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SummaryFragmented bronze sculpture is an important category of finds from the provinces in the north of the Alps at the frontier of the Roman Empire. Portraits and statues of the emperors and their families were raised in a great number in the military camps and civil settlements along the Limes. Numerous fragments of these statues are still preserved, but they have never been systematically registered. The paper introduces the project and the first results. Three different techniques of gilding have been revealed: besides leaf and fire-gilding, the unusual tech-nique of diffusion gilding has been observed. Also vari-ous repair techniques, different to those already docu-mented for Mediterranean statues have been exposed.

IntroductionOne of the important categories of finds from the Roman border provinces between the north of the Alps and the North See are large roman bronze sculptures. Portraits and statues of the emperors and their families were raised in a great number in the military camps and civ-il settlements along the Limes in Germania inferior, in Germania superior and in Raetia. Though also in ritual contexts, there must have been bronze statues in con-siderable numbers. Many of these statues are likely destroyed by invading German troops in the middle of the 3rd century AD when the Limes was abandoned or in case of Damnatio memoriae when the emporer fall out of favor. (Fig. 1).

Large Roman Bronze statues from the UNESCO World Heritage Limes

Frank Willer, Roland Schwab & Kati Bott

Fig. 1: Fragments of bronze statues from Aalen. Archäologisches Landesmuseum Baden-Württemberg. Scale 33 x 353 cm

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Even if the fragments are often difficult to characterize and to categorize, they do constitute an important source for the reconstruction of imperial representation in the Limes forts and thus the importance of the imperial im-age in the Roman army.

Therefore, it is astonishing that they have never been compiled systematically. More than 4300 fragments from 124 different sites have been identified so far. Most of them have been archived for undefined old finds in mu-seums and other collections. Only a few significant frag-ments have been published in the serial “Der Oberger-manisch-Rätische Limes des Römerreiches” of the Reichslimeskommission since the end of the 19th cen-tury (see Sarwey et al. 1892). They were briefly reviewed in the works of Gustav Gamer (1969) and Oliver Stoll (1992).

Only a few better preserved pieces like a horse head from Augsburg (Bakker 1985; Bergemann 1990), the portrait of Gordian III from Niederbieber (Fig. 2) (Lahus-en & Formigli 2001), or the so-called “Trajan” from the Rhine near Xanten (Schalles 2010) have already been presented in individual articles. Comprehensive publica-tions are available of scrap fragments from Colonia Au-gusta Raurica in Switzerland (Janietz Schwarz & Rouill-er 1996; Janietz 2000) or recently of the well preserved statue of so called “youth from Xanten” (Schalles & Peltz 2012).

The project: aims, strategies and methods

The research project “Roman bronze Statues from the UNESCO World Heritage Limes” (2010-2014) is funded by the German VolkswagenStiftung and is initiated by the Archäologisches Landesmuseum Baden-Wuerttem-berg, the Institut für Archäologische Wissenschaften of the Goethe Universität in Frankfurt a. M. and the LVR-LandesMuseum Bonn (see Wessler 2011). Within the initiative „Research in Museums“, the project is fur-thermore supported by numerous partners of museums, universities and specialized research institutes in Ger-many, Netherlands and Switzerland.

Scientific investigations are performed by the Bunde-sanstalt für Materialforschung und -prüfung in Berlin, the Institut für Gießerei of the RWTH Aachen, the Instituut Collectie Nederland in Amsterdam, the Institut für Che-mie, Biochemie und anorganische Chemie of the Freie Universität in Berlin and the Curt-Engelhorn-Zentrum Archäometrie in Mannheim. The aim is to reappraise these important relicts of ancient bronze casting to make them open for research and to

the public comprises an archaeological-historical ap-proach as well as the technological aspects. Casting techniques, alloy compositions and surface working like gilding, chiselling, repair and joining are investigated by different scientific methods.

A lot of the technological details like casting flaws, re-pairs, joints and the positions of chaplets can be revealed by carful optical surface observations and conventional radiography. Computed tomography (CT) provides 3-D information (Fig. 3) of more complex casting techniques and makes it possible to investigate the interior surface. Based on the 3-D models casting simulations will be performed to visualize the entire process of casting, in-cluding solidification and shrinkage formation. For the determination of the chemical compositions by ener-gy-dispersive x-ray fluorescence analysis (EDXRF), 370 samples were taken until now. Initial investigations of the microstructure, corrosion, gilding and other surface

Fig. 2: Picture of the life-sized head of Gordian III from Nieder-bieber. Landschaftsverband Rheinland-LandesMuseum Bonn.

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decorations are performed by metallography with optical (OM) and scanning electron microscopy (SEM) with at-tached energy dispersive spectroscopy equipment (EDX).

Apart from the categorization of the bronze fragments themselves, the reconstructions of the original statues, their locations and functions as well as aspects of pro-duction techniques will become more interpretable. The results are documented and recorded in an internet da-tabase, which is available to all participants during the whole project and which will also be available for every-body at the end of the project.

In addition to the research on bronze fragments, the compilation and analysis of all known statue bases with inscriptions is in progress. They are, as far as they are no bases for stone statues, directly and functionally as-sociated with the bronze statues. They can provide im-portant information on the source for the assembly and disassembly, and they are essential for the historical context.

Alloy compositions, surface decorations, experiments and exceptional casting techniques

According to a recipe given by Pliny, it is thought, that a third part of the bronze used for statues was scrap (Caley 1970; Gazda & Hanfmann 1970). Bearing in mind,

that the prevalent alloy in Roman times is brass, one would expect most of the statues being of gunmetal. Actually the large statues are all of bronze (see also Caley 1970, Scott & Podany 1990; Oddy et al. 1990; Janietz Schwarz & Rouiller 1996; Janietz 2000; Lahus-en & Formigli 2001). Therefore we can follow Caley’s (1970) supposition, that most scrap used for casting statues is foundry waste or scrap coming from statues in turn.

Figure 4 shows the alloying components of samples analyzed so far and it attests the published data that Roman large statues are mostly made of leaded or even of highly leaded bronze. Consequently the well known leaf gilding (Fig. 5 a, b) is the predominately observed gilding technology of the statues (see Oddy et al. 1990; Oddy 2000). Only one fragment of an arm shows a pre-vious fire-gilded layer which was regilded by gold leafs (Fig. 6 a, b).

A very unusual and hardly observed technique of diffu-sion bonding has been revealed at a back fragment from a life size statute of a naked male deriving from an Ale-mannic scrap depot of the early 4th century from Gross-Gerau in Hessen (Wenzel 2003; 2009), which were recently conserved in the LVR-LandesMuseum

Fig. 3: Computed tomography of the head of Gordian III as shown in figure 2 with a view inside. III Bundesanstalt für Material-forschung und -prüfung, Berlin (BAM)

Fig. 4: Histograms showing the lead (a) and tin concentrations (b) of statues analyzed so far. Landschaftsverband Rhein-land-LandesMuseum Bonn and Curt-Engelhorn-Zentrum Archäo-metrie, Mannheim

a)

b)

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(Fig. 7a , b). The surface beneath the gilding is heavily corroded, but figure 8 clearly shows that interdiffusion of gold and bronze has taken place. It is not clear if this technique is an accidental or an intentional one, but it left uncompleted because of the interaction of the gold leaf with the lead of the alloy (Fig. 9). The eutectic point of Pb-AuPb3 is more than 100 °C lower than the melting point of lead (Okamoto 1993). Experimental reconstruc-tions (Bott 2011) of the diffusion gilding technology with highly leaded bronze revealed, that the eutectic is eas-ily formed and therefore the golden colour of the leaf suddenly changes into gray (Fig. 10)1. One would expect that a deliberate diffusion gilding of statues should have

been done only to lead free bronzes, but several exper-iments showed that there is already a good adhesion of the gold coating on leaded bronze after polishing a leaf onto the surface. The attachment can be improved by gentle heating the bronze, which is probably due to fast diffusion behavior of gold in lead forming the eutectic with lead globules near the surface (Fig 11).

On a bronze fragment from Groß-Gerau, a large surface repair technology with patches and copper rivets has been observed. This technology had previously been considered to be a later repair of the damaged statue (see Fig. 4a). The findings from Gross-Gerau now

Fig. 5a: Fragment of a garment (17 x 13 cm) from Groß-Gerau, with repair which has been done direct after casting.5b) Leaf gilding with adhering lead corrosion. Landschaftsverband Rheinland-LandesMuseum Bonn and Curt-Engelhorn-Zentrum Archäometrie, Mannheim

Fig. 6a: Fragment of a gilded live-sized bronze arm (23 x 12 cm) of a statue from Bonn. 6b) Scanning electron image (backscat-tered mode) shows porous fire gilding with two outer layers of gold leaf from regilding. Landschaftsverband Rheinland- LandesMuseum Bonn and Curt-Engelhorn-Zentrum Archäo-metrie, Mannheim

Fig. 7a: Fragment of a gilded human back (60 x 42 cm) from Groß-Gerau. 7b) Scanning electron image (backscattered mode) shows phase formation on the gold-base metal interface. Land-schaftsverband Rheinland-LandesMuseum Bonn and Curt-Engel-horn-Zentrum Archäometrie, Mannheim

Fig. 8: The X-ray dot map from the interface of the gilded back from Groß-Gerau (see figure 7) illustrates the interdiffusion of gold from the gold leaf and copper of the bronze. Curt-Engel-horn-Zentrum-Archäometrie, Mannheim

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demonstrate, that this repair was immediately executed after the failed casting. It has been identified at several other statue fragments from the limes region so that we can assume this technique for repair was probably pri-marily used for difficult moulded large sized statuary. It is well known, that flawless complex castings were hard-ly to produce with ancient technology and antique found-ry men usually decided to apply very time consuming repairs rather than melting down their spoiled castings (Lechtman & Steinberg 1970). Probably it was much more effort to produce a new mould and recast the hole statue, than repairing the defects.

The statues coming from the Limes differ significantly to manufacturing techniques known from Mediterranean sculpture. Rectangular patches are well documented for repairs of casting flaws and holes from the removed

chaplets of statues coming from the Mediterranean area (Lechtman & Steinberg 1970; Willer 1994; Lahusen & Formigli 2001; Schalles & Peltz 2011).

Regarding the finds discussed here, the holes are small-er and usually plugged by small, wrapped bronze beads or even left open (Fig. 12).

A very particular phenomenon has been observed on the outer surface of objects from Mainz, Bonn and Co-logne. A uniform hexagonal network with each hexagon being 1mm in diameter resembling honeycombs can be seen under the microscope (Fig.13).

An identical pattern is published by Scott & Podany (1990), observed on a bronze statue of Roma dated to 40-68 A.D. from the J. Paul Getty Museum in Malibu

Fig. 9: Lifting of a gold leaf by corroded lead on the surface of the back from Groß-Gerau (see figure 7. Scanning electron image (backscattered mode). Curt-Engelhorn-Zentrum-Archäometrie, Mannheim

Fig. 10: Experimental diffusion gilding with a high leaded bronze alloy. After heating around 200 °C the Pb-AuPb3 eutectic appears and the color of the gold leaf turns to grey. Landschaftsverband Rheinland-LandesMuseum Bonn

Fig. 11: Some experiment like in figure 9 after soft heating. The gold leaf leaves unaffected, but the bonding is only mechanical-ly and can easily detached. Landschaftsverband Rhein-land-LandesMuseum Bonn

Fig. 12: Rectangular hole on the surface of a fragment of a life-sized bronze statue. These holes are typical relicts from chaplets. Landschaftsverband Rheinland-LandesMuseum Bonn

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(Inv. Nr. 84.AB.671). Scott & Podany (1990) are discuss-ing this network in the context of the microstructure and composition of the bronze, which contains 6.5 % tin and 1.8 % lead. They suggest a relation of the hexagons to a slow cooling of the cast bronze and the reheating in the annealing process.

The fragments from the Limes with those networks on the surfaces are not yet analyzed, so it is too early to state any interrelationship. There is quite different pos-sibly how this network could have been produced and alloy composition, solidification and heat treatment are likely to affect this structure. Regular geometric corrosion is rarely reported (Werner 1967) and would also be de-pendent on the microstructure. Since the hexagonal pattern is only be seen in areas where cold work hasn’t been done and due to the fact that a metal object cast usually carries the structure of the mould. One idea is at the moment, if the moulds could have been construct-ed by slabs of honeycombs. We are currently looking for possibilities to check this hypothesis.

ConclusionThere is some discussion about similarities of particular statues and therefore about specialized workshops cast-ing large bronze statues (e.g., Janietz 2000; Schalles & Pelz 2011). It is most unlikely that heavy bronze castings have been transported several hundred of kilometers, but itinerant founders like early modern bell founders could have left their marks within certain regions. The chemical compositions of Roman large statues from the Limes give a quite uniform impression concerning the alloy compositions and impurity pattern. There are mar-ginal differences within the compositions between the

individual find spots. Technological features like repairs and founding do probably allow a more differentiated view and some specific techniques could be observed in certain regions so far. Discussing a chronological de-velopment is not possible at the moment, but the work is in progress. The quantitative estimation and chrono-logical classification of the different gilding techniques employed for example is restricted in terms of the small number of gilded fragments investigated so far, but leaf gilding seems to be the most popular one.

The destruction of the bronze statues and their post-Ro-man use as scrap metal is also important. Date and type of destruction can be examined at the fragments as well as the handling of scrap by Germanic looters or Roman scrap collectors for example. Marks of hammers, levers or chisels as well as scratches on the surface of gilded bronze are evidence of the demolition of sculptures by probably post-antique looters greedy for metal. Numer-ous small-scale fragments indicate the preparation for remelting (see e.g. Fig. 1).

By its interdisciplinary approach, the project first aims at continuation of the research on statuary representa-tion north of the Alps to a crucial extent. Second, it offers the possibility to work out the similarities or differences between the military-dominated provinces and the Italian or Mediterranean regions in Graeco-Roman tradition.

At the conclusion of the research project, an internation-al symposium is planned. Following, the original finds and the research results will be presented 2014 in a joint exhibition of the LVR-LandesMuseum Bonn, the Limesmuseum Aalen and the Museum Het Valkhof (NL) to the public. Last but not least the project offers a great opportunity to emphasize the importance of finds that are stored in museums for the UNESCO World Heritage Limes – exemplary with the outstanding group of Roman bronzes Statues.

Footnote1 All experiments were carried out by Frank Willer and Kathi

Bott at the conservation department of the Landschaftsver-band Rheinland-LandesMuseum Bonn by using experimen-tal produced alloys without any deliberate modern additions. The thickness of the used gold leafs were similar to original roman material (less than 3µm). The work has been part of a diploma thesis (Bott 2011).

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