline phases was done using JCPDS database cards.

Fig. 5 – Points of analysis by XRD: 1 – reverse; 2 – bulk, 3 - obverse

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Fig. 6 – XRD spectrum (talc) at point 1 (see Fig. 5)

Fig. 7 – XRD spectrum (calcite) at point 2 (Fig. 5)

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Fig. 8 – XRD spectrum (talc + calcite + quartz) at point 3 (fig. 5) Dressing material Wet Chemistry – Phosphorus Test In order to test the existence of phosphorus in the dressing material, this substance was dissolved in a solution of nitric acid and the phosphate ions, if present, were precipitated as crystalline yellow ammonium phosphomolybdate by a nitric acid solution of ammonium molybdate. (PO4)3- + 12(MoO4)2- + 3(NH4)+ + 24(H)+ → (NH4)3PO4.12MoO3 ↓ + 12H2O

Fig. 9 – Crystalline yellow ammonium phosphomolybdate precipitate

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In Fig. 9, these crystals (with a “globular” aspect in the photo) are displayed and, consequently, the presence of phosphorus in the dressing material is proved. EDXRF The black dressing material was also analysed in the Kevex 771 spectrometer using the direct excitation mode with 5 kV, 0.5 mA, vacuum and 300s of live time. The analysis identified the presence of phosphorus, and some other elements, constituents of the stone material or from the soil where it was buried (Fig. 10).

Fig. 10 – EDXRF spectrum of the black material Elemental Analysis (C, H, N) Carbon, nitrogen and hydrogen contents in the dressing material were determined using a CARLO-ERBA EA 1100 CHNS-O Elemental Analyzer. The values obtained: C 11.3 %; N 0.9 %; H 0.7 %, suggest that we are in presence of an organic substance. FTIR Three samples were taken from the black material on the mould’s surface using a microsurgical scalpel. The samples were analysed in a Thermo Nicolet Nexus® 670 FTIR spectrometer coupled to a Continuµm™ IR microscope. FTIR spectra were collected in transmission mode using the compression diamond Spectra-Tech™ µSample Plan cell. Once the sample was placed in the diamond cell, this was positioned on the IR microscope and the spectrum was acquired. Each FTIR spectrum is the average of 254 scans collected at 4 cm−1 resolution, in the region from 4000 cm-1 to 650 cm-1. Fig. 11 shows the FTIR spectrum of one of the samples. Besides the dressing material, this sample also contained a little portion of stone from the mould. The spectrum was compared with reference spectra and two materials were identified: talc and clinochlore, according to the result obtained from the library search performed using the OMNIC™ software. This spectrum also shows two weak intensity C-H stretching bands at 2922 cm-1 and 2853 cm-1, that reveal the presence of traces of an organic material.

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2853

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%Transmittance

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2922 ν (C-H)

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Fig. 11 – FTIR spectrum of a sample containing the black matter and a little portion of the mould’s stone (up), Talc (middle) and Clinochlore (down) FTIR reference spectra1.

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Fig. 12 – FTIR spectrum of the black matter on the mould’s surface.

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The reference spectra are from the U. S. Geological Survey Minerals library of FTIR spectra.

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1741 v(C=O)

2852 v(C-H) 3661 v(O-H) 2922 v(C-H)

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3398 v(O-H) 60 e c n at ti m s n ar T %

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Fig. 13 – FTIR spectrum of the black matter taken from a different point of the mould’s surface.

In Figs 12 and 13, the spectra of the other two samples are shown. The spectrum in Fig. 12 presents two bands at 1396 cm-1 and 876 cm-1, assigned respectively to C-O stretching and bending vibrations in carbonates, and a combination band at 2525 cm-1. These bands are due to the presence of calcium carbonate. Along with these bands, it can be seen two bands attributed to the presence of talc (1021 cm-1 and 671 cm-1 – Si-O stretching and bending vibrations, respectively) and one band at 1578 cm-1 that couldn’t be assigned to any particular vibration. This band is positioned in the region of the C=O stretching in carboxylic acid salts, but as there are no bands assigned to C-H stretching, this hypothesis can’t be considered. The band is probably due to the presence of an inorganic salt, or other inorganic material, that cannot be identified from a spectrum collect in a frequency region from 4000 cm-1 to 650 cm-1. The spectrum in Fig. 13 also presents bands due to talc and probably to other silicates – the O-H stretching band at 3661 cm-1 is probably due to hydration in a silicate. Furthermore, the spectrum shows bands assigned to C-H stretching in hydrocarbons with -CH2- groups (2922 cm-1 and 2852 cm-1), and two weak intensity bands assigned to C-H bending (1458 cm-1) and to C=O stretching in ester groups. FTIR analysis of these three samples does not allow to determine the exact composition of the black material on the stone mould’s surface. It is possible to identify talc and other silicates, calcium carbonate and an organic material. The bands in the spectra attributed to this organic material point out to hydrocarbons containing -CH2- and ester groups. Therefore, it can be a wax, oil or any kind of fat. The C-H stretching bands frequency in the spectra excludes the possibility of a natural resin. PY-GC/MS Samples (about 0.5 mg) were placed in a ¼″quartz boat pyrolysis tube and 5 µL of tetramethylammonium hydroxide (TMAH) solution were added when mentioned. Afterwards, the sample was placed in the pyrolysis coil and introduced in the pyrolysis interface, which was kept at 250 °C. Experiments were carried out with an integrated system composed of a CDS Pyroprobe 1000 heated filament pyrolyser

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(Analytical Inc., New York, USA), and a gas chromatograph Agilent 6890N (Agilent Technologies, Palo Alto, CA, USA) coupled to an Agilent 5975N mass spectrometer (Agilent Technologies) and equipped with pyrolysis injection system. A capillary column HP-5MS (5% phenyl–95% methylpolysiloxane, 30 m × 0.25 mm i.d., 0.25 µm film thickness, J&W Scientific/Agilent Technologies) was used in order to provide the adequate separation of components. Pyrolysis was performed at 600 °C for 10 s using a precalibrated Pt coil type pyrolyzer (CDS pyroprobe). The pyrolyser interface and the inlet were set at 250 °C. The samples were injected in split mode (split ratio 22:1). The chromatographic conditions were as follows: initial temperature of 40 °C held for 2 min, increased at 10 °C/min up to 170 °C held for 2 min, increased at 8 °C/min up to 200 °C held for 2 min, increased at 4 °C/min up to 300 °C held for 12.25 min and increased at 4 °C/min up to 310 °C held for 2.5 min. Helium gas flow was set at 1.0 ml/min. The electronic pressure control was set to constant flow mode with vacuum compensation. The MS transfer line temperature was 280 ◦C; the MS ion source temperature was kept at 230 ◦C and the MS quadrupole temperature at 150 ◦C. The mass spectrometer was operating in the EI positive mode (70 eV) and the mass range was from m/z 45 to m/z 500 (3.18 scans/sec). An Agilent Chemstation software G1701DA MSD was used for GC–MS control, peak integration and mass spectra evaluation. Tuning of the mass spectrometer was checked using perfluoro-tributylamine. EI mass spectra were acquired by total ion monitoring mode and peak area data from total ion current (TIC) chromatograms were used for quantitative analysis. Wiley Library of Mass Spectra and Nist98 were used for identifying compounds. The sample analysis without TMAH did not drive to any conclusions. The components were found in low quantity and therefore its identity could not be established (Fig. 14). A bundance TIC : M O LD E 600 S E MTM A H .D \ data.m s

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Fig. 14 – Chromatogram obtained without TMAH The sample analysis with TMAH shows the presence of some kind of fat. Palmitic acid (P) was detected as well as heptadecanoic acid methylated derivatives (Hd) and other components that we could not identify (Fig. 15). Another sample from lower layers was collected and analyzed with TMAH. Fat acids were not detected, but in turn, the presence of some siloxanes, probably from the stone or from the column, were verified (Fig. 16). The component detected at higher retention time (46.871 min) was thought to be cholesterol but that fact was not confirmed by comparison with a cholesterol reference.

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A bundance TIC : M O LD E 600 C O MTM A H.D\ data.m s

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Fig. 15 – Chromatogram obtained with TMAH Abundance TIC : M O LDE 600 C O MTM AH 2.D\ data.m s

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Fig. 16 – Another chromatogram also obtained with TMAH

Other metallurgical remains EDXRF The green minerals and the metallic artefacts recovered from the archaeological excavations of the pits were subject to EDXRF analyses as reported for the stone mould. Quantification was made using the EXACT program [1], which is based in the fundamental parameter method but uses also experimental calibration factors. These factors were calculated through the analysis of a standard reference material – Phosphor Bronze 551 from British Chemical Standards. The two minerals and the blade fragment were analysed in two different areas (e.g. the two sides of the blade fragment), while the bead was only submitted to one analysis due to its small dimension. The results are shown in Table 1:

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Table 1 – EDXRF results from analyses of metallurgical remains (minerals and artefacts) Reference Pit 5 (C.3)/Level 1

Material Green mineral

Cu 95.9 94.6

Sn 0.68 0.62

Pb n.d. n.d.

As 0.22 0.32

Sb n.d. n.d.

Fe 3.2 4.5

Pit 12/Level 1

Green mineral

89.7 87.6

0.18 0.29

n.d. n.d.

n.d. n.d.

n.d. n.d.

10.1 12.1

Pit 42/Q.A35/Level 1

Bead

87.8

10.5

n.d.

n.d.

n.d.

1.3

Pit 49/Q.A’/B’ 38/Level 1

Blade (?) fragment

66.5 69.3

30.2 27.9

1.3 1.2