Project

Heat treatment of stone raw materials

Goal: I am currently working on heat treatment of silcrete in the Middle Stone Age of southern Africa and of flint in the Upper Palaeolithic and Neolithic of Europe. My work is at the interface of prehistoric archaeology and mineralogy.

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K.G. Nickel
added 3 research items
The study of lithic raw material quality has become one of the major interpretive tools to investigate the raw material selection behaviour and its influence to the knapping technology. In order to make objective assessments of raw material quality, their mechanical properties (e.g., fracture resistance, hardness, modulus of elasticity) should be measured. However, such comprehensive investigations are lacking for the Palaeolithic of Kazakhstan. In this work, we investigate geological and archaeological lithic raw material samples of chert, porphyry, and shale collected from the Inner Asian Mountain Corridor (henceforth IAMC). Selected samples of aforementioned rocks were tested by means of Vickers and Knoop indentation methods to determine one aspect of their mechanical properties: their indentation fracture resistance (a value closely related to fracture toughness). These tests were complemented by traditional petrographic studies to characterise the mineralogical composition and evaluate the level of impurities that could have potentially affected the mechanical properties. The results show that materials, such as porphyry, previously thought to be of lower quality due to the anisotropic composition and coarse feldspar and quartz phenocrysts embedded in a silica rich matrix, possess fracture toughness values that can be compared to those of chert. Thus, it appears that different raw materials cannot be distinguished from the point of view of indentation fracture resistance, calling for detailed supplementary analyses of different fracture properties. This work also offers first insight into the quality of archaeological porphyry that was utilised as a primary raw material at various Middle and Upper Palaeolithic sites in the IAMC.
The study of lithic raw material quality has become one of the major interpretive tools to investigate the raw material selection behaviour and its influence to the knapping technology. In order to make objective assessments of raw material quality, we need to measure their mechanical properties (e.g., fracture resistance, hardness, modulus of elasticity). However, such comprehensive investigations are lacking for the Palaeolithic of Kazakhstan. In this work, we investigate geological and archaeological lithic raw material samples of chert, por-phyry, and shale collected from the Inner Asian Mountain Corridor (henceforth IAMC). Selected samples of aforementioned rocks were tested by means of Vickers and Knoop indentation methods to determine the main aspect of their mechanical properties: their indentation fracture resistance (a value closely related to fracture toughness). These tests were complemented by traditional petrographic studies to characterise the mineralogical composition and evaluate the level of impurities that could have potentially affected the mechanical properties. The results show that materials, such as porphyry possess fracture toughness values that can be compared to those of chert. Previously, porphyry was thought to be of lower quality due to the anisotropic composition and coarse feldspar and quartz phenocrysts embedded in a silica rich matrix. However, our analysis suggests that different raw materials are not different in terms of indentation fracture resistance. This work also offers first insight into the quality of archaeological porphyry that was utilised as a primary raw material at various Upper Palaeolithic sites in the Inner Asian Mountain Corridor from 47-21 ka cal BP. PLOS ONE PLOS ONE | https://doi.org/10.1371/journal.pone.
Birch tar was the first adhesive produced by humans. Its study has consequences for our understanding of human evolution or the development of specialised craftsmanship. One of the better‐documented birch tar making methods is the ‘double‐pot’ technique where two containers are used, one containing bark, the other collecting the tar. Birch tar made with double‐pots has low viscosity and bad adhesive properties. To obtain a usable adhesive, it must be reduced in volume by cooking. We investigate the evolution of tar’s mechanical properties during cooking. We use lap‐shear tests to investigate strength, stiffness and failure behaviour. We found that tar must be cooked for one hour and a half to acquire adhesive properties. When cooked for longer, strength and stiffness increase over a 30 min lasting time span. Cooking for even longer, beyond this 30 min window, produced a substance that could no longer be used as an adhesive. The implications are that tar cooking requires a high level of skill because specific signs indicating the desired properties must be recognised during the process. Tar cooking constitutes a supplementary investment in time and resources and appears to be an activity associated with a certain degree of risk.
K.G. Nickel
added a research item
Birch tar is one of the oldest adhesives known in human history. Its production has been discussed in the framework of early complex behaviours and sophisticated cognitive capacities. The precise production method used in the Palaeolithic remains unknown today. Arguments for or against specific production pathways have been based on efficiency or process complexity. No studies have addressed the question whether birch tar made with different techniques is more or less performant in terms of its properties. We therefore investigate the adhesive performance of birch tar made with three distinct methods: the open-air condensation method and two variations of underground structures that approximate the double-pot method in aceramic conditions. We use lap-shear testing, a standard mechanical test used for testing the strength of industrial adhesives. Tar made in 1 h with the condensation method has a shear strength similar to, although slightly higher than, tar made underground if the underground process lasts for 20 h. However, tars from shorter underground procedures (5 h) are significantly less strong (by a factor of about 3). These findings have important implications for our understanding of the relationship between the investment required for Palaeolithic birch tar production and the benefits that birch tar represented for early technology. In this regard, the simple and low-investment open-air condensation method provides the best ratio.
Patrick Schmidt
added a research item
The European Solutrean has yielded one of the oldest chert assemblages documenting intentional heat treatment. Many questions about social and economic factors arise in this context. For example, did Solutrean knappers heat-treat only specific types of chert? Were specific heating techniques adapted to specific types of chert? We investigate these questions by analyzing 43 chert artifacts from Laugerie-Haute. We use near-infrared spectroscopy to identify heat-treated artifacts made from four different types of chart, and to estimate their heating temperatures. We identify 13/37 test pieces as heat-treated. Only 5 of these 13 pieces (38%) would have been identified as such macroscopically (based on gloss contrast). The overall magnitude of gloss, cannot be used to identify heat-treated chert artifacts. These findings have implications for future studies of the prevalence of heat-treated artifacts in Solutrean assemblages. Visual identification based on surface gloss is likely to only recognize the smaller part of heat-treated artifacts.
K.G. Nickel
added 2 research items
Ifri n'Etsedda is a rock shelter in northeastern Morocco, at the southern flank of the Kebdana Mountains. The site was discovered in 2008 and excavated during three field campaigns between 2012 and 2014 and provides Epipalaeolithic and Neolithic deposits. From the Neolithic deposits we analyzed 30 pottery sherds by macroscopic determination, Infrared (IR) Spectroscopy, polarizing microscope and thermodilatometric analysis (TDA) in order to determine information about the clay and natural inclusions in the clay and temper materials used as well as the manufacturing process. In addition, we identified and sampled 12 potential raw materials sources during one survey campaign in 2015, which were analyzed by IR spectroscopy as well. We identified 8 groups by means of inclusions in the clay: extrusive igneous rocks (EIR), EIR with amphiboles, EIR with fragments of micro quartz, granite, dolerite or gabbro with fibrous quartz and unidentified spherulites, schist, organic limestone and grog. Trends in the succession of clay traditions could be identified. Most of the pottery was fired in an oxidizing atmosphere and were placed upside down in a pit. Burning temperatures range between 800 and 1000 °C. No local clay source seems to have been used for the pottery production at Ifri n'Etsedda. With regard to the small artifact samples size as well as the incomplete raw material survey, further studies are needed to shed more light on Ifri n'Etsedda pottery.
In archaeology, heat treatment of stone is the process of “making” a new material for tool production. Its invention in the African Middle Stone Age was an important step in the evolution of transformative technologies and the cultural evolution of early humans in general. Although the chemical and crystallographic transformations in silica rocks, the only material class heat-treated in the Stone Age, begin to be well known, many of the mechanical transformations and their chemical origins remain a subject of controversy. The difference between different silica rock categories is also only poorly understood. In this paper, we investigate the thermally induced changes of three mechanical properties in the two silica rock types chert and silcrete: fracture strength, indentation fracture resistance (approximating fracture toughness) and elastic modulus. These tests are complemented by statistical analyses (Weibull modulus) and a quantitative fracture surface analysis. The results show that heat treatment transforms these silica rocks in terms of their fracture toughness and the uniformity of fracture. A comparison with published data on the structural transformations in the same samples identified the loss of chemically bound water and subsequent defect healing to be the chemical mechanism behind these mechanical transformations. These findings have important implications for the study of the interactions between chemical and structural processes and the mechanics of natural rocks or ceramics.
Patrick Schmidt
added a research item
The Still Bay, with its carefully crafted bifacial points, is one of the most enigmatic technocomplexes in the later Middle Stone Age of the southern African subcontinent. Heat treatment of silcrete has been documented in the Still Bay but it has recently been suggested that its application was restricted to the later stages of the production of points. This would confer a special role to heat treatment in the Still Bay if compared to the following Howiesons Poort technocomplex. In this paper, we analyse the silcrete assemblage from Hollow Rock Shelter for heating proxies to provide a first picture of the prevalence of heat treatment in the Still Bay and to investigate whether points were treated differently in terms of heat treatment than other end-products. Our results show no evidence of later-stage heat treatment but, on the contrary, comprehensive data to support heat treatment in an early stage of reduction. Relatively less silcrete was heated in the Still Bay than in later Howiesons Poort, revealing technological differences between both phases. We found a significant number of silcrete pieces that exploded during heat treatment and were still knapped afterwards, indicating a heating process that involved fast heating rates. We also found that points were not treated differently than the other end-products. These findings have implications for our understanding of the fabrication of bifacial points and the Still Bay chaîne opératoire in general.
Patrick Schmidt
added a research item
The laurel-leaf points of the Volgu cache found in eastern France rank among the most remarkable examples of skilled craftsmanshipknown from the Solutrean period of the Upper Palaeolithic. In addition to pressure flaking, heat treatment may have helped in the making of the points, as both have been previously described in association withSolutrean assemblages. This study presents the results of an infrared spectroscopic analysis of seven artefacts from the Volgucache conducted to test this assumption. The findings show that heat treatment was not universally applied to this particular tool type, meaning that we must rethink the reasons why such a technique was used.
Patrick Schmidt
added a research item
The Early Mesolithic of southwestern Germany, the so-called Beuronian (9600–7100 BC), is a period of important transformations in the way people lived, in their subsistence and in the stone tools they produced. One of the perhaps most spectacular re-inventions of that time is heat treatment of stones prior to their manufacture into tools. Although heat treatment has been understood as one of the defining characteristics of the Beuronian of southwestern Germany, and although its existence has been known for almost 30 years now, relatively few systematic studies on it are available. In this paper, we present such a study, aiming to shed light on two questions: (1) what technique and heating parameters were used in the Beuronian and (2) how reliable are the macroscopic proxies traditionally used to identify heat treatment in this context? We investigate these questions using a non-destructive archaeometric technique for measuring past heating temperatures of heat-treated stones and a quantitative surface roughness analysis aiming to understand the relations between surface aspect and heat treatment. These methods are applied to 46 Jurassic chert artefacts from the site Helga-Abri located in the Swabian Alb region of southwestern Germany. Our results document that an opportunistic low-investment procedure was used to heat stone, probably relying on the use of the above-ground part of regular camp-fires. We also found that the traditionally used macroscopic criteria, such as colour and surface gloss, cannot be unambiguously used to identify heat treatment in assemblages made from Jurassic chert. These findings have important implications for our understanding of the Beuronian lithic chaîne opératoire in terms of the investment in time and resources necessary, and for the refinement of archaeological techniques used to identify heat treatment in the Mesolithic of the Swabian Alb.
Patrick Schmidt
added a research item
The earliest evidence of flint and chert heat treatment was found in the ~21.5–17 ka old European Solutrean culture. The appearance of pyrotechnology as part of the production of stone tools has important implications for our understanding of Upper Palaeolithic technological evolution and the specific adaptations during the last glacial maximum in Europe. However, the techniques and procedures used to heat-treat rocks during the Solutrean remain poorly understood. No direct archaeological evidence has so far been found and the most promising approach is to understand these techniques by determining the parameters with which flint and chert were heated at that time. In this study, we investigate the heating temperature of 44 heat-treated laurel-leaf points from Laugerie-Haute, using a non-destructive technique based on infrared spectroscopy. Our results document that most of the artefacts were heated to a narrow interval of temperatures between 250 °C and 300 °C. This indicates a standardized technique that allowed to created similar conditions during successive heating cycles. The implications of these results for our understanding of the technical complexity during the Solutrean must be discussed in the light of different heating techniques used at different places and periods.
Patrick Schmidt
added 2 research items
Heat treatment of silcrete was a major innovation in the southern African Middle Stone Age (MSA). It allowed for the first time to improve materials for tool knapping, and it may have represented an important step in the perception of natural resources as modifiable objects. Recognising heat treatment in silcrete assemblages is therefore a crucial step for archaeologists working on the MSA. Two different methods, gloss analysis and visual estimation of surface roughness, have so far been used to identify heat treatment. Although both methods have advantages in specific situations, only visual heating proxy classifications allow to count heated vs. not-heated artefacts in assemblages. However, no objective independent data on the reliability and reproducibility of visual classifications are available today. This paper presents a new and promising non-destructive way to measure surface roughness and to verify the reliability of visual classifications: the replica tape method. The results show a rather good reliability of visual classifications: only few pieces are misclassified (n = 3), and the results of both replica tape measurements and visual classification agree within a 3% error range. These results also lay out the foundations for future developments of replica tape measurements to make it a stand-alone method for identifying heat treatment within silcrete assemblages.
Infrared spectrum of WK-13-13 between 5500 cm-1 and 2000 cm-1 compared to a kaolinite reference spectrum in the region of fundamental OH stretching vibrations. Note that the broad H2O absorption band of WK-13-13 is overgrown by two sharp OH stretching vibrations at 3695 cm-1 and 3620 cm-1 on its high frequency side. The bands correspond to the two OH groups of the 1:1 clay that is present as an impurity in this silcrete sample. (TIF)
Patrick Schmidt
added a research item
Heat treatment was one of the first transformative technologies in the southern African Middle Stone Age (MSA), with many studies in the Cape coastal zone of South Africa identifying it as an essential step in the preparation of silcrete prior to its use in stone tool manufacture. To date, however, no studies have investigated whether heat treatment is necessary for all silcrete types, and how geographically widespread heat treatment was in the subcontinent. The aim of this study is to investigate experimentally whether heat treatment continued further north into the Kalahari Desert of Botswana and northernmost South Africa, the closest area with major silcrete outcrops to the Cape. For this we analyse the thermal transformations of silcrete from both regions, proposing a comprehensive model of the chemical, crystallographic and ‘water’-related processes taking place upon heat treatment. For the first time, we also explore the mobility of minor and trace elements during heat treatment and introduce a previously undescribed mechanism—steam leaching—causing depletion of a limited number of elements. The results of this comparative study reveal the Cape and Kalahari silcrete to respond fundamentally differently to heat treatment. While the former can be significantly improved by heat, the latter is deteriorated in terms of knapping quality. These findings have important implications for our understanding of the role of fire as a technical solution in MSA stone tool knapping, and for the extension of its use in southern Africa. Silcrete heat treatment—at least in the form we understand it today—may have been a strictly regional phenomenon, confined to a narrow zone along the west and south coast of the Cape. On the basis of our findings, silcrete heat treatment should not be added as a new trait on the list of behaviours that characterise the MSA of the southern African subcontinent.
Patrick Schmidt
added 15 research items
At the end of the fifth millennium bc, the development of a specialized lithic industry in the Chassey societies of south-eastern France and its dissemination as far as Catalonia and Tuscany attest to important socio-economic changes in the Mediterranean Neolithic societies. The lithic production was made on barremo-bedoulian flint that was heat-treated to improve the sharpness of the tools produced. Microscopic observations of archaeological and geological, heated and unheated barremo-bedoulian flint samples allowed us to highlight the heat-induced formation of fluid inclusions. Microthermometry analyses showed that these inclusions contain pure H2O, most probably resulting from the dehydration of length-slow (LS) chalcedony and the closure of narrow pores, according to the model proposed by Schmidt et al. (). Our results enable us to estimate the heating temperatures used by Chassey artisans to ≈ 230°C. We also propose the 'pressure cooker' model to explain the migration of liquid water in flint nodules heated to 230°C. Then, we discuss the ability of a particular type of flint to be heat-treated, and hence its value for Neolithic society, which depends on: (1) the amount of LS chalcedony that ensures the water release at relatively low temperature; and (2) on the total volume of porosity available to store the dehydration water.
Heating stone to enhance its flaking qualities is among the multiple innovative adaptations introduced by early modern human groups in southern Africa, in particular during the Middle Stone Age Still Bay and Howiesons Poort traditions. Comparatively little is known about the role and impact of this technology on early modern human behaviors and cultural expressions , due, in part, to the lack of comprehensive studies of archaeological assemblages documenting the heat treatment of stone. We address this issue through an analysis of the procedure used for heating and a technological analysis of a lithic assemblage recovered from one Howiesons Poort assemblage at Klipdrift Shelter (southern Cape, South Africa). The resulting data show extensive silcrete heat treatment, which adds a new dimension to our understanding of fire-related behaviors during the Howiesons Poort, highlighting the important role played by a heat treatment stage in the production of silcrete blades. These results are made possible by our new analytical procedure that relies on the analysis of all silcrete artifacts. It provides direct evidence of a controlled use of fire which took place during an early stage of core exploitation, thereby impacting on all subsequent stages of the lithic chaıˆnechaıˆne opé ratoire, which, to date, has no known equivalent in the Middle Stone Age or Middle Paleolithic record outside of southern Africa.
Elands Bay Cave (EBC) is a key South African site allowing discussion of technological change and adaptations that occurred from the Upper Pleniglacial to the Holocene. In 2011, we set out a new field campaign aiming to clarify the nature and chronology of the earliest Robberg occupations at the site, a technocomplex whose appearance closely relates to the Last Glacial Maximum. Our results document the appearance of the Robberg technology at ca. 19 398–18 790 cal BP, succeeding a phase commonly referred to as the Early Later Stone Age. In this paper, we further develop the definition of the Robberg by providing a technological and functional study of the MOS1 lithic assemblage at EBC, dated to 14 605–14 278 cal BP. Our results show that EBC occupants dominantly selected local quartz in addition to heat-treated silcrete that was introduced from distances greater than 30 km. Robberg inhabitants applied different reduction strategies combining bipolar/anvil and soft stone hammer percussion. Reduction sequences were oriented toward the production of a set of small artefacts (< 25 mm long) that can be generically classified as bladelets. The low incidence of retouched forms and the absence of geometrics, together with our functional study, testify to a flexible composite microlithic technology. We also discuss the raw material provisioning strategies of EBC Robberg inhabitants and develop the question of the intraand inter-assemblages variability. Finally, we attempt to discuss its temporal trends and conclude on the originality of the Robberg technology within the context of other Late Pleistocene microlithic traditions.
Nicholas John Conard
added an update
We made fires, heated different lithic raw materials and discussed science over beer.
 
Patrick Schmidt
added a project goal
I am currently working on heat treatment of silcrete in the Middle Stone Age of southern Africa and of flint in the Upper Palaeolithic and Neolithic of Europe. My work is at the interface of prehistoric archaeology and mineralogy.