Formation of Fluid Inclusions during Heat Treatment of Barremo‐Bedoulian Flint: Archaeometric Implications

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. ( 2012 ). 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.     

DETECTING AND QUANTIFYING HEAT TREATMENT OF FLINT AND OTHER SILICA ROCKS: A NEW NON‐DESTRUCTIVE METHOD APPLIED TO HEAT‐TREATED FLINT FROM THE NEOLITHIC CHASSEY CULTURE,SOUTHERN FRANCE

Heat treatment of lithic raw material is known from the Middle Stone Age to the Neolithic. These findings require archaeometric techniques and methods for detecting the heat‐induced effects within lithic artefacts. However, the existing methods are often cost‐intensive and time‐consuming, and most of them are destructive. Here, we present a new method using the infrared spectroscopic measurement of the strength of H‐bonds formed between surface silanole groups (SiOH) and H₂O molecules held in open pores of the samples. The reduction of H‐bond strength in chalcedony is shown to be strongly correlated with the loss of open pores induced by heat treatment. Hence, the method is based on measuring one of the transformations aimed for by the instigators of the heat treatment: the reduction of porosity that modifies the rock's mechanical properties. A first application to heat‐treated material from the Neolithic Chassey culture (southern France) shows that flint was heated to temperatures between 200°C and 250°C in this period. This has important implications for the study of the procedures used and the heating environments. Our new method is non‐destructive, rapid, cost‐effective and allows for detection of the used annealing temperatures.     

Evaluation of Fe isotope values as a provenance tool for chert artefacts from the north-eastern United States

Iron (Fe) isotope compositions of prehistoric stone tools and geological sources were compared to evaluate the robustness of this isotopic fingerprinting technique. Artefacts and source materials were collected from the Hatch site in central Pennsylvania, United States, where both veined chert (Bald Eagle chert) and stone tools coexist within several metres. Yellow artefacts (δ⁵⁶Fe = 0.38 ± 0.1, n = 7) and source materials (δ⁵⁶Fe = 0.42 ± 0.1, n = 8) isotopically matched within error. The source values also overlap yellow chert samples from three other Bald Eagle chert locations in the area. These values are different from six other chert locations in the north-eastern United States. These data suggest that the Fe isotope composition of chert artefacts reflect distinct geological sources. To enhance the mechanical characteristics of the stone tools, the chert experienced heat treatment, which induced a phase shift of the Fe oxide mineral goethite to hematite. This phase shift changes the colour of the chert to red. The red chert artefacts and source material also have overlapping Fe isotope values, but are 0.2‰ higher than the initial yellow chert. Experimental work where cherts were heated with different clays shows that Fe exchange with site soils induces the +0.2 fractionation. These data demonstrate that predictable Fe isotope fractionation occurs during heating, resulting in isotopically distinct artefacts.     

Heat treatment in the South African Middle Stone Age: temperature induced transformations of silcrete and their technological implications

It was recently found that silcrete raw material was heat-treated during the South African Middle Stone Age (MSA) for altering its flaking properties. This finding led to hypotheses about the implications for the MSA hunter-gatherers such as the cost of thermal treatment in terms of investment and firewood. To date, these hypotheses lack a solid basis, for data on the thermal transformations of South African silcrete and, hence, the necessary heating procedure and heating environment, is missing. In order to produce such data, we conducted an experimental study within the framework of the Diepkloof project. This work is based on the petrographic, mineralogical and structural analysis of South African silcrete from the West Coast and its thermal transformations. Our results shed light on the nature of these transformations, the ideal heating temperatures and the tolerated heating speed. The processes occurring in silcrete are comparable to flint, i.e. the loss of chemically bound ‘water’ and the formation of new Si–O–Si bonds, but their intensity is less pronounced. Effective heating temperatures are significantly higher than for flint and the heating speed tolerated by South African silcrete is relatively fast. These findings imply that silcrete heat treatment cannot be directly compared with flint heat treatment. Unlike flint, heating silcrete does not require the setup of a dedicated heating environment and may have been performed in the same time as other fire related activities. This would represent only a minor supplementary investment in time and firewood. These results have broad implications for the discussion about technological evolution and the acquisition of specialised knowledge in the MSA.     

MICROSTRUCTURE OF FLINT AND OTHER CHERT RAW MATERIALS*

The crystallite size, microstrain, stacking disorder and moganite content of flint and other chert raw materials were studied by X‐ray powder diffraction profile analysis. The broadening of the X‐ray reflections of all microfibrous quartz varieties is due to both anisotropic small crystallite size and anisotropic microstrain. The microstructure varies for samples of different origin. The moganite content and the number of structural defects are higher in flint than in older chert and petrified wood. The X‐ray diffraction pattern of the cortex of flint differs from that of the core material, whereas those of the patina and the unweathered interior of the rocks do not.     

A NEW THERMOLUMINESCENCE DATING TECHNIQUE FOR HEATED FLINT*

A new thermoluminescence (TL) technique for determining the age of heated flint artefacts from archaeological sites is presented. It is a variant of the SAR protocol, which is usually used for OSL dating of sediment, but it is not based on a presumed model for fitting the dose–response curve. Dose recovery tests as well as comparisons with standard protocols show the accuracy of the new technique. It is found that the sensitivity of the thermoluminescence (TL) signal of flint in the orange–red waveband does not show severe changes due to the heating process while measuring the TL. This allows the application of a short SAR procedure, which requires only two dose points. The technique does not require as much instrument time as other SAR techniques, and thus is advantageous for dating very old samples. The major advantage of this new technique is the small amount of sample material required, which allows the dating of samples that are too small for standard TL dating techniques.     

Carnelian,agate, and other types of chalcedony: the prehistory of Jebel al‐Ma'taradh and its semi‐precious stones,Emirate of Ra's al‐Khaimah

The Jebel al‐Ma'taradh and its surroundings contain exceptional deposits of lithic raw materials, including flint and chert, but especially chalcedony, agate, carnelian, and chrysoprase. These deposits were intensively exploited during the Neolithic, and some of the artefacts produced entered the trade network that included settlements on the coast and inland, sometimes as far as 300 km. During earlier periods, probably as early as the Pleistocene, only flint was used. Between the sixth and the fourth millennia, carnelian and agate were exploited to make beads, which are found in the necropolises and settlements of the UAE.     

Experimental Heating of Moravian Cherts and its Implication for Palaeolithic Chipped Stone Assemblages

The applicability of colour, magnetic susceptibility and remanent magnetization measurements for the identification of heated or accidentally burned Palaeolithic cherts from Moravia (Czech Republic) is tested in this study. We conclude that all these methods are usable. Colour changes are best detectable in materials heated at between 250–350°C, while magnetic susceptibility and remanent magnetization change at higher temperatures, closer to 600°C. This latter temperature, however, is usually destructive for cherts and is probably evidenced by accidentally burned artefacts. With the use of the three methods, we managed to identify three probably heated chert artefacts from two early Upper Palaeolithic assemblages from Moravia.     

Ideas no longer written in antler

In the course of the Upper Paleolithic, antler debitage techniques seem to have followed a linear evolution. The earliest one, fracturing by cleaving, appeared during the Aurignacian and is considered by some specialists to be ineffective. According to them, it was not until the invention of the groove and splinter technique during the Gravettian that antler debitage became efficient. Nonetheless, during the Solutrean, fracturing once again became the most common technique, but by splitting. Based on a study of 102 Solutrean pressure tools and experimentations, we reach the conclusion that splitting is a very effective technique that can produce blanks with the same qualities as those made by the groove and splinter technique. The splitting technique was nonetheless excluded in previous studies. We explore the reasons for this and the particularities of the different antler debitage techniques evidenced in the Western Upper Paleolithic.     

PAUL‐LOUIS CYFFLÉ'S (1724–1806) TERRE DE LORRAINE: A TECHNOLOGICAL STUDY

Fragments of four Terre de Lorraine biscuit figurines were subjected to porosity analysis, X‐ray fluorescence analysis, X‐ray diffraction analysis, backscattered‐electron image analysis—coupled with energy dispersive spectrometry—and electron backscatter diffraction analysis to determine the porosity, bulk, major, minor and trace element compositions, and the composition and the proportion of their constituent phases. Cyfflé's Terre de Lorraine wares embrace two distinct types of paste, a calcareous and an aluminous–siliceous one. Both are porous (9–25% water adsorption). The former consists of a mixture of different proportions of ground quartz or calcined flint, ground Pb‐bearing glass and calcium carbonate with a refractory clay. The firing temperature was between 950 and 1050°C. For the latter, Cyfflé mixed ground pure amorphous SiO₂, ground quartz or calcined flint, ground porcelain, ground Na–Ca‐glass and coarse‐grained kaolinite with a fine‐grained kaolinitic clay. The figurines were fired below 1000°C. The result was a porous, hard paste porcelain‐like material. Cyfflé's recipes for both pastes can be calculated from the chemical and the modal analyses.     

The Upper Paleolithic Rock Art of Iberia

Nearly 200 rock art sites of Upper Paleolithic age are currently known on the Iberian Peninsula, in both caves and the open air. Over half are still concentrated in Cantabrian Spain and they span the period between c. 30–11 kya, but–tracking the course of human demography in this geographically circumscribed region–many of the images were probably painted or engraved during the Solutrean and, especially, Magdalenian. Dramatic discoveries and dating projects have significantly expanded the Iberian rock art record both geographically and temporally in recent years, in close coincidence with the growth of contemporaneous archeological evidence: cave art loci in Aragón and Levante attributable to the Solutrean and Magdalenian, many cave art sites and a few open-air ones in Andalucía and Extremadura that are mostly Solutrean (in line with evidence of a major Last Glacial Maximum human refugium in southern Spain), the spectacular Côa Valley open-air complex in northern Portugal (together with a growing number of other such loci and one cave) that was probably created during the Gravettian-Magdalenian periods, and a modest, but important increase in proven cave and open-air sites in the high, north-central interior of Spain that are probably Solutrean and/or Magdalenian. Despite regional variations in decorated surfaces, themes, techniques and styles, there are broad (and sometimes very specific) pan-Iberian similarities (as well as ones with the Upper Paleolithic art of southern France) that are indicative of widespread human contacts and shared systems of symbols and beliefs during the late Last Glacial. As this Ice Age world and the forms of social relationships and ideologies that helped human groups survive in it came to an end, so too did the decoration of caves, rockshelters and outcrops, although in some regions other styles of rock art would return under very different conditions of human existence.     

What causes failure (overheating) during lithic heat treatment?

Heat treatment of lithic raw material, i.e. the intentional alteration of silica rocks for improving their knapping quality, is a process that may require great care and precisely controlled conditions in order to avoid failure due to overheating. The physical causes of overheating remain poorly understood leading to problems in the interpretation of heat-treated artefacts and/or fire-related taphonomic alteration of different types of silica rocks. This driving force of overheating is investigated by a set of experimental heat treatment sequences with different ramp rates and different volumes of flint with a well-defined mineralogical composition, porosity and water content. The results of this experiment show the main cause of heat-induced fracturing to be the vapour pressure in fluid inclusions within the rocks. Heterogeneous thermal expansion could be discarded. The interdependence between volume and heating rate is also shown. These results have implications for the study of archaeological heat-treated rocks, the understanding of taphonomic heat-induced fracturing of silica rocks and experimental flint knapping.     

Refuting the technological cornerstone of the Ice-Age Atlantic crossing hypothesis

The “North Atlantic Ice-Edge Corridor” hypothesis proposes that sometime during the Last Glacial Maximum, roughly 26,500–19,000 years ago, human populations from southern France and the Iberian Peninsula made their way across the North Atlantic and colonized North America. A key element of that hypothesis is the apparent similarity between stone-tool-production techniques of Solutrean peoples of Western Europe and Clovis and purportedly pre-Clovis peoples of eastern North America, most especially the supposed intentional use of “controlled overshot flaking,” a technique for thinning a bifacial stone tool during manufacture. Overshot flakes, struck from prepared edges of the tool, travel across the face and remove part of the opposite margin. Experimental and archaeological data demonstrate, however, that the most parsimonious explanation for the production of overshot flakes is that they are accidental products created incidentally and inconsistently as knappers attempt to thin bifaces. Thus, instead of representing historical divergence, overshot flakes in Clovis and Solutrean assemblages mark convergence in the use of the same simple solution for thinning bifaces that produced analogous detritus.     

ELECTRON SPIN RESONANCE THERMOMETRY APPLIED TO QUARTZITE COBBLES FROM VERGELEGEN SLAVE LODGE,SOMERSET WEST,SOUTH AFRICA

This paper describes the use of electron spin resonance spectroscopy to estimate the degree of heating of quartzite cobbles from hearths on the floor of the archaeological remains of an eighteenth-century Dutch colonial slave lodge. A novel technique based on the comparison of line intensities for the E’and O^-₂ centres in quartz distinguished successfully between cobbles which had been heated to estimated temperatures ranging from 300 °C to 450° C and controls from an adjacent stream bed. This inexpensive and simple technique could be applied to a wide range of archaeological problems involving the thermal history of objects consisting of or containing quartz.     

Heat treatment of Polish flints

In Poland three types of flint (chocolate, spotted and banded) were intensively mined from the Terminal Palaeolithic to the Early Bronze Age. Although heating of flint to improve its flaking properties was practised across the world from ∼110,000 years ago to the recent, particularly in southwestern Europe, heat treatment of flint in Poland is known from only two sites.     

AN X-RAY DIFFRACTION STUDY OF HEAT-TREATED CHERT AND ITS ARCHAEOLOGICAL IMPLICATIONS

It has been established that on occasion prehistoric man heat-treated siliceous stones in order to improve their flaking qualities. In this work we have examined samples of heat-treated chert by means of X-ray diffraction and have established that, in most cases, heat treatment causes an X-ray diffraction line broadening which can be interpreted as a decrease in range of crystalline order of up to 40%, in the most extreme case. The implications of this to an understanding of the fundamental mechanisms of fracture and as a tool for archaeological studies are discussed.     

The chert from the Castelltallat Formation (south-central Pyrenees): archaeometric characterisation and archaeological implications

Chert from the limestones and marly limestones of Castelltallat Formation (Ebro Basin) was widely used throughout prehistoric times in north-eastern Iberia to produce stone tools due to its properties and accessibility. A rough estimate indicates that this rock—either as raw material or as lithic products—was distributed mainly to the north of the outcrops, over an area of 6000–8000 km². However, other rocks in the area have similar characteristics which can lead to confusion in the interpretation of its prehistoric use and distribution. In order to establish useful archaeometric criteria for differentiating this chert from other similar, the Castelltallat chert is characterised in petrographic, mineralogical and geochemical terms. The chert nodules are found to be homogenous at the macroscopic and microscopic level, with a significant presence of bioclasts, thus indicating they might be formed in a freshwater lake environment by the early diagenetic replacement of carbonates in shallow waters. The mineralogical composition is homogeneously uniform and of a flint type, characterised by an almost exclusive predominance of quartz, without any opaline phases, and a variable proportion of moganite. The iron oxide content is very low, whereas its chemical composition is unusually high in uranium which correlates positively with carbonate content and negatively with silica. Archaeometrical parameters are provided to reach a proper identification of tools knapped with this chert. This way, chert from Castelltallat Formation turns out to be a valuable lithological marker to evaluate the range of mobility of the human groups who lived in the north-eastern Iberia and their contacts with neighbouring areas.     

Isothermal Time‐Series Determination of the Rate of Diffusion of Water in Pachuca Obsidian*

Friedman and Smith's (1960) article introducing an exciting, potentially precise and inexpensive method of dating obsidian artefacts has thus far failed to reach its potential. Numerous efforts to refine, improve and even redevelop the method since that time have similarly failed to achieve the original promise. Only within the last eight years have significant improvements been made, due to both improved analytical techniques and a better understanding of the hydration process. However, most of our mechanistic understanding of the interaction of water with rhyolitic glass is based on experiments performed on melts and glasses at temperatures above their glass transitions, conditions inappropriate for investigation of near‐surface environmental conditions. Unfortunately, studies detailing the temporal evolution of the diffusion profile at low temperatures are rare, and few useful data are available on the low‐temperature diffusive hydration of silicate glasses. This paper presents data on the experimental hydration of obsidian from the Pachuca source (a.k.a. Sierra de las Navajas, Basin of Mexico) at 75°C for times ranging from 3 to 562 days, and compares these results with data for samples obtained from a stratigraphic excavation of the Chalco site in the Basin of Mexico. Samples have been analysed using secondary ion mass spectrometry (SIMS) to provide concentration/depth data. While 75°C is still significantly above the temperatures at which archaeological obsidians hydrate, it is well below the glass transition temperature (approx. 400°C) and thus processes are likely to be similar to those that occur in nature, but fast enough to be observed over a laboratory timescale. The results demonstrate that a simple square‐root‐of‐time model of the evolution of the diffusion profile is not adequate to describe the diffusion process, as measured diffusion profiles exhibit the effects of concentration‐ and time‐dependent, non‐Fickian diffusion. With progressive hydration, characteristic diffusion coefficients first decrease, then increase with time. Surface concentration increases with time, but an intermediate plateau is observed in its time evolution that is consistent with results obtained from the suite of Chalco samples. Both of these effects have been observed during diffusion in glassy polymer systems and are associated with the build‐up and relaxation of self‐stress caused by the influx of diffusing material.     

Learning by Experience: The Flint Mines of Southern England and Their Continental Origins

Since the discovery of prehistoric flint mines across Europe during the nineteenth century, mining has been recognized as a central component of the Neolithic ‘package’. In the south of Britain a small group of mines date to the early fourth millennium BC, posing a problem for traditional interpretations of the Early Neolithic, as they appear a significant period of time before other Neolithic monuments. This paper will look at evidence preserved in the mines of southern England, examining whether these sites demonstrate flint‐mining techniques already practised in Continental Europe. Central to the research is a notion that complex activities, such as mining, involve long periods of trial and error before evolving into an accomplished working methodology.