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Keywords:

  • ornaments;
  • beadwork;
  • symbolism;
  • Still Bay;
  • heat treatment;
  • SEM–EDS;
  • Raman;
  • microscopy

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

Colour plays an eminent role in beadwork. Colour modifications are reported on early shell beads from Middle Stone Age sites. However, identifying the colouring agent and demonstrating the intentional nature of the colouring process is not straightforward. Here, we provide analytical data on colour and structural modifications observed on Nassarius kraussianus (Nk) collected in modern thanatocoenoses and on shells of the same species experimentally heated in oxidizing and reductive atmospheres. Comparison with Nk shell beads from the 72 ka old Middle Stone Age levels of Blombos Cave, South Africa, and contextual analysis of other malacological remains from the same levels that were not used as ornaments identify the mechanisms responsible for the change of colour in modern Nk thanatocoenoses and heated shells, and show that although some Nk shell beads were heated, intentional heat treatment of shell beads is not demonstrated.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

The earliest known evidence for the use of personal ornaments consists of perforated marine and estuarine shells found at sites from northern and southern Africa, as well as western Asia, dated to between 100 and 70 ka (Henshilwood et al. 2004; d'Errico et al. 2005, 2008, 2009; Vanhaeren et al. 2006, 2013; Bouzouggar et al. 2007; Bar-Yosef Mayer et al. 2009; Eiwanger et al. 2009). A characteristic trait of these ornaments is that they belong, at each site, to a single species (d'Errico and Vanhaeren 2009). This personal ornament tradition apparently disappears at the end of the last interglacial (c.70 ka). Around 44 ka, beads reappear almost simultaneously in Africa and Eurasia. In Africa, they take the form of ostrich eggshell beads (OESB) (Ambrose 1998; d'Errico et al. 2012; Gliganic et al. 2012); and in Eurasia, of dozens of discrete types identifying regional patterns (Vanhaeren and d'Errico 2006).

Personal ornaments represent a behaviour specific to humans in which items are displayed on the physical body to project meaning that can be interpreted by members of the same and possibly other groups. For this reason, early instances of bead use are commonly interpreted as evidence for the existence of symbolic communication systems created by human societies comparable to ours (Ambrose 1998; McBrearty and Brooks 2000; Vanhaeren and d'Errico 2006; Kuhn and Stiner 2007; Henshilwood and Dubreuil 2011). This argument is not universally accepted and it has been argued that due to their apparent simplicity, the earliest beads do not necessarily reflect symbolic systems and a degree of cognitive sophistication qualitatively comparable to those recorded in present-day and historically known human societies (Wynn and Coolidge 2007; Botha 2008; Klein 2008; Dissanayake 2009; Coolidge and Overmann 2012; Klein and Steele 2013). The first beadworks may, however, be more complex than one could at first sight believe. Bead type is but one factor that plays a role in beadwork codes. Bead size, number, arrangement, colour and location on the body may convey meaning as much as bead type, and these elements have only recently started to be investigated in the earliest beads. Experimental reproduction of use wear recorded on Nassarius kraussianus shell beads from Blombos Cave has recently shown that a clear change in the way of stringing beads and the visual appearance of the resulting beadwork occurred between the lower and upper Still Bay layers (Vanhaeren et al. 2013). Also, the presence of pigment residues (Henshilwood et al. 2004; d'Errico et al. 2005, 2008, 2009; Vanhaeren et al. 2006, 2013; Bouzouggar et al. 2007; Bar-Yosef Mayer et al. 2009; Eiwanger et al. 2009) on many well-preserved shell beads from Middle Stone Age (MSA) and Middle Palaeolithic (MP) sites suggests that colour may have played a role in the way early beadworks conveyed meaning. Dark grey to black colouring, interpreted as due to heating, has been observed on marine shell beads from the MSA at Blombos Cave (d'Errico et al. 2005) and Sibudu (d'Errico et al. 2008) in South Africa, the MP at Grotte des Pigeons, Rhafas and Ifri n'Ammar in Morocco (Bouzouggar et al. 2007; d'Errico et al. 2009; Nami and Moser 2010), on the ostrich egg shell beads (OESB) from the Early Later Stone Age levels at Border Cave (d'Errico et al. 2012), the Later Stone Age of Geelbek in South Africa (Kandel and Conard 2005) and the shell beads from the Upper Palaeolithic/Mesolithic site of Franchthi Cave in Greece (Lange et al. 2008; Perlès and Vanhaeren 2010). It has been suggested (Kandel and Conard 2005; Lange et al. 2008, d'Errico et al. 2009; Perlès and Vanhaeren 2010) that dark beads were submitted to heat treatment for intentional modification of their colour, possibly to enhance their visual impact or convey meaning through colour codes, similarly to what is known from present-day beadwork (Schoeman 1983; Wickler and Seibt 1995). The identification of such technology in the MSA and the MP would come as no surprise. Intentional colour modification of pigment through heating has been proposed for the MP sites of Qafzeh (Godfrey-Smith and Ilani 2004) and Skhul (d'Errico et al. 2010; Salomon et al. 2012). Heat treatment of silcrete for tool production is attested at Pinnacle Point site PP5/6 as early as 164 ka and has been recently identified in the Still Bay levels of Blombos Cave dated to 72 ka (Brown et al. 2009; Mourre et al. 2010; Schmidt et al. 2013). Controlled use of fire was also involved in the production of adhesive to haft tools in Europe and Africa (Mazza et al. 2006; Wadley et al. 2009; Pawlik and Thissen 2011).

However, the question of whether controlled use of fire was applied to change the colour of beads remains open. Burnt or burnt-looking marine shells may result from darkening due to diagenetic processes, accidental burning (loss of shells in an active hearth, hearth affecting pre-deposited sediments containing shells), heating for subsistence activities (cooking of edible species), heat treatment to purposely change the colour of the shells or to facilitate its manufacture, waste burning or a combination of these factors (Hartzell 1991; Claassen 1998; Stiner 1999; Kandel and Conard 2005; Lange et al. 2008; d'Errico et al. 2009; Perlès and Vanhaeren 2010). In this study, we analyse light and dark Nassarius kraussianus (Nk) shell beads from the Still Bay layers of Blombos Cave, modern dead Nk darkened by taphonomic processes, and modern experimentally heated shells of the same species in order to reach a better understanding of the mechanisms responsible for colour change in shells (Lange et al. 2008; Perlès and Vanhaeren 2010), and to establish whether Blombos MSA shell beads were heated, and whether this process was done deliberately to change their colour.

The Archaeological Context

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

Blombos Cave is situated 300 km east of Cape Town (34°25′S, 21°13′E), and 100 m inland from the Indian Ocean (Fig. 1 (a)). Excavations conducted since 1991 (Fig. 1 (b)) by one of us (CSH) have identified a stratigraphic sequence (Fig. 1 (c)) with, from the top to the bottom, 80 cm of Later Stone Age (LSA) deposit, an archaeologically sterile layer of aeolian dune sand, referred to as the BBC Hiatus, and four MSA phases (BBC M1, BBC M2 upper, BBC M2 lower and BBC M3) (Henshilwood et al. 2001a,b, 2011; Jacobs et al. 2003a,b, 2006, 2013; Jacobs 2004; Henshilwood 2008a,b; Henshilwood and Dubreuil 2009, 2011; Villa et al. 2009; Mourre et al. 2010; Thompson and Henshilwood 2011). LSA layers have been radiocarbon-dated to c.2 ka bp. Multiple- and single-grain OSL and TL methods have provided dates for the sterile sand layer lying on top of the MSA layers and for each of the MSA phases (Fig. 1 (c)): c.70 ka for the sand layer, c.78–72 ka for the M1 and upper M2 phase, c.84 ka for the lower M2 phase, and c.100 000 ka for the M3 phase (Jones 2001; Henshilwood et al. 2002, 2011; Jacobs et al. 2003a,b, 2006, 2013; Tribolo 2003; Jacobs 2004; Tribolo et al. 2006; Thompson and Henshilwood 2011). Bifacial foliate points typical of the Still Bay technocomplex interpreted as spear points (Villa et al. 2009; Mourre et al. 2010), bone tools including awls and fully shaped spear points (Henshilwood et al. 2001a; d'Errico and Henshilwood 2007) are cultural markers of the M1 and upper M2 phases. The M2 lower phase is a low-intensity occupation in which no Still Bay cultural markers are found. Ochre pieces, often with traces of utilization, are present in all MSA layers and abundant in the M3 phase (Henshilwood et al. 2009). Ochre pieces engraved with geometric patterns come from the M1, M2 and M3 phases (Henshilwood et al. 2009). Possible engravings on bone (d'Errico et al. 2001; d'Errico and Henshilwood 2007) consisting of parallel striations and sets of joining lines also come from the M1 and upper M2 phases.

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Figure 1. (a) The location of Blombos Cave. (b) A map of the site, with excavated areas indicated in white. (c) The south section of Blombos Cave, showing layers, phases and available OSL, TL and U/Th age ranges for the MSA (modified after Henshilwood et al. 2011).

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Sixty-eight Nk shell beads (Fig. 2 and Table 1) were found in the Still Bay layers (Henshilwood et al. 2004; d'Errico et al. 2005; Vanhaeren et al. 2013). All derive from the M1 phase (layers CA, CB, CC and CD), with the exception of two specimens from the upper M2 phase (layer CF) in an area where slumping of overlaying layers occurred. Fifty-six of the 68 Nk beads published so far (Vanhaeren et al. 2013) were found in seven groups (1–7) of 2–24 beads, each group being recovered in a single square (1 × 1 m) or in two adjacent sub-squares (50 × 50 cm each) during a single excavation day (Table 1). This led us to propose that each group was originally part of a single beadwork item, lost or disposed of during a single event (d'Errico et al. 2005). The 12 remaining shell beads are isolated recoveries unearthed in different sub-squares or at different dates (Table 1).

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Figure 2. Perforated Nassarius kraussianus from the Middle Stone Age Phases M1 and M2 at Blombos Cave. The numbers refer to Table 1.

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Table 1. Contextual and descriptive data on Nassarius kraussianus shell beads from the Middle Stone Age levels of Blombos Cave
Number in Figure 2Date of excavationMSA phaseLevelSquare and sub-squareGroupDorsal side colourVentral side colourSurface featureaBurnt
  1. a

    PDD, post-depositional damage; HC, heat cracks; HD, desquamation; N, none.

  2. b

    Isolated recovery.

 102/02/1999M1CAAF5cbOrangeLight beigePDDno
 209/02/1998M1CAE5b1BeigeBeigeNno
 309/02/1998M1CAE5b1Light orangeBeigeNno
 409/02/1998M1CAE5b1BeigeBeigeNno
 509/02/1998M1CAE5b1BeigeBeigeNno
 609/02/1998M1CAF5a1OrangeBeigeNno
 709/02/1998M1CBAE51Dark orangeBeigePDDno
 809/02/1998M1CBAE51OrangeBeigeNno
 909/02/1998M1CBF5a2Light orangeBeigeNno
1009/02/1998M1CBF5a2Light orangeBeigeNno
1128/01/1999M1CBF6bbDark beigeBeigeNno
1205/02/1999M1CBF6abDark brownLight greyNyes
1306/02/1998M1CBE5bbLight orangeBeigeNno
1405/02/1997M1CA-CCF3bDark greyDark greyHC, HDyes
1506/02/1997M1CA-CCE2bLight orangeBeigePDDno
1611/11/1997M1CA-CCE43Light orangeBeigeNno
1711/11/1997M1CA-CCE43Dark greyDark brownHDyes
1807/02/1997M1CA-CCE44Light brownLight orangeNno
1907/02/1997M1CA-CCE44BeigeBeigePDDno
2007/02/1997M1CA-CCE44BeigeBeigePDDno
2107/02/1997M1CA-CCE44Light beigeBeigePDDno
2207/02/1997M1CA-CCE44BeigeBeigePDDno
2307/02/1997M1CA-CCE44Light orangeBeigePDDno
2407/02/1997M1CA-CCE44Light brownBeigeNno
2507/02/1997M1CA-CCE44Light brownBeigePDDno
2607/02/1997M1CA-CCE44Light brownBeigePDDno
2707/02/1997M1CA-CCE44BrownLight brownHCyes
2807/02/1997M1CA-CCE44OrangeBeigeNno
2907/02/1997M1CA-CCE44BeigeBeigePDDno
3010/02/1997M1CA-CCE45BeigeLight beigePDDno
3110/02/1997M1CA-CCE45BeigeBeigePDDno
3210/02/1997M1CA-CCE45Light brownBeigeNno
3310/02/1997M1CA-CCE45Light brownBeigeNno
3410/02/1997M1CA-CCE45Light brownBeigeNno
3516/02/2000M1CCH5a6Dark brownBrownHC, HDyes
3616/02/2000M1CCH5a6Dark greyDark greyHC, HDyes
3716/02/2000M1CCH5a6Dark greyGreyHC, HDyes
3816/02/2000M1CCH5a6Dark greyGreyHCyes
3916/02/2000M1CCH5c6Dark orangeOrangeHC, HDyes
4006/02/1998M2CFAE4abBeigeLight beigeNno
4109/02/1998M2CFE5bbLight brownBeigeNno
4216/02/2000M1CCH5c6Dark orangeBeigeHC, HDyes
4316/02/2000M1CCH5c6Dark brownDark brownHCyes
4416/02/2000M1CCH5c6Dark beigeBeigeNno
4516/02/2000M1CCH5c6Dark beigeLight orangeHCyes
4616/02/2000M1CCH5c6OrangeLight orangeHCyes
4716/02/2000M1CCH5c6Dark brownBrownHCyes
4816/02/2000M1CCH5c6OrangeOrangeNno
4916/02/2000M1CCH5c6Dark beigeBeigePDDno
5016/02/2000M1CCH5c6Light orangeBeigePDDno
5116/02/2000M1CCH5c6Dark beigeLight orangePDDno
5216/02/2000M1CCH5c6Dark greyDark greyNyes
5316/02/2000M1CCH5c6Dark greyGreyNyes
5416/02/2000M1CCH5c6Dark greyDark brownNyes
5516/02/2000M1CCH5c6OrangeLight orangePDDno
5616/02/2000M1CCH5c6Dark brownLight brownHCyes
5716/02/2000M1CCH5c6Dark brownDark brownHCyes
5816/02/2000M1CCH5c6Dark beigeDark orangeNyes
5916/02/2000M1CCH5c6OrangeLight orangeHC, HDyes
6016/02/2000M1CCH5c6OrangeLight orangePDDno
6124/04/2002M1CCH6b7BeigeOrangePDDno
6224/04/2002M1CCH6b7GreyBrownHC, HDyes
6324/04/2002M1CCH6b7GreyBrownHC, HDyes
6424/04/2002M1CCH6b7Dark beigeOrangeNno
6522/04/2002M1CCH6bbLight orangeOrangePDDno
6615/04/2004M1CCI5cbLight orangeOrangeNno
6717/04/2002M1CAH6bbDark brownBrownHCyes
6820/04/2004M1CDh2I5cbBeigeBeigeHC, HDyes

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

To identify possible causes of colour variation in Blombos Nk, two heating experiments were performed on Nk collected alive, emptied of their meat and cleaned. In the first experiment, 60 shells were equally distributed in 20 porcelain crucibles. In 10 crucibles, shells were sandwiched between leaves and humic soil. Shells were placed in the other 10 crucibles without additions. Step heating in a Vecstar furnace used 10 temperature intervals of 100°C, ranging from 100°C to 1000°C. One crucible from each group was removed from the furnace at each step after the set temperature was reached for 10 min. In the second experiment, three groups of 10 shells were heated in the soil underneath an open fireplace for approximately 1 h. Ten shells were stuck into three capsules of Carpobrotus edulis fruits (South African sour figs), 10 shells wrapped in leaves and 10 buried in the soil. The compact sticky flesh of the sour fig presents the advantage of homogeneously embedding the Nk shells with organic material, avoiding oxidation during the heating process.

Modern unmodified and experimentally heated specimens were prepared for analysis of their chemical and microstructural composition as follows: the lateral aspect of the shell opposite to the lip was abraded with an ESCIL 300 GTL lapping and polishing machine, using an 800 grit paper, until a 3 × 2 mm perforation was created. The perforation edge was then polished using a cloth covered with a fine diamond polishing solution. The resulting surfaces were examined with an optical microscope in reflected light and polishing was repeated until all visible scratches were removed. Shells were then etched in a 0.1 M solution of HCl in distilled water for 3 s, rinsed in distilled water and air dried.

Unetched and etched shells were analysed and photographed using a motorized Leica Z6 APOA microscope equipped with a DFC420 digital camera and a Leica Application Suite equipped with the Multifocus Module. To determine their elemental composition, five etched modern specimens were analysed using a JEOL 840A scanning electron microscope (SEM) equipped with an energy-dispersive X-ray system (EDAX). The modern shells included one unheated specimen from the Goukou Estuary biocoenosis, one unheated specimen from a thanatocoenosis of the same estuary showing a dark grey colouring, and six more specimens from the Goukou biocoenosis: one heated with leaves underneath an open fireplace and five heated in a furnace at temperatures of 400°C, 600°C and 800°C respectively, with leaves and soil, and 300°C and 600°C as such.

To determine the structural composition in a non-destructive way, four unetched modern shells as well as two archaeological Nk—a beige (Table 1, no. 42; Fig. 2, no. 42) and a dark grey (Table 1, no. 53; Fig. 2, no. 53) specimen from the Blombos Still Bay level CC—with fractures showing the internal shell structure, were analysed using a Jobin Yvon T64000 Raman spectrometer operated in triple subtractive mode. The 514.5 nm line of an argon ion laser was used as the excitation source. Backscattered spectra were collected via an Olympus BX40 microscope Raman attachment, and the light dispersed via 1800 lines per millimetre gratings on to a liquid nitrogen–cooled CCD detector. Power for the sample was kept fairly low (1.2 mW) to minimize localized heating effects. A narrow bandpass filter was used to remove laser plasma lines from the spectra. The modern shells included one specimen heated within a sour fig beneath an open fire, and three specimens heated in a furnace, two between leaves and humic soil at 400° and 800°C respectively, the third as such at 300°C.

Three shades of beige, brown, orange and grey were used to account for colour variation on the dorsal and ventral sides of unmodified and experimentally heated modern Nk shells and Nk shell beads from the MSA levels at Blombos Cave (Vanhaeren et al. 2013). For comparative purposes, colour variation was also recorded on a representative sample of malacofauna, not used as ornaments, from the same MSA level (CC) that yielded most of the dark grey Nk shell beads. The comparative sample comes from four 50 × 50 cm sub-squares (H5a H5c, H6b and I5c). The presence of cracks and desquamations was systematically recorded using a Wild M3C microscope equipped with a Coolpix 995 digital camera.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

The colour and shade of the Blombos MSA Nk shells varies between beige, orange, brown and grey, depending on the observed shell aspect (Table 2) and the stratigraphic level in which the shell was found (Table 3). An opposition appears to exist within this variation between shades of beige, orange and light brown on the one hand and shades of darker brown and grey on the other hand, as single shells may display various shades within these two broader groups but never combine shades from both (Table 2). The largest colour variation is observed on the ventral aspect of Nk shells from layer CC (Table 2). This layer also yielded all the brown- and grey-coloured shells, with the exception of one shell from the intermediate level CB that had a light grey ventral and a dark brown dorsal side (Table 2).

Table 2. The contingency table of colour variation on the dorsal and ventral aspect of Blombos Middle Stone Age Nassarius kraussianus shellsThumbnail image of
Table 3. The frequency of colour variation on the dorsal and ventral aspects of Blombos Middle Stone Age Nassarius kraussianus shells according to level and spatial grouping
 Dorsal colour*
L. beigeBeigeD. beigeL. orangeOrangeD. orangeL brownBrownD. brownL. greyGreyD. greyTotal
Layer
CA4116
CAA11
CB1315
CBA112
CC1625262630
CDh211
CF11
CFA11
CA-CC163171221
Total113799381802868
Group
131217
222
3112
414114112
5235
651525624
7123
Isolated323112113
Total113799381702868
 Ventral colour*
L. beigebeigeD. beigeL. orangeOrangeD. orangeL brownBrownD. brownL. greyGreyD. greyTotal
  1. *L, light; D, dark.

Layer
CA66
CAA11
CB415
CBA22
CC46511533230
CDh211
CF11
CFA11
CA-CC116111121
Total334075125413368
Group
177
222
3112
4101112
5145
646211233224
7123
Isolated2621113
Total334075125413368

Colour variations of this type are not observed on modern Nk collected alive, as these display a combination of green, khaki and yellow shades (Fig. 3). Shades of beige, orange, brown and grey are, however, observed in modern Nk thanatocoenoses (Fig. 3). SEM observation and elemental characterization, through energy-dispersive X-ray spectroscopy (EDAX) of a modern shell collected alive and a modern dark grey shell collected dead, indicates that 5–10 μm long particles of pyrite (iron sulphide), migrating through the more porous dorsal surface, probably as a result of bacterial activity (Silverman 1967), are responsible for the black colouring of Nk shells found in modern thanatocoenoses (Figs 4 and 5).

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Figure 3. Colour variation on the dorsal and ventral aspects of Nassarius kraussianus shells collected alive at the Duiwenhoks estuary, Cape Province, South Africa (a), dead at Goukou estuary, Cape Province, South Africa (b), and collected alive and heated after cleaning them at different temperatures in an oxidizing (c) and reducing atmosphere created by sealing the shells with organic material (d).

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Figure 4. A polished and etched section of a dark grey Nassarius kraussianus from a thanatocoenois collected at the Goukou estuary, observed in optical (a) and SEM microscopy (b, c). Note in (a) the presence of black particles penetrating the shell matrix from the dorsal outer surface, on the left of the photograph, and in (b, c) the same particles highlighted by the etching of the shell surface. The arrows indicate the particles analysed by EDAX in Figure 5 (d).

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Figure 5. SEM–EDAX spectra of the surface of a light-coloured unmodified Nassarius kraussianus (Nk) collected alive in the Duiwenhoks estuary (a), a dark-coloured Nk from the Goukou estuary thanatocoenosis (b), Nk heated at 300°C (c) and 600°C (d) in an oxidizing atmosphere, Nk heated at 400°C (e), 600°C (f) and 800°C (g) in a reducing atmosphere created by sealing the shells with organic material, and of a Nk heated within the capsule of a South African sour fig in the soil underneath an open fireplace (h). The peaks of gold, chlorine and sodium are due to etching and metal coating for SEM analysis. The aluminium, sulphur and iron peaks in (b) are most probably due to permineralization, replacement and weathering phenomena affecting shells in thanatocoenoses.

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SEM observation and EDAX analysis did not identify particles of pyrite on MSA Nk shell beads. In addition, Raman spectroscopy of a dark grey MSA shell did not show any evidence for the presence of iron sulphide (peaks at 342 and 379 cm−1; McGuire et al. 2001) indicating that this diagenetic process is not responsible for the colouring of the MSA shells (Fig. 6).

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Figure 6. Raman spectra of (a) four different shell layers in modern unmodified Nassarius kraussianus (1, outer shell layer; 2, 3, middle shell layers; 4a, 4b, inner shell layer); (b) modern Nassarius kraussianus heated at different temperatures in oxidizing (1, 300°C, inner shell layer; 2, 300°C, middle shell layer; 3, 300°C, outer shell layer) and reducing atmospheres (4, 400°C, inner shell layer; 5, 400°C, outer shell layer; 6, 800°C), (c) modern Nassarius kraussianus heated in the soil underneath an open fireplace, and (d: 1a, 1b) a light (Table 1, no. 42) and (d: 2a–c) dark (Table 1, no. 53) coloured Nk from Middle Stone Age level CC of Blombos Cave. The peaks correspond to aragonite (at 205 cm−1), calcite (at 280 cm−1) and carotenoid pigments (at 1133 and 1526 cm−1) present in the shell (Urmos et al. 1991; Withnall et al. 2003).

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Experimental heating of modern Nk produced shells that had colours and shades that differed relative to temperature and atmosphere (Fig. 3). Heating shells in an organic compound within a reductive atmosphere resulted in the shell colour changing into dark grey/black at temperatures above 300°C and produced a glossy black shell surface at temperatures above 500°C. The coating of and penetration into the shell of amorphous carbon released by the leaves and humic soil is probably responsible for the dark-grey/black colouring of the shells at temperatures above 300°C. The glossy metallic appearance of this black coating on shells heated above 500°C can possibly be explained by structural modification of the layer due to release of O and H, previously reported to occur between 450°C and 500°C (Ray et al. 2003). Heating in an oxidizing atmosphere produced larger colour variation. The ventral sides of the shells became light beige, beige, light brown, grey and white at 200°C, 300°C, 400°C, 500°C and 800°C respectively. The black colour observed on the dorsal aspect of shells heated at 300°C to 400°C in an oxidizing atmosphere, and on both shell aspects of shells heated at 400°C to 500°C in a reductive atmosphere, is due to a superficial layer of soot obscuring the underlying shell colour.

Optical and scanning electron microscopy analysis coupled with EDAX and Raman spectroscopy shows that heating Nk causes structural modifications. These analyses reveal that Nk shells are composed of four layers of aragonite crystals with a crossed lamellar layered microstructure (Fig. 6) that degrade into calcite with an irregular prismatic microstructure at temperatures above 300°C (Figs 7 and 8). At 800°C in an oxidizing atmosphere, the calcium carbonate is transformed in calcium oxide (lime), leading to the decomposition of the shell (Fig. 6). At the same temperature in a reducing atmosphere, the calcite undergoes a densification with elimination of porosity and net shrinkage (Fig. 8 (b)). Elemental analysis (Fig. 5) shows that Nk heated in a reducing atmosphere and in the presence of organic material become enriched in carbon, probably in the form of amorphous carbon, which is responsible for the blackening of the shells.

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Figure 7. Optical (a) and SEM (b–f) photographs of the cross-section of an unheated modern Nassarius kraussianus (Nk) shell (a–c), and of a Nk shell heated in a reducing atmosphere at 300°C (d), 400°C (c) and 600°C (f).

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Figure 8. SEM photos of the cross-section of a Nassarius kraussianus (Nk) shell heated at 800°C in a reducing atmosphere (a, b) and of a Nk heated in the soil underneath an open fireplace (c, d).

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Raman analysis shows that dark MSA Nk are composed of calcite and the light-coloured Nk, found in the same stratigraphic levels, of aragonite (Fig. 6). The presence of heat cracks on the surface of dark MSA Nk (Fig. 9) confirms that they have been heated. On the basis of the experimental results presented above, dark MSA Nk shells were heated in a reductive environment at a temperature between 300°C and 500°C, a process responsible for their colouring.

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Figure 9. Heat cracks on experimentally heated Nassarius kraussianus (Nk) shells (a, b) and on Nk from the Middle Stone Age levels of Blombos Cave (c, d).

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Burnt marine shells other than Nk do occur in the same sub-squares in which burnt and unburnt Nk were recovered (Table 4). The proportion of burnt shells increases with their degree of fragmentation. The only difference between Nk and edible species is that unlike Nk, the latter systematically include remains of calcined shells resulting from prolonged heating at high temperature in an oxidizing environment. Like Nk, inedible species and water-worn shell fragments show evidence of burning but not of charring.

Table 4. The frequency of unburnt (‘Unbur.’), burnt (‘Bur.’) and calcinated (‘Calc.’) shells and shell fragments of marine shell species from layer CC. Large fragments are >2 cm, medium fragments between 1 and 2 cm, and small fragments are <1 cm. Residues correspond mainly to microflakes of nacre, probably of Perna perna or Turbo sarmaticus. Incidental shells correspond to small gastropods, and water-worn shells to rounded fragments of undetermined shell species
 Sub-square
H5aH5cH6bI5c
n (%)n (%)n (%)n (%)
Unbur.Bur.Calc.Unbur.Bur.Calc.Unbur.Bur.Calc.Unbur.Bur.Calc.
  1. * >2 cm; † between 1 and 2 cm; ‡ <1 cm; § microflakes of nacre, probably Perna perna and/or Turbo sarmaticus; ¶ small gastropods; ∥ rounded fragments of undetermined shell species.

Perna perna
MNI3384264159
Complete or almost2
Large fragments*1540311
Medium fragments123631044817
Small fragments18152844322681143089
    (65)(33)(2)    (25)(75)
Turbo sarmaticus
MNI413711322033
Complete or almost
Large fragments*4313126224511
Medium fragments3125122273
Small fragments7412146154171313465
    (72)(28) (90)(10)(<1)(24)(26)(50)
Scutalasta argenvillei
MNI1112
Complete or almost2
Large fragments*1
Medium fragments
Small fragments12
Cymbula oculus
MNI10.50.5261
Complete or almost141
Large fragments*3411
Medium fragments72468
Small fragments95582894
Dinoplax gigas
MNI0.50.5110.50.5
Complete or almost   
Large fragments*31
Medium fragments1271
Small fragments19411
Haliotis sp.
MNI111
Complete or almost
Large fragments*2
Medium fragments1
Small fragments2
Donax sarra
MNI11
Complete or almost
Large fragments*
Medium fragments31
Small fragments2
Diloma sp.111
Residues§13050254207n/an/an/an/an/an/a
 (72)(27) (55)(45)       
Barnacles229193
Incidentals4132352
Water-worn101414421
Nassarius kraussianus41271211

Finally, no clear correlation appears when examining the spatial distribution by layer of burnt/unburnt Nk shell beads and the location of hearths in the same layers (Fig. 10). A consistent number of unburnt shells are present in areas in which large hearths were found, particularly in layer CC, and burnt shells occur in sub-squares in which no hearths were recorded. Groups of shells found together in a sub-square associated with a hearth include burnt and unburnt shells. Also, the shell distribution does not fit the hypothesis that shell burning is due to heating produced by overlying hearths.

figure

Figure 10. The spatial distribution of Nassarius kraussianus shell beads recovered at Blombos Cave according to contextual and descriptive data. The dark grey and grey sub-squares indicate the location of hearths and ash areas (data on hearths from Haaland 2012).

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Discussion and Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

Microscopic, Raman and EDAX analyses of experimentally heated Nk shells, dark Nk shells from modern thanatocoenoses and MSA Nk shell beads identified firm criteria to distinguish heated from unheated Nk beads, and showed that the dark colouring of the latter is due to heating. In order to formally demonstrate that the heating of the Nk was the consequence of a deliberate process seeking to produce dark ornaments, one should either find no traces of burning on other shell remains found in the same squares/levels or record on the latter traces of burning in proportions significantly different from those observed on the shell beads. The Blombos data do not conform to the first expectation, and match the second with such a degree of ambiguity that it is difficult, at present, to reach a firm conclusion. The absence of calcined Nk and the substantial presence of calcined shells from other species are consistent with the intention of darkening the Nk in a controlled reductive environment, but cannot be considered, at this stage, as a convincing proof that Nk shells were intentionally heated. The spatial distribution of burnt and unburnt shell beads supports, to some extent, intentional heating of the beads. If MSA Blombos Cave inhabitants wore only unburnt shell beads and hearths were responsible for the blackening of lost shell beads, one would expect to find all burnt shells in the hearths, which is not the case. On the contrary, if MSA Blombos Cave inhabitants wore intentionally heated and unburnt shell beads, one would expect to find black and light shells in and outside hearths. This is due to the fact that hearths may have not been functional at the moment in which light beads were lost. This is the pattern that we observe at Blombos Cave. In order to verify this hypothesis, future research will need to focus on high-resolution analysis of the site formation processes to evaluate the degree to which the spatial distribution of small items such as Nk shells may have been affected by syn- and post-depositional displacement.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. The Archaeological Context
  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References

We thank Gérard Blanc, Pierre Guibert, Kathrin Lange, Marlize Lombard, Renata Garcia Moreno, Catherine Perlès, Alain Queffelec, Ina Reiche and Jörg Schäfer for helpful discussions, Elisabeth Sellier for assistance with the SEM analysis, and Cécile Bossy and Alain Queffelec for giving us access to the muffle furnace and the polishing machine. This research was funded by the European Research Council (ERC) under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 249587, the PROTEA French – South African research programme, the Groupe de Recherche Internationale STAR of the CNRS, the Wenner-Gren Foundation and the Projet Région Aquitaine Origines III. CSH was funded by a National Research Foundation/Department of Science and Technology supported Chair at the University of Witwatersrand, South Africa, and by a joint Norwegian Research Council/South African National Research Foundation grant.

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  5. Materials and Methods
  6. Results
  7. Discussion and Conclusions
  8. Acknowledgements
  9. References
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