Ochre is a common feature at Middle Stone Age (MSA) sites and has often been interpreted as a proxy for the origin of modern behaviour. However, few ochre processing tools, ochre containers, and ochre-stained artefacts from MSA contexts have been studied in detail within a theoretical framework aimed at inferring the technical steps involved in the acquisition, production and use of these artefacts. Here we analyse 21 ochre processing tools, i.e. upper and lower grindstones, and two ochre-stained artefacts from the MSA layers of Porc-Epic Cave, Dire Dawa, Ethiopia, dated to ca. 40 cal kyr BP. These tools, and a large proportion of the 4213 ochre fragments found at the site, were concentrated in an area devoted to ochre processing. Lower grindstones are made of a variety of raw materials, some of which are not locally available. Traces of use indicate that different techniques were employed to process ochre. Optical microscopy, XRD, μ-Raman spectroscopy, and SEM-EDS analyses of residues preserved on worn areas of artefacts show that different types of ferruginous rocks were processed in order to produce ochre powder of different coarseness and shades. A round stone bearing no traces of having been used to process ochre is half covered with residues as if it had been dipped in a liquid ochered medium to paint the object or to use it as a stamp to apply pigment to a soft material. We argue that the ochre reduction sequences identified at Porc-Epic Cave reflect a high degree of behavioural complexity, and represent ochre use, which was probably devoted to a variety of functions.

Funding: Research by DR was funded by the Generalitat de Catalunya (Ajuts per a la contractació de personal investigador novell, FI-DGR), the Doctoral Research scholarship Programme of the Martine Aublet Foundation, and the Eiffel Excellence Scholarship Programme of the French Ministry of Foreign Affairs and International Development. Research by AP was funded by the Generalitat de Catalunya (Beatriu de Pinós postdoctoral fellowship, 2014 BP-A 00122). This research was conducted with the financial support of the Wenner-Gren Foundation (Gr. 8786), the LaScArBx research programme, supported by the ANR ANR-10-LABX-52, and the European Research Council Advanced Grant, TRACSYMBOLS No. 249587 awarded under the FP7 program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2016 Rosso et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

Evidence for systematic exploitation of ochre has been reported at several Middle Stone Age (MSA) sites from North and South Africa [1–9], as well as Mousterian and Châtelperronian sites in Europe [10–16] and the Middle East [17,18]. Here, we define "ochre" as rocks containing a high proportion of iron oxides, often mixed with silicates and other mineral substances, which are red or yellow in colour, or are streaked with such shades [19]. The use of these iron-rich minerals has often been interpreted to reflect high cognitive functions and symbolic thinking [1,3,9,20,21]. However, this view has been contested, as some evidence indicates that ochre may have also been used for functional purposes [19,22–29].

With the exception of a few sites in Sub-Saharan and North Africa, information on the way ochre was selected, processed, stored, and used is still scarce. This complicates the detection of behavioural similarities and what such behaviours may represent in terms of cognition and cultural complexity. Analysis of artefacts stained with ochre and involved, to varying degrees, in the treatment, storage and use of ochre have been conducted on Middle Stone Age and Mousterian knapped lithics (possibly stained by hafting or ochre processing) [27,28,30,31], shell containers [5,16], and personal ornaments [16,32–35]. Such analyses remain few in number due to the rarity of these objects in the archaeological record and the methodological challenges associated with the analysis of microscopic ochre residues.

In order to gain a better understanding of ochre processing and use in the East African MSA, and to evaluate the degree of behavioural complexity reflected by these activities, we present the first detailed analyses of ochre processing tools (OPT), namely upper and lower grindstones, and ochre-stained artefacts (OSA), consisting of stained pebbles and cobbles, recovered by Kenneth D. Williamson [36] in 1975 and 1976 from the MSA layers of Porc-Epic Cave (Dire Dawa, Ethiopia). The interest of these objects lies in their number, variety, excellent state of preservation of surface modifications, consistent presence of ochre residues, and the fact that they are associated with the most abundant collection of ochre pieces ever found at a Palaeolithic site [37]. In addition, research on the spatial distribution of pigment lumps and ochre processing tools has shown that concentrations of these artefacts are present at the site. The location of these concentrations shift through time, thereby offering the possibility of documenting temporally changing behavioural patterns. Thus, one has a unique opportunity to comprehensively reconstruct technical processes involved in the treatment and use of ochre in an area of the African continent virtually unexplored in this respect and for a key period for hominin cultural and biological evolution.

Early ochre processing tools and ochre containers Ochre processing tools, ochre containers, and ochre-stained artefacts from MSA and Middle Palaeolithic contexts are, in most cases, only briefly mentioned in the literature and have rarely been analysed in detail [5]. Additionally, they often present a poor state of preservation, with little or no trace of residue. In Africa, the earliest tools that may have been used to process ochre are found in early MSA contexts. At the site of GnJh-15, in the Kapthurin formation, Kenya, possible grindstones stained with ochre were found in layers dated to 500–284 ka [8,38]. A quartzite cobble with ochre stains, interpreted as an ochre processing tool, was recovered at Twin Rivers, in Zambia [1,39]. At Sai 8-B-11, Sai Island, Sudan, sandstone mortars shaped by knapping and small chert pebbles with residues of red and yellow ochre are reported from levels dated to ca. 180 ka [40,41]. At Blombos Cave, South Africa, two toolkits used for the production and storage of ochre-rich compounds were recovered from layers dated to 100 ka [5]. These toolkits include two large abalone shells containing an ochre-rich compound composed of ochre powder and microflakes of two types of ferruginous siltstone (composed of quartz, hematite, muscovite/illite, and goethite), fragments of crushed trabecular bone, crushed compact bone, charcoal, and fragments of grindstones made of quartz, quartzite and silcrete. The two shells were found in close proximity to utilized ochre lumps, bones, as well as upper and lower grindstones. Two rhyolite grinders, and a faceted quartz mortar with ochre residues are reported from Ngalue Cave, in Mozambique, in levels dated between >42 ka and 105 ka. One of these shows the presence of possible starch residues [42,43]. At Klasies River, South Africa, a piece of tabular quartzite, battered on one edge, and bearing possible ochre residues, was found in shelter 1A in levels dated to 80–65 ka [44]. Two ochre-stained upper grindstones (one of which is quartzitic sandstone) and several ochre-stained artefacts are reported from middle–late MSA layers at Die Kelders, South Africa [45–47]. Nine backed tools with ochre on the cutting edges were found at Rose Cottage Cave in layers dated to 68–60 ka [31]. At Sibudu, South Africa, the presence of cemented hearths with ochre powder deposits was observed in layers dated to ca. 58 ka, suggesting that they were used as receptacles for ochre powder or as work surfaces on which grindstones were placed during the processing of ochre pieces [48]. Sandstone slabs, dolerite and hornfels tools with yellow or red residues were also recovered at the site [49]. Scrapers and flakes from late MSA layers with ochre residues on their working edges were interpreted as ochre processing tools [26,28,30]. Two diorite chunks and one diorite cobble with pigment residues that suggest grinding or scoring were found in MSA layers (dated to ca. 119–46 ka) at Yserfontein, South Africa [50]. Six broken pieces of quartzite with ochre residues, interpreted either as bearing paint or as "ochre-smeared slabs of non-artefactual stone" were found in MSA layers at Umhlatuzana Rock Shelter, South Africa [51]. At Bushman Rockshelter, South Africa, several broken grindstones, some with traces of ochre, were found both in MSA (ca. 47–43 14C kyr BP) and LSA levels [52–55]. One sandstone fragment coated with red ochre was found in the MSA layers of Sehonghong, Lesotho [56]. Grindstones stained with ochre were also reported in Botswana, in the late MSA levels of ≠Gi [8]. In Mali, MSA levels of Songona I, dated to 55–35 ka, yielded sandstone artefacts with smoothed areas interpreted as possible ochre processing tools [57,58]. In Zimbabwe, granite slabs with ochre residues are found at Nswatugi and Pomongwe, in late MSA layers [8,59,60]. In East Africa, two flakes with traces of red ochre and one small ochre-stained lower grindstone were found at Enkapune Ya Muto, Kenya [61]. Grindstones have been recovered from other East African late MSA sites where the presence of ochre lumps is reported. However, it is not specified whether these tools bear ochre residues. Cases in point are Mochena Borago Rockshelter, in Ethiopia [62], and Mumba and Nasera rockshelters, in Tanzania [38,63]. In the Middle East, a possible ochre processing tool was found at Qafzeh cave, Israel, in layers dated to 100–90 ka. A centripetal recurrent mode Levallois core displays a concentration of ochre residues in the concavity of a large negative scar. This is interpreted as a core recycled into an ochre receptacle [18]. Glycymeris shells found in the same layers [64] have been interpreted as possible receptacles for ochre by some authors [35], or as ochered shell beads by others [65]. In Europe, grindstones found in early Mousterian levels (250–200 ka) at Beçov I, Czech Republic, were apparently used to process pigments [66,67]. In Germany, sandstone slabs with modifications attributed to the grinding of mineral material are reported in late middle Pleistocene levels of Rheindahlen [13,68]. At Cioarei-Boroşteni Cave, Romania, eight concave fragments of stalagmites and stalagmite crusts, showing ochre residues in concave areas, as well as scraping and polishing marks, were found in levels dated to ca. 52–45 ka BP are thought to be ochre containers [69–71]. In the same site was found an apparently painted geode, in levels dated to ca. 48 ka BP [70]. Grinding stones possibly used for mineral processing were also reported at Barakaevskaya Cave, in southern Russia [72,73]. In Spain, grindstones possibly used for ochre processing are reported from Mousterian levels at Cueva del Castillo and Cueva Morín [8,54], but the presence of ochre residues on these objects is not documented. At Cueva de los Aviones, Spain [16], in levels dated to ca. > 50 cal kyr BP, ochre residues were found on the inner side of an upper valve of a Spondylus gaederopus shell and have been interpreted as evidence for use of this shell as an ochre container. A use as ochre containers was also suggested for a Callista chione and two lower valve fragments of Pecten maximus. However, it has been argued that S. gaederopus upper valves have a limited volumetric capacity, which is insufficient for use for ochre processing and storing. A perforated Glycymeris shell with red residues identified as hematite was also found. In addition to these finds, an unmodified ancillary metatarsal of Equus sp. with orange residues on one extremity is reported from this site, suggesting that it functioned as a tool for the preparation or application of ochre [16]. At Pech-de-l'Azé I, France, a sandstone slab with black residues and diagnostic use-wear of grinding was found in Mousterian of Acheulean Tradition (MTA) levels, which are older than 43 cal kyr BP [15,74–76]. A limestone slab with pigment residues was also found in MTA levels at Le Moustier, France [12]. The absence of smoothed areas or homogenous pigment stains, suggested to the excavator that it was a painted rock rather than a grindstone. At Grotte de Néron, also in France, a limestone block interpreted as an ochre receptacle, possibly modified along its base by knapping and characterized by a central pit with ochre residues, was found in late Mousterian context [53,77]. The Châtelperronian levels of Grotte du Renne, France, have yielded an abundant collection of grindstones with red and black residues [14,53,78]. The earliest evidence of ochre processing tools in Sahul may date back to more than 50 kya. At Madjedbebe (Malakunanja II), in northern Australia, grindstones that sometimes show streaks of red residues were recovered in layers dated to ca. 55–45 ka [79]. At Nauwalabila I, also in northern Australia, levels dated to ca. 53 ka yielded a grindstone made of quartzite with flaked edges and abrasion marks on one face [80,81]. It shows no traces of pigment, but was stratigraphically associated with a large piece of worked hematite. Ochre processing tools and ochre containers become ubiquitous at LSA and Upper Palaeolithic sites, including rock art contexts [53,82–84]. However relatively few artefacts and associated residues have been subjected to detailed analyses.