a mysterious black ornamentation on an early bronze age dagger from schoolbek (kosel),...

Journal of Archaeological Science: Reports 5 (2016) 407–421

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A mysterious black ornamentation on an Early Bronze Age dagger fromSchoolbek (Kosel), Schleswig-Holstein, Germany

Daniel Berger a,⁎, Christoph Berthold b, Jan-Heinrich Bunnefeld c, Melanie Keuper b

a Curt-Engelhorn-Zentrum Archäometrie gGmbH, D6, 3, D-68159 Mannheim, Germanyb Eberhard Karls University Tübingen, Department of Geosciences, Wilhelmstraße 56, D-72074 Tübingen, Germanyc Landesamt für Denkmalpflege und Archäologie Sachsen-Anhalt, Landesmuseum für Vorgeschichte, Richard-Wagner-Straße 9, D-06114 Halle (Saale), Germany

⁎ Corresponding author.E-mail address: daniel.berger@cez-archaeometrie.de (

a b s t r a c t

Article history:Received 7 October 2015Received in revised form 1 December 2015Accepted 7 December 2015Available online xxxx

Early Bronze Age societies of northern Germany and Scandinavia are well-known for skilfully crafted and richlydecorated metal objects. In this paper a Nordic bronze dagger from Schoolbek farm, Schleswig-Holstein,Germany, dating from themid-2ndmillennium BC (Period II) is investigated in detail. Besides chased ornamentsthe object carries a conspicuous decoration pattern of black coloured stripes and rivets on the hilt that contrastswith the dagger's green patina. Chemical andmineralogical analyses as well as examinations with an optical mi-croscope show that the black areas are largely identical in structure and composition with the green corrosioncrusts. Therefore, tinning or comparable decoration techniques can be excluded. It appears instead that thestripes are either the result of local artificial patination treatment or, more likely, of the application of a pastyma-terial such as tar, pitch or resin to the bronze surface. Tiny gold residues indicate that gold foils could possiblyhave been attached this way. This would have provided deviating corrosion conditions than for the undecoratedobject parts and thus lead to differently coloured areas.

Keywords:Early Nordic Bronze AgeBronze daggerMetal decorationFoil gildingX-ray diffraction analysisRaman spectroscopyCorrosionHydrated stannic oxide

1. Introduction

The Schoolbek farm is located in themunicipality of Kosel in the dis-trict of Rendsburg-Eckernförde in Schleswig-Holstein, northernGermany (Fig. 1). The dagger under study was excavated by a memberof public in 1970 on a sandy low ridge in a burialmound SE of Schoolbekfarmwhichwas heavily damaged by ploughing. Thewestern end of themoundwas removed and its diameter was still 12 to 15mwith a resid-ual height of 0.30 to 0.40 m. The mound had been built up in severalconstruction phases. During one of those a tree trunk coffin withoutstone covering was deposited in NE–SW orientation in the mound'scentrewhich once contained a corpse thatwas totally decayed. The dag-ger – as the onlyfind in this burial –was found in a distance of ca. 0.40mfrom the SW-end of the coffin with its hilt pointing to SW (Aner andKersten, 1978, 195 no. 2520 I A).

In its present condition the object from the collection of theArchäologisches Landesmuseum Schloss Gottorf, Schleswig, Germany(inv.-no. K.S. B 166a), is heavily corroded and 320 mm long. Its bladeis broken three times so that the whole find now consists of four pieces(Fig. 2). By applying four rivets, the hilt plate of the blade had been

D. Berger).

mounted to the dagger's hilt which was cast in one piece by lost-waxcasting after shaping a wax model over a clay core (cf. Drescher, 1961,57; Bunnefeld and Schwenzer, 2011, 212–215). The core is still insidethe hilt today as is also the single rivet for positioning the core. The re-sult of the casting process was a thin-walled objectwith an entirely hol-low hilt and pommel (Fig. 3).

The partially fractured circular pommel plate is ornamented, startingfrom the pommel knob in the centre, by a row of small chased triangles,a row of ten spirals and a groove. The hilt runs dished from the pommelto the lower hilt part. It is decorated on both faces by eight horizontaland black coloured stripes that are connected at their ends by verticaland similarly black coloured stripes. The entire ornament thus resem-bles some kind of framework (Fig. 4a). Looking in more detail, allthese stripes appear not to be completely solid, but are composed ofeven thinner, rectangular oriented lines with regularly recurring open-ings between each other (Fig. 4b). The resulting pattern looks like a lad-der band which is well-known on many metal artefacts of the NordicBronze Age as cast or chased ‘pearl band pattern’ (ger. Perlband)(Fig. 5; e.g. Nørgaard, 2015). One such chased Perlband is also presenton the dagger from Schoolbek just underneath the pommel plate anddiffers from the remaining Perlband parts regarding the missing blackcolouration and its deepened appearance (Fig. 4c). The black ornamentis finally taken up again at the lower part of the hilt where it follows thearc-like edge towards the blade (Fig. 4d).

408 D. Berger et al. / Journal of Archaeological Science: Reports 5 (2016) 407–421

The find itself belongs to the group of the so-called Nordic solid-hilted swords and daggerswhich arewidely distributed across southernScandinavia and northern Germany (Ottenjann, 1969; Bunnefeld,2014). Due to its shape, ornamentation and manufacturing techniquethe dagger is dated from period II of the Nordic Bronze Age, followingthe modified chronological scheme of O. Montelius (Montelius, 1885;Kersten, 1936). In absolute dates this means a period between ca.1500 and 1330 BC (Hornstrup et al., 2012, 48 tab. 1). Regarding thetype of artefact, the dagger of Schoolbek would just be an ordinary ex-ample, but it stands out for its exceptional decoration with the blacklines. This decoration is the topic of this paper. A range of analyticalmethods is used in order to disclose the nature of the decoration andits manufacture.

2. Materials and methods

2.1. The black decoration of the dagger

During reappraisal of Bronze Age solid-hilted swords and daggers inDenmark and Schleswig-Holstein, Germany (Bunnefeld, 2014), one ofthe authors (J.-H. B.) rediscovered the black ornamentation on thedagger's hilt. Although the decoration had already been described byAner and Kersten (1978, 195), they did neither account for the conspic-uous black colour of the stripes nor that it differs from the usual chased

Fig. 1. Location of Schoolbek farm in northern Germany (

or cast Perlband patterns by its even appearance. The black stripes are atthe exactly same level like the shiny green and smooth patina thatcovers large object parts. Additionally, they are cracked to the same de-gree, and cross-border cracks occur along the entire decorated regionsuggesting a close relationship regarding the formation history of thedifferently coloured areas (cf. Fig. 4b). On closer inspection, the edgesof the stripes turned out to be not well-defined, but rather blurred orrunny just as if someone had applied a liquid or a paste on the metal'ssurface. Because of these observations and because no indicationscould be found on X-ray images of the dagger (cf. Fig. 3), a corrodedmetal inlay such as it was sometimes applied on Early and MiddleBronze Age swords and axes in central and northern Europe can beruled out (Schwab et al., 2010; Berger et al., 2010; Armbruster, 2010;Berger, 2012; Berger et al., 2013; Berger, 2014, 2016). For the same rea-son, inlays of dark coloured resin or pitch are unlikely as well. EspeciallyNordic Bronze Age weapons, dress accessories and jewellery are oftendecorated with resin or pitch, and even numerous swords show similarframe-like recesses on their hilts, filled with organic inlays (Ottenjann,1969; Bunnefeld, 2014). So, if the ornament on the Schoolbek daggerdoes not represent some kind of inlay technique, it at least picked upand imitated a well-known decoration scheme of contemporaryweapons.

So far, no other metal object of early date is known to the authorsthat seems to be ornamented by a similar technique. One of the nearest

map: C. Frank, D. Berger, made with Natural Earth).

Fig. 2. Front (a) and back face (b) as well as hilt plate (c) of the dagger from Schoolbek, dating from Period II of the Nordic Bronze Age (1500–1330 BC). Total length: 320 mm; weight:322 g; hilt plate not to scale (photographs: C. Dannenberger, Archäologisches Landesmuseum, Schleswig, Germany).

409D. Berger et al. / Journal of Archaeological Science: Reports 5 (2016) 407–421

analogues is a Late Bronze Age situla from Brâncoveneşti, Mureş,Romania, that has superficial black coloured triangles, stripes and zigzaglines contrasting with the object's green patina (Soroceanu, 2005).Giumlía-Mair (2005) proved the black ornamentation to be the resultof tinning and subsequent corrosion under burial conditions. Tinning

Fig. 3. X-ray image of the dagger's hilt and pommel region seen from two differe

was also identified by Zhihui and Shuyun (2009) for the black anddot-like patterns on the blades of several Chinese Bronze Age swords.In contrast, black coloured patterns (also triangles) on two situla vesselsfrom the Early Iron Age princely tombs in Kleinklein, Styria, Austria,yielded analytical evidence for the use of birch pitch (pers. comm. S.

nt positions (images: R. Aniol, Archäologisches Landesmuseum, Schleswig).

Fig. 4. Detailed views of the dagger showing the patina and black stripes (a), the Perlbandmuster of one black stripe (b), chased Perlband just below the pommel (c) and the dark colouredrivet as well as the black stripe at the arc-like edge of the hilt (d) (photographs: a: C. Dannenberger, Archäologisches Landesmuseum, Schleswig, Germany; b–d: D. Berger).

Greiff, Römisch-Germanisches Zentralmuseum Mainz, Germany). For-mer examinations considered similar ‘painting’ with pitch on theBrâncoveneşti situla aswell (Roska, 1928, 251), but also artificial patina-tion, niello and partial wax covering were proposed (Miclea andFlorescu, 1980, 125 Nr. 507; Gogâltan, 1991, 18; Petrescu-Dîmbovi a,1995, 77 Kat. 4). This shows the large range of decoration techniquesthat could possibly have been applied to the dagger from Schoolbek.The following parts account for these aspects.

2.2. Analytical procedure

After a first thorough optical inspection of the dagger, its patina andthe black stripes, drill samples from the uncorroded metal core were ex-tracted from the blade, the hilt and one out of four rivets. The latter stuckout by its dark, almost black corrosion comparedwith surroundingpatina(Fig. 4d)whose origin to clarifywas a secondary, even though not less in-teresting question besides the primary issue. The other three samplesshould provide information on the metal composition and its relation

to contemporary metal objects. Corrosion material that accrued duringsampling was separated and collected for phase analysis.

Compositional metal analysis was performed using an ARL QUANT'Xenergy dispersive X-ray fluorescence spectrometer (EDXRF) fromThermo Scientific at the Curt-Engelhorn-Zentrum Archäometrie Mann-heim, Germany (CEZA). The analytical parameters and device settingsare specified in Table 1. The analytical procedure and quantification ofthe X-ray spectra are geared to that described by Lutz and Pernicka(1996).

Because sampling of the black stripes was not allowed, examinationhad to be taken place non-destructively and thus in situ on the object'ssurface. An Eagle III XXL μ-EDXRF device (Roentgenanalytik SystemeGmbH & Co. KG) and an EVO MA 25 scanning electron microscope(Zeiss) with an energy dispersive X-ray analyser (SEM-EDX) fromBruker AXS at the CEZA were chosen for chemical surface analysis ofthe green patina and the stripes as well as the black corrosion of therivet. Point analyses and mappings were performed without carboncoating (Table 1; Fig. 6a). Micro X-ray diffraction analysis (μ-XRD2)

Table 1Conditions and parameters of the analytical facilities employed in the study.

Analyticalmethod

Device type, company Parameters and conditions

EDXRF ARL QUANT'X, ThermoScientific

X-ray tube with Rh anode, twoexcitations: 28 kV, Pd filter, 1200 smeasuring time, 50 kV, Cu filter, 800 smeasuring time, Peltier cooled Si(Li)detector, analysis on drill samples,standard related quantification,empirical data correction

μ-EDXRF Eagle III XXL,RoentgenanalytikSysteme GmbH & Co. KG

X-ray tube with Rh anode, 30–40 kV,0.3–0.4 mA, monocapillary with 50 μmspot, Si(Li) detector, in situ analysis, 60 smeasuring time for each spot,standardless quantification

μ-XRD2 D8 Discover GADDS,Bruker AXS

X-ray tube with Co anode, 30 kV, 30 mA,HOPG monochromator, polycapillaryoptics with 50 μm pinhole collimator, 2D

Fig. 5. Three swords from Period II contexts with chased Perlbandmuster: (a) Gottrupel and (b) Haurup, Schleswig-Flensburg, Schleswig-Holstein, Germany, (c) Hjerpsted, Syddanmark,Denmark (photographs: J.-H. Bunnefeld).

appeared as a promising method to unveil the mineralogical nature ofthe stripes and the surrounding patina regions. For this purpose, aBruker D8 Discover GADDS μ-XRD2 device equipped with a cobalt X-ray source at the Department of Geosciences, University of Tübingen,Germany, was used. This diffractometer allowed non-destructive, fastand locally resolved phase analysis with high lateral resolution(≈50 μm FWHM) due to the used focusing capillary optic and thelarge 2D X-ray detector covering 40° 2θ and Ψ (Berthold et al., 2009).Moreover, the device settings allow rotation of samples for better crys-tallite statistics (cf. Table 1; Fig. 6b). The same device also served for in-vestigation of corrosion specimens that emerged as a consequence ofmetal sampling.

The measurements with μ-XRD2 were supplemented by surface ex-aminations with an inVia Reflex confocal Raman microscope fromRenishaw (cf. Table 1; Fig. 6c) since diffraction analyses are able to de-tect only crystalline and semi-crystalline phases. Measurements withRaman spectroscopy instead can also determine amorphous com-pounds with high spatial resolution that are often present on archaeo-logical bronze artefacts. Especially tin minerals are known to growamorphously (Nord et al., 1998; Robbiola et al., 1998; Piccardo et al.,2007, 2013). Ramanmicroscopy further offers the possibility to identifyorganic substances, even if infrared spectroscopy (IRS) is not available.Due to reasons of organisation it was not possible to include in situIRS which, in fact, would have been the most suitable non-destructivemethod for examination of potential organic remains.

VÅNTEC-500 detector, 10 ° incidence forin situ analysis on object surface,software: PANalytical BV, PDF 2

Ramanmicroscopy

inVia Reflex confocalRaman microscope,Renishaw

Edge filter, laser wavelength, λ=532 nm(Nd:YAG), CCD detector, spectralrange: 150–4000 cm−1, measuringtime: variable

SEM EVO MA 25, Zeiss Tungsten filament, 12–25 kV, highvacuum (10−5 Pa), no carbon coating,SE and BSE detectors, variableoperating distances, software:SmartSEM V05.03.05

EDX Quantax 400, Bruker AXS Silicon drift detector, 100–250 smeasuring time (mapping: 3600 s),standardless quantification, software:Esprit 1.8.2.2167

3. Results and discussion

3.1. Metal composition

Table 2 shows the elemental composition of the sampled objectparts. The tin contents of the blade and the hilt are identical at 8.8 wt.%(MA-130950 and 51). This is largely in agreement with contemporarysolid-hilted swords and daggers from Nordic period II, but the contentis somewhat lower than the mean value of 11.0 ± 1.7 wt.% (Fig. 7a).Irrespective of this observation, the metals of both parts exhibit compa-rable impurity patterns except for silver and antimony which makes itunlikely that the hilt and the blade derive from the same metal batch.

Nevertheless, the trace elements suggest the use of the same coppersource because they are diagnostic for copper and not for the additionof tin. Compared with other solid-hilted swords (Period II) fromnorthernGermany and Scandinavia, the determined trace elements – es-pecially nickel, arsenic, silver and antimony – are well comparable(Fig. 7b–c). Silver and antimony are usually fairly low (0.013 ±

Fig. 6.Maps showing thedistribution ofmeasuringpoints of μ-EDXRF analysis (a), μ-XRD2 analysis (b) and μ-Raman spectroscopy (c). Numbering corresponds to Tables 3, 5 and 6 (images:C. Dannenberger, Archäologisches Landesmuseum, Schleswig, Germany, D. Berger).

0.013 wt.% and 0.107 ± 0.081 wt.%) while nickel and arsenic show dis-tinctly higher concentrations (0.62 ± 0.32 wt.% and 0.36 ± 0.16 wt.%).This uniform type of metal is also characteristic for other Nordic objectsand had been termed as ‘AsNi’ by Liversage and Liversage (1991) andLiversage (2000). As previous studies showed it was also a commonalloy base in large regions across Europe during the later EBA and MBA(Carpathian Basin, eastern Alps and Bohemia (‘rib ingots’), Switzerland)(Otto and Witter, 1952, Tab. 21; Junghans et al., 1968; Butler, 1978;Rychner, 1988; Rychner and Kläntschi, 1989). From the elementalpattern (including Fe)most likely chalcopyrite oreswith nickel and arse-nic bearing minerals are inferred whose origin has be seen repeatedly inthe eastern Alps and more precisely in the Mitterberg region, Salzburg,Austria (Otto and Witter, 1952, 76; Liversage, 2000, 79–80; Pernicka,2010). A possible provenance from the Slovakian Ore Mountains was,however, also suggested on the basis of lead isotope ratios (Bunnefeld,2014).

The same origin of metal must certainly be considered for typologi-cally and chronologically comparable swords from eastern Germanywhich had been investigated by Wüstemann (2004). They are almostsimilar in chemical composition, but compared with swords fromnorthern Germany and Scandinavia they display somewhat higher sil-ver (0.03 ± 0.03%) and tin contents (12.2 ± 1.9 wt.%) (cf. Fig. 7). Thisobservation is probably owed due to the analytical method used(atomic absorption spectrometry; Wüstemann, 2004, 259).

Table 2Chemical composition of the sampled object parts, normalised to 100%. Zn (0.1%), Se(0.01%), Te (0.005%) and Bi (0.01%) were sought, but their values lay below the detectionlimits of the EDXRF device denoted in brackets. All data is given in wt.%.

Sample no. Origin Fe Co Ni Cu As Ag Sn Sb Pb

MA-130950 Blade 0.16 0.05 0.80 90 0.44 0.050 8.8 0.059 0.02MA-130951 Hilt 0.21 0.05 1.00 89 0.54 0.017 8.8 0.110 0.03MA-130952 Rivet 0.84 0.05 1.46 92 0.82 0.095 4.3 0.060 0.02

The rivet of the Schoolbek dagger clearly differs from the other twoparts regarding its tin content (MA-130952). Only 4.3wt.% tin had beenadded to the copper which also seems to be related to chalcopyrite oresbut certainly derived from another ore load. This is suggested by deviat-ing nickel, arsenic and iron contents (Table 2; Fig. 7b–c). The lower tinconcentration on its part can perhaps be explained from themetallurgi-cal viewpoint.While the blade and the hilt were cast and saw slightme-chanical reworking (hammering) at best, the rivet had to be driven intoholes by extensive hammering in order to join the two parts securely.High tin contents would have been disadvantageous at this state of pro-duction because such bronze work-hardened much faster. Annealingwould thus have been required to soften the bronze again and toallow further deformation.With less tin annealingmust not necessarilyhave been carried out and the rivet attached at once. The low tin contentseems therefore to be adjusted to manufactural requirements otherthan most of the other Nordic swords. Their rivets display equal oreven higher tin concentrations than their blades and hilts (Bunnefeldand Schwenzer, 2011; Bunnefeld, 2014). The same applies tomany cen-tral European solid-hilted swords of Middle and Late Bronze Age dating(e.g. Wüstemann, 2004, 294–328). Therefore, their fabrication wasprobably subjected to other manufactural or metallurgical motifs.

On examination of the drill shavings of the rivet (MA-130952) a highgold content was initially observed. After inspection of the sample withan optical microscope, tiny gold particles were discovered amongst thebronze shavings. Since they cannot be associated with the bronze theymust derive from a former gilding of the rivet which had not beenrecognised during the microscopical inspection of the dagger. The goldappears to be totally embedded into the corrosion material. SEM-EDXanalyses on several gold specimens reveal silver (31.3 ± 0.6 wt.%) andcopper contents (7.5 ± 0.3 wt.%) that are rather high compared withNordic Bronze Age gold objects (Fig. 8; Hartmann, 1982, Tab. 22–24).They are more comparable with gold from Aegean contexts(Hartmann, 1982, 32–33, Tab. 31).

Fig. 7.Histogram of tin contents of Period II dating solid-hilted swords (a) and logarithmic plots of trace elements As–Sb (b) and Ag–Ni (c). All data inwt.% (diagrams: D. Berger with datafrom Liversage, 2000; Wüstemann, 2004; Bunnefeld and Schwenzer, 2011; Bunnefeld, 2014 and this study).

3.2. Phase analysis of the corrosion crusts and the black stripes

As mentioned above, the whole object is covered by a thick patinalayer. It had developed smooth and lustrous on someparts of themetal'ssurface (Type I corrosion after Robbiola et al., 1998), but inmost regionscrusty minerals lie on top (cf. Fig. 4). The latter display various yellow-

Fig. 8.X-ray spectra (a) and chemical data (mean values and standard deviations) obtained fromare shown in the BSE images on the right hand side (b–d). Scale bar applies to all three images

green and brown colours while the smooth layer – commonly referredto as ‘noble’ patina (e.g. Gettens, 1970) – has a bright green and moreeven appearance. This again contrasts with light green tinges of corro-sion products that can be found in pits where the original surface(characterised by the smooth layer) had been disrupted by pitting cor-rosion (Type II corrosion after Robbiola et al., 1998). This corrosion

gold shavings of sampleMA-130952 during examinationwith SEM–EDX. Three shavings.

Fig. 9. Detailed view of the dagger's hilt marking the area for SEM analysis with strips ofcopper (photograph: D. Berger).

Table 3Chemical composition (semi-quantitative) of green patina, black stripes and rivet ob-tained by in situ μ-EDXRF neglecting oxygen and other light elements. Data normalisedto 100% is given in wt.%. Numbering of measuring points corresponds to Fig. 6a.

No. Position P Fe Ni Cu As Sn

F1 Black stripe 7.3 – – 40 – 53F2 Bright green patina 8.5 – – 34 – 57F3 Black stripe 9.0 0.7 1.5 22.5 – 66F4 Bright green patina 8.0 1.4 1.2 20.7 – 69F5 Black stripe 9.6 1.1 1.3 22.3 – 66F6 Black stripe 11.8 0.9 1.0 16.7 0.1 69F7 Light green patina 11.0 0.7 0.5 14.2 0.2 73F8 Light green patina 8.8 0.8 0.6 18.8 – 70F9 Dark patina rivet 13.6 6.9 0.4 19.0 – 60F10 Bright green patina 9.2 0.5 1.1 18.4 – 71

Table 4Semi-quantitative results of analyses with SEM–EDX from four areas covering the greenpatina and the black stripes (cf. Fig. 9). No. S1 obtained from black stripe area, nos. S2–4from ‘noble’ patina and pit corrosion. Data normalised to 100% is given in wt.%.

No. O Na Mg Al Si P S Cl K Ca Fe Cu Sn

S1 35.8 – – 3.3 2.2 2.8 – 0.8 – – 1.8 22.1 31.2S2 35.6 – – 0.9 0.4 4.7 – 0.7 – – – 31.6 26.1S3 27.9 – – 6.4 0.3 12.0 – – 1.9 1.0 – 48.0 2.6S4 23.4 1.5 0.3 1.5 0.2 5.5 0.2 1.2 0.8 1.5 – 63.7 –

phenomenon was perhaps made possible because of localiseddepassivation of small metal areas during burial in soil (Angelucciet al., 1978; Meeks, 1993, 266). Pitting also affected the black stripesshowing oncemore that they can hardly be some kind of inlaidmaterial.

The patina and the stripes had first been examined by μ-EDXRF onthe surface (Fig. 6a). The results are listed in Table 3 and reveal veryhigh tin contents in the different green patina parts with up to 73 wt.%.Besides low copper contents (14–34 wt.%) high concentrations of phos-phorus (8–11 wt.%) are striking which indicate formation of metal phos-phates. Since the dagger belonged to a burial originally containinghumanremains it is likely thatmetal phosphates developed after decompositionof the bones in its vicinity (Geilmann, 1956, 206; Scott, 2002, 243).Fertilisation as implicated by Aner and Kersten (1978, ‘überpflügteGrabhügel’) must, however, also be considered as possible source of thephosphorus.

Phosphorus is observed in the black stripes to the same degree(Table 3). This applies to tin and copper contents as well, and also thetrace elements from the metal substrate (Fe, Ni, As), that can be re-trieved in the patina, show similar concentrations as in the green patinaparts. This observation strongly supports the idea that the black stripesrepresent corrodedmaterial. Simultaneously, the data rules out the pos-sibility of niello inlay and tinning because there are no compositionaldifferences between stripes and surrounding patina. Such differencesin tin and sulphur contents, however, would be expected if the surfacehad been embellished partially with tin or niello (Giumlía-Mair, 2005;Thomas, 2011).

SEM-EDX examination confirms and slightly adds to the findings ofEDXRF. Table 4 shows the composition at four measuring positions in-cluding oxygen that could not be accounted for during XRF measure-ments. No. S1 represents the composition in the area of one blackstripe while measuring no. S2 was carried out at the green patina. Com-pared with each other, the results are almost identical again suggestingcopper and tin corrosion products at both positions. These products ap-pear to have incorporated considerable amounts of soil elements (Al, Si,K, Ca, P). Tin contents are significantly lower with respect to the XRF re-sults, but since the penetration depth of the electron beam is muchlower than of the X-ray beam the differences are easily explainable. Atother positions almost tin-free corrosion products of copper with phos-phorus have been determined showing the heterogeneous character ofthe corrosion crust.

Lacking differences in chemical composition of the black stripes andadjacent green corrosion layer are illustrated by SEM elemental map-ping. The recorded area in Figs. 9–10 covers both surface parts, but thestripes that can clearly be distinguished in the photograph (Fig. 10, bot-tom right) could not be recovered by the element distribution patterns.Although there are some local variations (e.g. C, Al, Cl or Cu content), noelement convincingly correlates with the trend of the stripe. It is there-fore demonstrated again that the green patina and the black stripes arechemically identical.

In order to explain the colour phenomenon by mineralogical differ-ences diffraction and Raman spectroscopic analysis were included. Thecorrosion powder that had been collected during metal samplingyielded stannic oxide, cassiterite, SnO2, as the main compound of theblade's patina during μ-XRD2 measurements. This corrosion product isfrequently observed on bronzes from burial contexts (e.g. Gettens,1970) which is not true for basic copper sulphate, brochantite,CuSO4·3Cu(OH)2, whose presence on the blade was proved by Ramanspectroscopy. Its development on buried copper-base alloys is disad-vantaged the formation of malachite (Scott, 2002, 164–165), the mostcommoncorrosion product in soil environments besides (par)atacamiteand cuprite (Robbiola et al., 1998; Scott, 2002, 106). The latter was theonly compound that could be recognised through μ-XRD2 on the pow-der samples of the hilt and the rivet.

More information on the complex nature of the corrosion crusts isobtained from localised μ-XRD2 analyses. Line scans running over thegreen and black coloured parts and comprising several measuringpoints were carried out at three object regions (Fig. 6b). Judging fromthe diffractograms, the corrosion products in the smooth (‘noble’) pa-tina and the stripes are in general poorly or non-crystallised. This ispointed out by only few and broad diffraction peaks besides a highamorphous background that contrasts with a singular and well-defined peak of the coarse-grained bronze substrate (Fig. 11; Table 5).The broad XRD peaks are attributable to stannic oxide in most caseswhich appears to be present in a poorly crystallised or hydrated form(Abello et al., 1998). This impression is substantiated by the performedμ-Ramanmeasurements (Fig. 6c). A broad and asymmetric band occur-ring in the spectra of several green and almost all black areas around

Fig. 10. Elemental distribution maps in false colours obtained from the surface area with black stripes shown in Fig. 9 and on BSE image and photo bottom right (images: R. Schwab, D.Berger). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

585 cm−1 (Fig. 12a–b) can be assigned to a surfacemode of nanometrictin dioxide rather than to cassiterite with conventional crystal sizes. Thelatter always exhibits its strongest band at 635 cm−1 (volume mode)that is only observed as very weak shoulder here (Zuo et al., 1994).The asymmetric peak in the dagger's spectra is always accompaniedby a second somewhat weaker but broad shoulder between 470 and500 cm−1. This Raman shift also belongs to cassiterite of small crystal-lite size (Zuo et al., 1994). It seems possible that the shoulder and theasymmetric peak at the same positions described from the patinas ofseveral archaeological bronze objects by Piccardo et al. (2013) derivefrom the samemineralogical compound. In contrast to others, however,these authors link the peaks with Sn–O and Sn–C stretching vibrationsrather thanwith a particle size effect (Zuo et al., 1994). This is the reasonwhy they assume an organic character of the identified tin compound('organotin'). As further argument, Piccardo et al. (2013) note Ramanshifts at 1444 and 2930 cm−1 deriving from methyl (-CH3) vibrations.These shifts are present in the spectra of the Schoolbek dagger's patinaas well (Fig. 12a–b). Since the object was, however, treated with ace-tone shortly before examination in order to remove a Paraloid protec-tive layer from restoration, we do not want to exclude the possibilitythat the weak peaks observed here together with another one at790 cm−1 trace back to residual solvent (Dellepiane and Overend,1966).

Some other broad peaks at 3230 and 3450 cm−1 always occur withthe asymmetric peak at 585 cm−1, which provides further mineralogi-cal information. They conform to the reference spectrum in Fig. 13aand to spectra reported in another study of Piccardo and co-workerson archaeological bronzes (Piccardo et al., 2007). They are associatedwith O–H stretching vibrations and in this case with tin corrosion prod-ucts which had incorporated crystallisation water or hydroxyl groups(Bouchard and Smith, 2003). Taking all spectral and chemical

information and the results of previous studies (Zuo et al., 1994;Huang et al., 2002; Piccardo et al., 2007; Bernardi et al., 2009) into ac-count, it is very likely that the dagger's smooth patina and the blackstripes exhibit a superficial layer of hydrated stannic oxide or tinoxyhydroxide (‘Sn(OH)4’/SnO2·nH2O) of nanometric or quasi-amorphous character. According to Robbiola et al. (1998, 2105) andPiccardo et al. (2007, 242), such tin compounds easily develop onbronzes which are rather difficult to identify by means of XRD andRaman spectroscopy.

Unlike the black stripes yielding indications of tin compounds only,corrosion products of copper could be identified from the Raman spectraof the other patina parts. Libethenite, Cu2PO4OH, was observed in boththe bright green (‘noble’) and the light green patina within the pits(Table 6, R10 and R14)). In contrast to the first area, the mineral appearsmuch better crystallised in the latter (Fig. 12b). It explains the highamount of phosphorus detected during XRF and SEM–EDX analysis.Brochantite was also observed, however, almost exclusively in thelight green patina parts (Fig. 12b; Table 6, R3). Atacamite, Cu2(OH)2Cl, and malachite, CuCO3·Cu(OH)2, were found in two cases (R3and R9) alongwith light green pit corrosion, but not in the ‘noble’ pa-tina. XRD analysis proves, however, the presence of malachite allover the various coloured patina parts even within the black stripes(Fig. 11; Table 5). In contrast to the light green pit corrosion, themin-eral is in general poorer crystallised in the black and bright green('noble') areas. This also applies to libethenite that had been verifiedby μ-XRD2 as well (Table 5; Fig. 11). Better crystallisation is thus ageneral feature of the light green patina regions which developedsecondary after breakdown of the 'noble' patina.

A common feature of almost all Raman spectra (of the green patinaand the black stripes) is two bands at ≈1390 and 1590 cm−1 respec-tively with a weak shoulder around 1290 cm−1 (Fig. 12a–b). Second

Fig. 11. Selected diffractograms from black stripes (a) and green coloured patina regions(b) as well as from the rivet (X25). Symbols signify peaks of cassiterite (▲), malachite (▼),libethenite (●), atacamite (■) and α tin bronze (◆). Numbering of measuring points corre-sponds to Fig. 6b and Table 5 (diffractograms: C. Berthold, D. Berger).

order overtones between 2800 and 3100 cm−1 always accompanythese first order shifts. They resemble the so called D and G bands(and their overtones) which are characteristic for carbon materials oflow structural order. This could be graphite itself, but charcoal andsoot (carbon black) show the same spectral features as well (Tuinstraand Koenig, 1970; Montes-Morán et al., 2002; Sadezky et al., 2005).Trąbska et al. (2011) recorded the D and G bands from an opaque,black residue on a Bronze Age ceramic pot from Odense County,Denmark, and linked them to the decorative application of birchtar. Yet, the presence of such carbonaceous material would be diffi-cult to explain for the Schoolbek dagger where the spectral featurewas observed from all over the surface. Another origin of theRaman shifts is thus more likely (e.g. a restoration material), but tothe present state of knowledge we have no convincing explanation,not least because none of conventional corrosion products on ar-chaeological objects matches the spectral features (Frost, 2003;Bouchard and Smith, 2003).

In the dark patina of the dagger's rivet the same carbon-relatedRaman shifts are recorded. In addition, the corrosion crust exhibits

all those peaks attributed to hydrated stannic oxide or tinoxyhydroxide earlier this paper (Fig. 12a, R16). μ-XRD2 analysisshowed no presence of cassiterite in this case, but some peaks thatbelong to coarse-grained atacamite (Fig. 11a, X25). µ-EDXRF analysisand elemental mapping with SEM–EDX illustrates that the patina ofthe rivet is enriched in iron relative to surrounding corrosion (Table3, F9; Fig. 14). The iron content most likely derived from the rivet'siron-rich metal underneath. Since it is a well-understood featurethat patina colour is strongly dependent on incorporation of impu-rity or soil elements such as silicon, phosphorus, chlorine and iron(Robbiola et al., 1998, 2094–2097), we believe that the rivet's darkcolour is linked with the elevated iron (and phosphorus) content.In this respect, the rivet differs from the black stripes which showno significant difference from the green patina (Tables 3 and 4; Fig.10). Nevertheless, subtle variations not recorded here could playsome role in colouration.

4. Conclusions

The Early Bronze Age dagger (Period II) from Schoolbek consists ofbronzewith 8.8wt.% tinwhich is slightly low comparedwith themajorityof contemporary Nordic bronze weapons (Liversage, 2000; Bunnefeld,2014). The impurity pattern of the metal conforms to the ‘AsNi’ typeand indicates the use of copper ores rich in arsenic and nickel. Such chal-copyrite related coppers were frequently used throughout Early NordicBronze Age, but despite intensive research (Liversage, 2000; Ling et al.,2013, 2014; Bunnefeld, 2014) their true provenance remains as yetunidentified.

The results of the chemical and mineralogical analyses of thedagger's patina and the conspicuously black stripe ornamentationallow no definite statement regarding the origin of the stripes andtheir underlying fabrication technique. At least, data from μ-XRD2 mea-surements rule out the possibility of tinning and niello because neithersulphur andmetallic tin, intermetallic copper-tin compounds (ε- and η-phases) nor localised typical tin corrosion products (e.g. cassiterite) andsulphides were observed (Giumlía-Mair, 2001; Thomas, 2011). Thematching chemical composition between black and green coloured ob-ject parts revealing tin surface enrichment further underlines this con-clusion. Together with the visual appearance and surface texture(cracks running from green to black areas) one can conclude that thedifferently coloured surface parts are identical from the physical andchemical point of view. This is demonstrated by phase analyses carriedout with μ-XRD2 and μ-Raman spectroscopy. According to the results,poorly crystallised tin corrosion products prevail in both the blackstripes and the smooth bright green patina which correlates with ahigh tin content (tin enrichment). From Raman spectra most likelynanocrystalline or quasi-amorphous hydrated tin dioxide or tinoxyhydroxide can be identified as major component. Copper detectedin the patina suggests, however, the presence of copper corrosion prod-ucts aswell. μ-XRD2measurements of the smooth green and black partsprovide evidence of poorly crystallised copper corrosion from whichmalachite can sometimes be deduced; in some cases well crystallisedlibethenite andmalachite are present, primarily in regionswhere corro-sion pitting occurred (light green corrosion in pits). Cuprite could not beobserved during in situ μ-XRD2 and μ-Raman analysis, but this is per-haps because the mineral often develops as thin internal layer onbronzes. Analyses carried out on patina powder specimens confirmedthe presence of copper(I) oxide.

Taking together all results from chemical and mineralogical in-vestigation there is no reasonable indication that could explainthe black colour of the stripes. They appear chemically and miner-alogically identical with the surrounding green patina. We there-fore can only speculate at present that the area of the now darkcoloured stripes once was covered by some kind of material thatprovided other corrosion conditions than for the remaining metal.Tar, pitch and resin, for instance, were well-known materials with

Table 5Results from in situ μ-XRD2 analysis on green patina, black stripes and rivet. Numbering of measuring points corresponds to Fig. 6b.

No. Position Identified mineralogical compounds Remarks

X1 Black stripe Cassiterite Poorly crystallised, amorphous backgroundX2 Green patina ‘noble’ Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX3 Black stripe Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX4 Black stripe Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX5 Green patina ‘noble’ Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX6 Black stripe Bronze substrate Amorphous backgroundX7 Light green patina (pit) Malachite, atacamite Well crystallisedX8 Black stripe Bronze substrate Amorphous backgroundX9 Light green patina (pit) Malachite Well crystallised, with preferred orientation or coarse-grainedX10 Light green patina (pit) Malachite Well crystallised, coarse-grainedX11 Green patina ‘noble’ Cassiterite, malachite, bronze substrate Poorly crystallised, amorphous backgroundX12 Green patina ‘noble’ Cassiterite, malachite, bronze substrate Poorly crystallised, amorphous backgroundX13 Green patina ‘noble’ Cassiterite, malachite, bronze substrate SnO2 poorly crystallised, malachite with preferred orientation, amorphous backgroundX14 Black stripe Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX15 Black stripe Cassiterite, bronze substrate Poorly crystallised, amorphous backgroundX16 Green patina ‘noble’ Cassiterite, malachite, bronze substrate SnO2 poorly crystallised, malachite well crystallisedX17 Light green patina (pit) Malachite Well crystallisedX18 Black stripe (green pit) Libethenite, malachite, bronze substrate Well crystallisedX19 Black stripe (green pit) Libethenite, bronze substrate Well crystallisedX20 Green patina ‘noble’ Malachite, bronze substrate Poorly crystallised, amorphous backgroundX21 Green patina ‘noble’ Bronze substrate No compounds identified, amorphous backgroundX22 Green patina ‘noble’ Bronze substrate No compounds identified, amorphous backgroundX23 Black stripe Bronze substrate No compounds identified, amorphous backgroundX24 Light green patina (pit) Malachite, atacamite?, bronze substrate Well crystallised, amorphous backgroundX25 Dark patina, rivet Atacamite Well crystallised, coarse- grainedX26 Dark patina, rivet Atacamite Poorly crystallised, amorphous background

Fig. 12. Representative Raman spectra frommeasurements at the black stripes (a) and the rivet (R16) aswell as the green corrosion products (b). Specifiedwavenumbers are explained inTable 6. Numbering of measuring points corresponds to Fig. 6c and Table 6 (diagrams: M. Keuper, D. Berger).

Fig. 13. Reference Raman spectra of typical copper and tin corrosions species. Spectral data taken from rruff.info except hydrated stannic oxidewhichwas synthesised following a slightlymodified protocol like that described by Lu and Schmidt (2008) (diagrams: M. Keuper, D. Berger).

conserving properties that had often been used for decoration ofbronze surfaces. Because of their pasty consistency these sub-stances could be applied locally enough in order to obtain diminu-tive ornamentations like the Perlband pattern on the dagger. Therunny appearance of the edges of the pattern reinforces thishypothesis.

An elaborated explanation is offered by the gold particles foundwithin themetal shavings of the rivet. They suggest former gold plat-ing on top of the rivet that would be unique in the Early NordicBronze Age. Such gold foil could have been attached to the surfaceby some kind of adhesive, for instance animal glue or again resin orpitch. The black appearance of the patina with its elevated iron con-tent could then be linked with deviating corrosion conditions com-pared with the remaining surface of the dagger. The same may beassumed for the black stripes, although we did not find the sameiron enrichment. At these positions, embossed gold strips thatlooked like Perlbänder could have been applied to the bronze surfacewith glue on their undersides. The resulting patterns would look likethe black stripes today.

Unfortunately, we have no opportunity to prove our theory notleast because no gold foils were excavated along with the dagger.Gold plating was, however, often practised during the Nordic BronzeAge, especially on bronze swords and daggers (Hartmann, 1982;

Jørgensen and Petersen, 1998; Armbruster, 2010). There is thus noreason to refuse that the observed gold particles derive from foilgilding. Whether these foils had been attached with glue or mechan-ically remains an open question. Finally, artificial patination proce-dures as practised in the Mediterranean area at the same time (e.g.Giumlía-Mair, 2013) must also be considered as possible origin forthe black ornamentation. Such technique would, however, be almostimpossible to prove since artificially and naturally grown patinas aremineralogical often identical.

Acknowledgements

This article is an outcome of the research project ‘TechnischeUntersuchungen an älterbronzezeitlichen Vollgriffschwertern ausDänemark und Schleswig-Holstein’ under direction of K.-H. Willroth,Georg August University Göttingen, Germany. The work was financiallysupported by the German Research Foundation (WI 691/19-1). Wewish to thank the Stiftung Schleswig-Holsteinische LandesmuseenSchloss Gottorf — Archäologisches Landesmuseum, Schleswig,Germany, who made the examination of the dagger possible for us.We are also grateful to R. W. Aniol, Archäologisches Landesmuseum,Schleswig, for conducting theX-ray images at Yxlon International, Ham-burg, Germany, and for transferring thefind toMannheim and Tübingen

Table 6Results of μ-Raman spectroscopic analysis on the dagger's surface. The specified Raman shifts can be allocated to hydrated tin dioxide (grey), brochantite (underscored), atacamite(underscored, double), libethenite (underscored, dashed), malachite (underscored, bold), acetone? (regular font), carbonaceous material (bold), unidentified shifts (italic) and aredeveloped strong (s), weak (w), broad (b), as shoulder (sh) or ‘very’ (v). Numbering of measuring points corresponds to Fig. 6c.

No. Position Raman shifts Identified compounds

R1 Black stripe 480(bsh), 585(b), 790(w), 1042(w), 1394, 1445(w), 1582(b), 2853(w), 2931(w), 3058(w), 3226(vb), 3467(vb)

Hydrated tin dioxide, carbonaceous material

R2 Green patina ‘noble’ 243(w), 316(bw), 497(bsh), 583(b), 791(vw), 987(bw), 1290(wsh), 1398, 1448(w), 1599(b), 2856(w), 2929(w), 3058(w), 3239(vb), 3452(vb)

R3 Light green patina (pit) 195(w), 242(w), 364(w), 399, 420(w), 447, 482, 507, 595, 607(w), 624(w), 723(w), 912, 974(s), 12071(w), 1098(w), 1126(w), 1393, 1571(b), 2272, 2280, 2303, 2314, 3249(bw), 3350, 3366,

Brochantite, atacamite, carbonaceous material

3400, 3435, 3564, 3588

R4 Green patina ‘noble’ 494(bsh), 576(b), 790, 986(w), 1045(w) , 1296(wsh), 1393, 1445(w), 1578(b), 2863(w), 2929(w), 3237(vb), 3459(vb)

R5 Green patina ‘noble’ High fluorescence, no peaks detected

R6 Black stripe 178(w), 481(b), 590(b), 790(w), 988(bw), 1049(bw), 1291(wsh), 1359(w), 1390, 1445(w), 1585(bs), 2103(b), 2856, 2933, 3056, 3230(vb), 3422(vb)

R7 Black stripe 279(bw), 610(vb)?, 1388(bw), 1585(bs),2102(vbw), 3234(vbw), 3440(vbw)

Hydrated tin dioxide?, carbonaceous material

R8 Black stripe 178(w), 475(b), 588(b), 790(w), 993(bw), 1048(b), 1293(wsh), 1390, 1443, 1490(w), 1580(bs), 2112(b), 2848, 2925, 3060, 3225(vb), 3453(vb)

R9 Light green patina (pit) 150, 180, 216, 266, 355, 404, 434, 512, 584(b), 631(w), 716(w), 754(w), 793(w), 819, 911, 982, 1045(w), 1057, 1088, 1370, 1391, 1446, 1492, 1581(b), 2850, 2881, 2930, 3330(sh), 3350(s), 3435(s)

Atacamite, malachite, hydrated tin dioxide, carbonaceous material

R10 Yellowgreen patina 163(vw), 193, 227, 248(vw), 298(s), 304(sh), 387(w), 558(vw), 585(bw), 625(vw), 647(vw),790(w), 972(s), 1008(sh), 1018, 1049(w), 1293(w), 1392, 1444, 1590(bs), 1950(bw), 2115(b), 2880(bw), 2930, 3059(w), 3478

Libethenite, hydrated tin dioxide, carbonaceous material

R11 Black stripe High fluorescence, no peaks detected

R12 Green patina ‘noble’ 240(bw), 314(bw), 491(bw), 598(b), 1287(wsh), 1393, 1442(w), 1587(b), 2929(w), 3228(vbw), 3449(vbw)

R13 Green patina ‘noble’ 497(bsh), 593(b), 631(wsh), 790(w), 995(w), 1043(w), 1290(wsh), 1391, 1445(w), 1581(bs), 2860(w), 2928(w), 3222(vb), 3460(vb)

R14 Green patina ‘noble’ 191(w), 225(w), 248(vw), 297, 304(sh), 383(w), 585(vw), 621(vw), 643(vw), 972, 1017, 1048(w), 1394(w), 1585(b), 3474(b)

Libethenite, carbonaceous material

R15 Green patina ‘noble’ 180, 206, 290, 407(w), 434(w), 479(s), 507(s), 573(w), 788(w), 811(w), 1395(w), 1590(b), 2119(b)

Carbonaceous material

R16 Dark patina, rivet 175(w), 491(sh), 585(bw), 789(w), 1045(w), 1294(wsh), 1390, 1442(w), 1584(b), 2867(bw), 2853(vw), 2930(w), 3061(w), 3228(vb), 3470(vb)

Hydrated tin dioxide

R17 Black stripe 2094, 2848(w), 2926(w), 3058(w), 3236(bw), 3464(bw) (high fluorescence)

Hydrated tin dioxide?, carbonaceous material?

two times. J. Lutz and R. Schwab, Curt-Engelhorn-Zentrum für

Fig. 14. Elemental distribution maps in false colours obtained from the rivet and surrounding patina shown in the BSE image and the photo bottom right (images: R. Schwab, D. Berger).(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

for performing the analyses with µ-EDXRF and SEM as well as M.Uckelmann and B. Roberts, Durham University, UK, for reviewing thelanguage.

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