Examinations on small bronze items from the Hallstatt period burial ground at Mitterkirchen in Upper Austria

1 Introduction

The archaeological site near Mitterkirchen in Upper Austria is a large burial ground from the Hallstatt period dated to the 8^th/7^th century BC (Ha C). Inhumation was predominant in the respective burial ground. Owing to intensive agriculture, archaeological objects were repeatedly found and the site was discovered. However, some areas of the burial ground had already been destroyed before excavations began. Nevertheless, around 80 graves containing numerous burial gifts were excavated during the archaeological excavations between 1980 and 1990 [1, 2]. The most significant finds are the approximately 3000 buttons of a ceremonial cloak and three chariot burials [3, 4, 5, 6].

The metallurgy/mining of copper and bronze in prehistoric times has already been described in detail [7, 8]. Material science investigations on metallic artifacts aim at gaining detailed information on the manufacturing process and the alloy composition [9].

2 Experimental procedure

To identify inhomogeneities, the objects were examined using micro-computed tomography (micro-CT) at a resolution of 10 μm [10, 11]. Before the metallographic examinations, images of the objects were also taken using 3D digital microscopy (3D-DM).

The metallographic examinations paved the way for subsequent more in-depth microstructural examinations. In the first preparation step, the samples were vacuum cold mounted to avoid pores. Since the samples were rather small, the preparation involved gradual material removal, polishing, and documentation. The gross removal was carried out using a P500 diamond grinding wheel (30 μm). The sample was subsequently gradually diamond ground and polished up to a grit of 1 μm. Unetched samples and samples etched with Klemm II solution [12] were examined using a light optical microscope (LOM), scanning electron microscope (SEM), and energy dispersive X-ray analysis (EDS). The chemical compositions of the samples were also determined using X-ray fluorescence analysis (XRF).

3 Results and discussion

3.1 The triangular-headed nail

The triangular-headed nails were first examined using micro-CT. Those images did, however, not show any peculiarities. Overall views of the nails, in the 3D-DM and SEM, are shown in Figure 1. Both nails exhibit a uniform green patina.

Figure 1a to f

Overall views of the triangular-headed nails: a–c) triangular-headed nail A; d–f) triangularheaded nail B; a, b, d, e) 3D-DM; c, f) SEM.

Bild 1a bis f: Gesamtansichten der Dreieckskopfnägel: a–c) Dreieckskopfnagel A; d–f) Dreieckskopfnagel B; a, b, d, e) 3D-DM; c, f) REM.

The triangular-headed nail A was used for the metallographic examinations. It was prepared layer by layer. Figure 2 shows a section from about the center of the nail.

Figure 2a to k

Triangular-headed nail A: a, b) overview images of the sections, LOM; c–k) different microstructural areas; b, d, f, i) Klemm II etching, LOM; e, g, j) Klemm II etching and polarized light; c, h, k) SEM.

Bild 2a bis k: Dreieckskopfnagel A: a, b) Übersichtsbilder der Schliffe, LOM; c–k) unterschiedliche Gefügebereiche; b, d, f, i) Klemm 2 Ätzung, LOM; e, g, j) Klemm 2 Ätzung und polarisiertes Licht; c, h, k) REM.

In the polished condition (Figure 2a), corrosion spots can be recognized on the edges of the head and on the tip. The etched sample exhibits a very coarse structure with dendritic solidification cells up to 500 μm in size (Figure 2b). A metallic area as homogeneous as possible (Figure 2c) was selected and analyzed using EDS to characterize the bronze microstructure. Approx. 15 wt. %Sn and 1.9 wt. %S were found. While the bright areas are Cu₄₁Sn₁₁, the dark gray spots are Cu₂S. Figures 2d and 2e show the transition from head to shaft. Some areas are characterized by a dendritic solidification structure. The dendritic solidification cells are particularly easy to recognize in the polarized light and attain a size of up to 500 μm (Figures 2e, g, h).

Corroded areas can be found in the edge area, where a Sn-rich layer is noticeable on the surface. It appears to be Cu₄₁Sn₁₁ (Figures 2f–h). Owing to its bright color, this layer is clearly visible in the SEM (Figure 2h). In an EDS element distribution, the different Sn-enriched areas are clearly recognizable (Figure 3): the outer layer, presumably Cu₄₁Sn₁₁, the corroded area with an enrichment of SnO₂ [13, 14], and Cu₄₁Sn₁₁ phase precipitated in the metal. The point-like enrichment of S can also be observed. In the area of the tip, the eutectically solidified areas are slightly deformed (Figures 2i, j) and Sn enrichments can also be found in the corroded areas (Figure 2k).

Figure 3

EDS element distribution of an edge region of triangular-headed nail A.

Bild 3: EDX Elementverteilung eines Randbereichs von Dreieckskopfnagel A.

The metallographic section was also subjected to XRF analysis (Table 1) revealing a content of about 24.4 wt. %. 0.2 wt. %S and 0.5 wt. %Pb were also found. The Sn values are very high and can be explained by the enrichment of Sn during corrosion.

Table 1

XRF measurements on the metallographic sections (wt. %).

Tabelle 1: RFA Messungen an den metallographischen Schliffen (Gew. %).

Element / Element	Triangular-headed nail A / Dreieckskopfnagel A	Lamellar Buckel A / Lamellenbuckel A
Cu	74.42	75.29
Sn	24.38	24.04
Pb	0.50	0.13
S	0.23	0.20
Ni	0.06	n.n.
Fe	0.07	0.10
As	n.n.	n.n.
Si	0.23	0.24
P	n.n.	n.n.
Ca	0.12	n.n.

To illustrate the issue, locally measured values of the analysis are compiled in Figure 4. The original bronze alloy probably contained about 14 wt. %Sn.

Figure 4

Sn contents (wt. %) from EDS surface analyses at different locations of the triangular-headed nail.

Bild 4: Ergebnisse der Sn-Gehalte (Gew. %) von EDX-Flächenanalysen an verschiedenen Stellen des Dreieckskopfnagels.

So far, such triangular-headed nails have only been found in Undlingen (Germany) near the famous Heuneburg site [15].

3.2 The lamellar “Buckel”

Overall views and SEM images of the lamellar Buckel are assembled in Figure 5.

Figure 5a to f

Overall views of the lamellar Buckel: a–c) lamellar Buckel A; d–f) lamellar Buckel B; a, b, d, e) 3D-DM; c, f) REM.

Bild 5a bis f: Gesamtansichten der Lamellenbuckel: a–c) Lamellenbuckel A; d–f) Lamellenbuckel B; a, b, d, e) 3D-DM; c, f) REM.

Lamellar Buckel A has a smooth, grayish surface (Figures 5a–c) which may probably be attributed to thorough cleaning. Lamellar Buckel B has a partially flaked off green patina (Figures 5d–f).

The micro-CT images of both objects show areas of severe corrosion and also cracks, especially in the bends (Figure 6).

Figure 6

Micro-CT of lamellar Buckel A and B.

Bild 6: Mikro-CT der Lamellenbuckel A und B.

Lamellar Buckel A was subjected to metallographic examinations (Figure 7). Figure 7a is an overview image. It already shows clear signs of corrosion.

Figure 7a to k

Metallography of lamellar Buckel A: a, b, g, j) Klemm II etching, LOM; f, i) polished, LOM; c, d, e, h, k) SEM.

Bild 7a bis k: Metallographie von Lamellenbuckel A: a, b, g, j) Klemm 2 Ätzung, LOM; f, i) poliert, LOM; c, d, e, h, k) REM.

The bronze structure has a uniform grain size of up to 100 μm with some deformation twins also present (Figure 7b, c). Some dark lines can be seen in the middle of the image (also the middle of the bronze sheet) which, according to EDS analysis, could be Cu₂S. According to the respective surface analysis (Figure 7c), this area contains about 13 wt. %Sn and 2 wt. %S. There is hardly any intermetallic Sn phase Cu₄₁Sn₁₁ present.

The values for sulfur correspond approximately to those of the XRF measurement, but the Sn values are clearly too high (Table 1). Increased corrosion can be found at the bends. Cracks probably formed first with corrosion subsequently spreading around the cracks (Figure 7f–i). In one portion of the Buckel, a depression was found (Figures 7j, k) which could also have been a feature of the original aspect.

It can be assumed that the initial sheet used for this lamellar Buckel was manufactured by fine forging. The process caused the sulfides in the center of the sheet to elongate. The sheet was probably subjected to an intermediate annealing step to reduce cold deformation. The twins observed were probably introduced when the lamellar Buckel was eventually attached to a piece of textile. The cracks on the sheet’s inside are unusual, as they should have formed on the outside.

4 Conclusions

Both bronze objects examined consist of Sn bronze with S and Pb impurities.

The triangular-headed nail contains about 15 wt. %Sn. The dendritic cast structure can be observed. The eutectoid phase Cu₄₁Sn₁₁ is evenly distributed in the micro-structure, but layers of Cu₄₁Sn₁₁ were found in some areas of the surface. In corroded areas of the bronze, Sn contents of more than 55 wt. % were measured, which can be attributed to the enrichment of SnO₂ during corrosion.

The bronze of the examined lamellar Buckel contains about 13 wt. %Sn. Therefore, only very little Cu₄₁Sn₁₁ is present in the microstructure. The microstructure is recrystallized and exhibits only a few twins. This means that heat treatments were also carried out. Elongated Cu₂S precipitates indicate that the initial sheet used for the lamellar Buckel was manufactured by fine forging. Corrosion predominantly occurred at existing cracks.

The findings show that the casting and processing of small items by fine forging were well established at that time.

1 Einleitung

Die archäologische Stätte bei Mitterkirchen in Oberösterreich ist ein großes Gräberfeld aus der Hallstattzeit, das auf das 8./7. Jahrhundert v. Chr. (Ha C) datiert wird. Im Gräberfeld war die Körperbestattung vorherrschend. Die Fundstelle wurde entdeckt, weil infolge der intensiven Landwirtschaft immer wieder archäologische Objekte gefunden wurden. Allerdings waren bereits vor Beginn der Ausgrabungen einige Bereiche des Gräberfeldes zerstört. Trotzdem wurden bei den archäologischen Ausgrabungen zwischen 1980 und 1990 ca. 80 Gräber ausgegraben, die zahlreiche Grabbeigaben enthielten [1, 2]. Die bedeutendsten Funde sind die etwa 3000 Knöpfe eines Prunkmantels, sowie drei Wagenbestattungen [3, 4, 5, 6].

Die Metallurgie der Kupfer- und Bronzegewinnung in der Urgeschichte wurde bereits ausführlich beschrieben [7, 8]. Ziel materialwissenschaftlicher Untersuchungen an metallischen Artefakten ist es, Details über den Herstellungsprozess sowie die Legierungszusammensetzung herauszufinden [9].

2 Experimentelles

Mittels mikro-CT wurden die Objekte mit einer Auflösung von 10 μm untersucht, um Inhomogenitäten zu erkennen [10, 11]. Vor den metallographischen Untersuchungen wurden Bilder der Objekte auch mittels 3D-Digitalmikroskopie (3D-DM) aufgenommen.

Die metallographischen Untersuchungen erlaubten danach die detaillierteren Gefügeuntersuchungen. Als erster Schritt der Präparation erfolgte eine Vakuum-Kalteinbettung um Poren zu vermeiden. Da die Proben eher klein waren wurde schrittweise abgetragen, poliert und dokumentiert. Der grobe Abtrag erfolgte mit einer Diamantschleifscheibe P500 (30 μm) und danach wurde stufenweise bis 1 μm Diamant geschliffen bzw. poliert. Ungeätzte und mit Klemm 2 Lösung [12] geätzte Proben wurden mittels Lichtmikroskop (LOM), Rasterelektronenmikroskop (REM) und energiedispersiver Röntgenanalyse (EDX) untersucht. Die chemischen Zusammensetzungen der Proben wurden auch mittels Röntgenfluoreszenzanalyse (RFA) bestimmt.

3 Ergebnisse und Diskussion

4 Schlussfolgerungen

Beide untersuchten Bronzeobjekte bestehen aus Sn-Bronze mit Verunreinigungen an S und Pb.

DerDreieckskopfnagelenthältetwa15 Gew. %Sn und es ist die dendritische Gussstruktur zu erkennen. Die eutektoide Phase Cu₄₁Sn₁₁ ist gleichmäßig im Gefüge verteilt, jedoch wurden in manchen Bereichen der Oberfläche Schichten aus Cu₄₁Sn₁₁ festgestellt. In korrodierten Bereichen der Bronze wurden Sn-Gehalte von über 55 Gew. % gemessen, was auf die Anreicherung von SnO₂ bei der Korrosion zurückzuführen ist.

Die Bronze des untersuchten Lamellenbuckels enthält etwa 13 Gew. %Sn, wodurch nur sehr wenig Cu₄₁Sn₁₁ im Gefüge vorliegt. Das Gefüge ist rekristallisiert und weist nur wenige Zwillinge auf. Dies bedeutet, dass auch Wärmebehandlungen durchgeführt wurden. Länglich ausgebildete Cu₂S Ausscheidungen weisen darauf hin, dass das Ausgangsblech für den Lamellenbuckel durch Feinschmieden hergestellt wurde. Korrosion fand vorwiegend an bereits vorhandenen Rissen statt.

Die Ergebnisse zeigen, dass zu dieser Zeit das Gießen von Kleinteilen und die Verarbeitung durch Feinschmieden gut etabliert waren.