- Patrick SchmidtPatrick SchmidtEberhard Karls University of Tübingen
In archaeology, heat treatment is the intentional transformation of stone (normally sedimentary silica rocks) by fire to produce materials with improved fracture properties. It has been documented on all continents, from the Africa Middle Stone Age up to subrecent times. It was an important part of the Mediterranean Neolithic and it sporadically appeared in the Paleolithc and Mesolithic of Asia and Europe. It may have been part of the knowledge of people first colonizing North and South America, and it played an important role for toolmaking in the Australian Prehistory. In all these contexts, heat treatment was normally used to improve the quality of stone raw materials for tool knapping; especially its association with pressure flaking has been highlighted, but a few examples also document the quest of making tools with improved qualities (sharper cutting edges) and intentional segmentation of large blocks of raw material to produce smaller, better-usable modules (fire fracturing). Two categories of silica rocks were most often heat-treated throughout prehistory: relatively fine-grained marine chert or flint and more coarse-grained continental silcrete. The finding of stone heat treatment in archaeological contexts opens up several research questions on its role for toolmaking, its cognitive and social implications, and the investment it required. Important venues for research are, for example: Why did people heat-treat stone? What happens to stones when heated? How can heating be recognized? By what technical means were stones heated? Which cost did heat treatment represent for its instigators? Answering these questions allows light to be shed on archaeologically relevant processes like innovation, reinvention, convergence, or the advent of complexity. The methods needed to produce these answers, however, often stem from other fields such as physics, chemistry, mineralogy, or material sciences.
A Historical Overview and Relevant Research Topics
In prehistoric archaeology, heat treatment of stone to produce tools by controlled fracturing (stone knapping) was one of the earliest efforts of humankind to deliberately alter the properties of naturally available materials. Its first occurrence dates back to the southern African Middle Stone Age (MSA; ~300–30 ka), where locally available silcrete was intentionally modified with fire. At our current state of knowledge (see, e.g., Delagnes et al. 2016; Schmidt et al. 2015; Schmidt and Högberg 2018), it appears clearly that heat treatment played an important role for tool production from the local Stillbay and Howiesons Poort cultural horizons on, and it was even proposed to be as old as 164 ka (Brown et al. 2009). This early appearance, roughly coinciding with the arrival of anatomically modern humans in southern Africa (Dusseldorp, Lombard, and Wurz 2013), confers a special role to stone heat treatment. Indeed, the perhaps most debated research focus in current MSA archaeology lies in the advent of what some authors have called “modern behaviors” (McBrearty and Brooks 2000), others “behavioral variability” (Shea 2011), and still others “complex cognition” (Wadley 2013). These transformations taking place in the MSA are documented by markers of distinct behaviors (sometimes called traits) from which greater interpretations on cultural evolution can be derived. Early heat treatment may be one of these markers (see, e.g., Sealy 2009).
The implications of stone heat treatment are, however, not limited to these early periods. There are examples from a variety of cultural contexts throughout all continents, dating from the Upper Paleolithc to subrecent ethnographically documented periods. Figure 1 synthetically shows archaeological contexts that have yielded evidence of stone heat treatment. Prominent example are the European Upper Paleolithc Solutrean (Bordes 1969; Tiffagom 1998), several Mediterranean Neolithic complexes (Léa 2005; Binder 1984; Terradas and Gibaja 2001; Santaniello et al. 2016), parts of the Australian Prehistory (Hanckel 1985; Hiscock 1993; Flenniken and White 1983), the North American Clovis culture (Wilke, Flenniken, and Ozbun 1991), and later Paleo-Indian groups (Hester 1972; Shippee 1963). In these periods, heat treatment is often interpreted as a marker of high technical skill (Tiffagom 1998; Inizan and Tixier 2001) or specialized craftsmanship (Léa et al. 2012). For example, it has been found that during the 5th and 4th millennia bc, highly specialized communities in southern France locally heat-treated chert cores to export them via an extensive trading network, not sharing their knowledge with the communities that imported their heat-treated products and who needed well-heated chert for their tool production by pressure flaking (Léa 2004; Léa et al. 2007; Léa et al. 2012; Binder and Gassin 1988).
Regardless of geographic and temporal position, all these techno-contexts have in common that their makers did not content themselves with the natural resources available to them but invested time and thought in creating new materials with previously unavailable properties. Investigating these processes thus requires methods and techniques focused on the material evidences that make these technocomplexes comparable: the heated stones themselves. Although a variety of silica rocks like sandstone (Hurst, Cunningham, and Johnson 2015), quartzite (Ebright 1987), or silicified tuff (Kononenko, Kononenko, and Kajiwara 1998) are sporadically reported as being heated, the majority of all heat-treated archaeological assemblages is made of two categories of rocks: (1) flint or chert, used for most heat-treated assemblages throughout the world, and (2) silcrete, used in the southern African MSA and the Australian Prehistory. Both are sedimentary silica rocks with similar mineralogical composition but very different structures (cf. Cayeux 1929; Summerfield 1983). Even within each of these two categories, the structures of different rocks are not always alike (Flörke 1967; Füchtbauer 1988). Comparing heat-treated assemblages from different parts of the world and from different time periods is therefore not always straightforward.
Another impediment for our understanding of stone heat treatment is that relatively few researchers have spent time on systematic and holistic investigations since its discovery. Knowledge of heat treatment dates back to the 1960s (Crabtree and Butler 1964; Shippee 1963) and the map in Figure 1 is constantly receiving new spots (for recent additions, see, e.g., Santaniello et al. 2016; Weiner et al. 2015; Zhou et al. 2014), but archaeological interpretations are still often based on assumptions rather than systematic knowledge. For example, an often-repeated tenet of the archaeology of heat treatment is that it is a complicated process. This view is as old as the first publications on flint heat treatment (e.g., Crabtree and Butler 1964; Mandeville and Flenniken 1974), which found that heating must be gradual and that successful treatment depends on the meticulous set-up of a complex heating environment. When silcrete heat treatment was first discovered in the South African MSA, excitement was great because the same complex heating procedure as required for flint was assumed for this early period (Brown et al. 2009). Several theories about the necessity of large amounts of wood fuel for heat treatment (Brown and Marean 2010) or the complexity of cognitive processes necessary to perform heat treatment (Wadley 2013) were brought forward. These theories were not based on a theoretical understanding of silcrete heat treatment but on an analogy with chert heat treatment. Following this, other works (e.g., Schmidt, Porraz, et al. 2013) questioned the comparability between the thermal transformations in flint and silcrete, also questioning the then available interpretations of early MSA heat treatment.
Thus, archaeological interpretations must not be based on a priori assumptions but on hard data. It is unfortunate but well known that the actions and gestures performed during past procedures do not fossilize, rendering their reconstruction a complicated endeavor. The question therefore really is: what data can we retrieve from the material vestiges associated with heat treatment and which archaeological research questions can we answer with them? Many hard data can be retrieved using traditional archaeological methods but often, the only potential source of information available is a macroscopically undiagnostic stone assemblage. Modern studies of archaeological heat treatment must therefore aim to extract the information contained in these stones using an interdisciplinary approach combining archaeology, mineralogy, crystallography, and material sciences. Main research topics in this context are studying the transformations occurring in stone, recognizing heat treatment in archaeological assemblages, investigating the reasons for heat treatment, or interpreting the technical behaviors associated with it.
What Happens during Heat Treatment in Stone?
Fine-Grained Silica Rocks like Chert and Flint
Several nomenclatures describing fine-grained silica rocks are available and the terms “flint” (for cretaceous rocks formed in chalk by metasomatism) and more generally “chert” (for metasomatic rocks from limestone host rocks) appear regularly throughout the literature. The term chert is henceforth used here to address fine-grained sedimentary silica rocks in general, while stressing that no particular geological background can be inferred from it. These are normally marine rocks made of chalcedony, a nano-crystalline quartz texture with a fiber-like structure (Cady, Wenk, and Sintubin 1998; Rios, Salje, and Redfern 2001; Flörke et al. 1991). Most chalcedony in chert is length-fast (LF) -chalcedony (Cayeux 1929) but some samples also contain varying amounts of other petrographic quartz textures and minerals: namely, length-slow (LS) -chalcedony, that is, quartzine (see, e.g., Cady, Wenk, and Sintubin 1998; Flörke et al. 1991), and the minerals opal-CT (see, e.g., Langer and Flörke 1974; Graetsch 1994; Flörke et al. 1976) and moganite (Flörke, Flörke, and Giese 1984; Miehe, Flörke, and Graetsch 1986; Schmidt et al. 2014). Micro-crystalline quartz and other minerals of the SiO2 system have high densities of crystal defects (Miehe, Graetsch, and Flörke 1984; Graetsch, Flörke, and Miehe 1987) and contain “water” in the form of chemically bound water (silanol, SiOH) and H2O (Kronenberg 1994; Langer and Flörke 1974; Graetsch, Flörke, and Miehe 1985).
Since the first discovery of archaeological heat treatment (Crabtree and Butler 1964; Shippee 1963), several authors have worked on the thermal transformations in such silica rocks. A relatively large number of theories describing these transformations were proposed in the second half the 20th century. For example, impurities held interstitially between quartz grains were thought to melt at temperatures between 350°C and 400°C, increasing the coherence between grains (Purdy and Brooks 1971), or to recrystallize above 300°C, creating supplementary void spaces that would improve knapping quality (Schindler et al. 1982). Another theory (Flenniken and Garrison 1975) proposed that heat-induced micro-fractures are responsible for better force transmission during knapping. Most of these early theories were refuted, either because they do not satisfactorily describe the observed transformations (for a response to Flenniken and Garrison, see Inizan, Roche, and Tixier 1976) or because alternative experiments produced different data (for a response to Schindler et al., see Patterson 1984). The most promising of these early theories was put forward by Griffiths et al. (1987), who proposed the migration of water in chert during heat treatment. Their theory provided interesting perspectives for the coming research on heat treatment. From the 1990s on, the dominant theory postulated recrystallization of quartz crystals (Domanski et al. 2009; Domanski and Webb 1992), although later works (Schmidt et al. 2012) argued against it. Thus, the majority of all theories proposed before 2010 had already been refuted or contradicted by other theories by then. These early works did, however, provide a valuable basis for the more recent hypotheses. For example, many of the early authors had noted the loss of mass induced by heating (Micheelsen 1966; Purdy and Brooks 1971; Inizan, Roche, and Tixier 1976; Griffiths et al. 1987) and, in most cases, attributed it to the loss of water. Some authors also noted that the quartz crystals in chalcedony themselves do not change upon heating (Purdy and Brooks 1971; Mandeville 1973; Flenniken and Garrison 1975). Based on these findings, some authors (Schmidt et al. 2012; Schmidt, Badou, and Fröhlich 2011) proposed that internal defect sites are healed during heat treatment and part of the intergranular pore space is closed. This is achieved by dehydroxylation of surface silanol that, before heat treatment, impeded on the formation of atomic bonds within and between quartz crystallites. This reaction (that can be structurally viewed as Si-OH ··· OH-Si → Si-O-Si + H2O) leads to increasing coherence of the rock’s structure but also produces the release of molecular water (Schmidt et al. 2012; Milot et al. 2017). The creation of new atomic bonds causes the rock’s resistance against fracture (fracture toughness) to decrease (Schindler et al. 1982; Domanski, Webb, and Boland 1994; Schmidt et al. 2019), and its elastic modulus and the predictability of its fracture behavior (Weibull modulus) to increase (Schmidt et al. 2019). These modifications of mechanical properties transform the heated rock into a material that requires less force to be knapped and that fractures in a more predictable way. The magnitude and minimal activation temperature of this reaction were found to be variable in different rocks, depending on their relative composition in terms of quartz textures (LF- and LS-chalcedony) and moganite (Schmidt, Slodczyk, et al. 2013), with minimal temperatures of the reaction’s onset ranging between 200°C and 300°C. The reaction kinetics of this dehydration reaction, however, was found to be very stable: holding a rock at a constant temperature for approximately one hour is sufficient to accomplish full dehydration (Schmidt, Paris, and Bellot-Gurlet 2016; Fukuda and Nakashima 2008). After such a one-hour dwell time, the only way to increase the transformations’ magnitude is to increase temperature. Heat-induced fracturing during the treatment is caused by the critical vapor pressure in H2O inclusions (Schmidt 2014) that, if heating rates are excessively fast, can lead to a spontaneous pressure release through internal fracturing (making the stone useless for tool knapping, thus “overheating”). The necessity to evacuate H2O before critical temperatures are reached explains why larger volumes must be heated with slower heating rates than smaller volumes (Schmidt 2014; Mercieca and Hiscock 2008).
Coarse-Grained Silica Rocks like Silcrete
Silcrete is a continental subsurface rock formed by the concretion of preexisting sediments (normally quartz sand, present as clasts in the rocks) by secondary quartz cement (Summerfield 1981, 1983). This cement may be chalcedony in groundwater silcrete (Nash and Ullyott 2007; Thiry 1991) but in pedogenic silcrete (Summerfield 1981; Roberts 2003), it more often is micro-quartz.
Compared to chert, very few studies on the thermal transformations in silcrete are available. Heat-induced color change was described (Corkill 1997) and the change of fracture pattern before and after heat treatment was documented (Cochrane et al. 2012) and analyzed by scanning electron microscopy (Rowney and White 1997). Some studies also showed that silcrete’s mechanical transformations are similar to those of chert (Domanski, Webb, and Boland 1994) and increasing rebound hardness was described after heat treatment (Brown et al. 2009). Others demonstrated that there is a relationship between volume and heat-induced fracturing in silcrete (Mercieca 2000; Mercieca and Hiscock 2008). No explicit theory of the crystallographic and structural transformations taking place in silcrete was available prior to 2010. When the first paper on Middle Stone Age (MSA) silcrete heat treatment appeared (Brown et al. 2009), a separate short communication was published along with it in the same issue (Webb and Domanski 2009). Although it was not explicitly written in either of the two papers, their appearance in the same issue may have suggested to some readers that Domanski and Webb’s theory on recrystallization in chert (Domanski et al. 2009; Domanski and Webb 1992) was also valid for silcrete. A dedicated theory of the thermal processes taking place in silcrete from South Africa was proposed between 2013 and 2017 (Schmidt, Porraz, et al. 2013; Schmidt et al. 2017). The nature of the transformations documented in chert was found to be the same in silcrete. The creation of new Si-O-Si bonds leads to decreasing fracture toughness and an increasing Weibull modulus (Schmidt et al. 2019). Only the magnitude of the reaction is different. In silcrete, the presence of quartz clasts within the structure limits the drop of fracture toughness, while significantly lower values are reached in chert without clasts (Schmidt et al. 2019). Higher absolute heating temperatures are therefore necessary to achieve greater magnitudes of the thermal transformations in silcrete (Schmidt, Porraz, et al. 2013), although the levels observed in chert cannot be attained in silcrete even at the highest temperatures. The results on maximum tolerated heating speed and temperature, previously obtained from chert, are not applicable to silcrete. Because of a significantly lower amount of chemically bound and molecular water that need to be evacuated from the rock to avoid thermal fracturing, and because of a significantly larger network of open pores that allows this evacuation, silcrete withstands higher heating temperatures and faster heating rates. Some authors have proposed that the quartz α → β phase transition near 573°C plays a role in the heat-induced transformations of silcrete (Wadley and Prinsloo 2014; Prinsloo, van der Merwe, and Wadley 2018), but others (Schmidt et al. 2019) argue against this. It has also been proposed (Schmidt et al. 2019; Prinsloo, van der Merwe, and Wadley 2018; Schmidt, Porraz, et al. 2013) that auxiliary minerals in silcrete (e.g., like anatase, TiO2) may play an additional role in silcrete’s transformations, but no consensus has been reached to date.
Why Heat-Treat Stone for Toolmaking?
The most widely accepted interpretation of why stone was heat-treated is to improve its knapping quality. A particular focus lies in the association of heat treatment and pressure flaking. In their pioneering work, Crabtree and Butler (1964) proposed that the transformed mechanical properties of heated chert are of interest for pressure retouch, allowing to produce longer removals. This view has been widely accepted among archaeologists, and heat treatment has been interpreted to provide suitable materials for pressure retouch in some of the major known heat treatment-bearing contexts: some of the bifacial laurel-leaf points of the European Solutrean were heated before a final step of pressure retouch (Tiffagom 1998; Schmidt and Morala 2018; Bordes 1969); a similar association was proposed for the North American Clovis points (Wilke, Flenniken, and Ozbun 1991); and heat treatment was proposed to have preceded a final step of pressure retouch of some southern African MSA Still Bay points (Mourre, Villa, and Henshilwood 2010). Other authors document heat treatment for the production of blades and bladelets by pressure knapping: chert was heated before bladelet debitage in the Siberian Dyuktai culture (Flenniken 1987), and in many contexts of the Mediterranean Neolithic, preforms and blade cores were heated before pressure knapping with the leaver technique (Darmark 2011). The advantages of heated stone for pressure flaking result from a loss of fracture toughness (Schindler et al. 1982; Domanski, Webb, and Boland 1994; Schmidt et al. 2019) that allows flakes to run longer through the material, even if little force is applied for flake initiation. However, heat treatment for improving knapping quality is not exclusively associated with pressure flaking. There are examples of silcrete, heated to obtain better-quality raw materials for freehand (Porraz et al. 2013) and bipolar (Porraz et al. 2016) knapping, from southern Africa and Australia (Hiscock 2008). In parts of the southern African MSA, heat treatment was applied to raw material blocks before knapping to provide materials suitable for hard and soft hammer percussion (Delagnes et al. 2016; Schmidt et al. 2015). In this context, it was also proposed that silcrete may have been heated to compensate for the lack of other good-quality raw materials like chert at some sites (Schmidt and Mackay 2016). A similar association of heat treatment and freehand knapping was proposed for Mesolithic chert from southwestern Germany (Eriksen 2006), where fire was used to prepare blocks in the early stages of reduction. Thus, the most widespread interpretation of the reasons for heat treatment is the improvement of a raw material’s quality for knapping.
There are, however, a few alternative interpretations that do not contradict the former but rather complement it in some contexts.
For example, at some sites of the French Mesolithic Sauveterrian culture, fire was used to split large chert blocks into smaller, better-usable pieces before knapping (Guilbert 2003; Guilbert 2001). Whether the improvement of the raw material blocks’ quality for knapping was also aimed for, or at least appreciated by the Sauveterrian knappers remains unclear. What appears certain is that the technique used for heat fracturing does theoretically not imply the same constraints as the techniques used to carefully heat-treat chert. In the latter case, heat-induced fracturing is to be avoided whereas in the former it is sought (i.e., heat fracturing requires faster heating speeds and higher temperatures). Several ethnographic records also document heat-aided fracturing or the use of fire for raw material extraction and mining in Australia (Binford and O’Connell 1984), Southeast Asia (Man 1883), or on the southern African subcontinent (Robinson 1938). A similar reason for the use of fire in connection with silcrete knapping was tentatively proposed for the southern African MSA (Delagnes et al. 2016; Schmidt et al. 2015) and Later Stone Age (LSA) (Porraz et al. 2016): alongside the obvious benefits of silcrete heat treatment for knapping, these authors also observed that the majority of all raw material blocks broke during heat treatment and were still knapped into tools afterward. In this context, both the traditional interpretation of heat treatment, the amelioration of knapping quality, and intentional heat fracturing before knapping may have coexisted for tens of thousands of years.
The last interpretation of the reasons for heat treatment is the production of more efficient stone tools. It was first noticed by Anderson (1978) that flakes produced from heat-treated chert have sharper edges than flakes of unheated chert. This observation opens up new ways of understanding why people heated their raw materials. Not only making the tools may have been a reason for heat treatment but also using them. An example comes from the French Neolithic Chassey culture. There, blades made from unheated chert were used as multipurpose tools, but bladelets made from heat-treated chert were exclusively used to cut meat (Gassin 1996; Gassin et al. 2010). Following this observation, mechanical testing of the cutting performance of tools similar to the Neolithic ones and real-world cutting experimentation were conducted to evaluate the effectiveness of heated and unheated bladelets (Torchy 2015). The results showed that bladelets knapped from heat-treated chert have sharper cutting edges than their unheated counterparts, being overall more efficient tools for meat processing (Léa et al. 2012; Torchy 2015). Although results of this kind are yet only rarely found in the current literature on stone heat treatment, they highlight a promising new approach in the field.
Recognizing and Quantifying Heat Treatment in Assemblages
Before summarizing the physical and chemical methods that can be used to recognize past heating, it is important to set the frame in which these methods inscribe themselves. In the archaeology of stone heat treatment, it is most important to demonstrate the intentional character of past heating events. Unfortunately, this is sometimes a difficult challenge. The inverse demonstration, the absence of intentionality, is often easier: intense cracking and crazing or pot-lid fractures may indicate post-depositional burning because they make the stone unknappable or worsen its knapping quality (Patterson 1995; Rondeau 1995). To date, there is only one known method that allows one to make statements on the intentionality of stone heat treatment: as detailed (see “What Happens during Heat Treatment in Stone”), heat treatment modifies the fracture properties of silica rocks in a sense that propagating fractures are less offset from their ideal conchoidal fracture path. This results in smoother fracture surfaces on all fracture scars produced after heating. In chert, this leads to surface “luster,” “gloss,” or “shine” (Crabtree and Butler 1964; Bordes 1969; Inizan, Roche, and Tixier 1976) because the surface-roughness relief is small enough to affect light reflection. In silcrete, it leads to visually smoother surfaces (Delagnes et al. 2016; Schmidt et al. 2015) because the surface relief is large enough to be appreciated visually. Therefore, the distinction between two proxies can be used to recognize heat treatment in archaeological stone assemblages, as follows: (1) preheating removal scars: relatively rough (for silcrete) or matt (for chert) fracture surfaces corresponding to the removal of flakes from unheated stone; and (2) post-heating removal scars: relatively smoother (silcrete) or shinier (chert) fracture surfaces, corresponding to the removal of flakes from heat-treated stone. Comparing (1) and (2) visually with a scanning electron microscope (Olausson and Larsson 1982) through surface gloss (Brown et al. 2009; Cochrane et al. 2012) or through measuring surface roughness (Boix Calbet 2012) allows one to set apart the sequences of knapping before and after the heating event. The demonstration of intentionality is based on the argument that stone was knapped after heat treatment. All other techniques can only demonstrate that stone was subjected to heat (not excluding unintentional processes like natural fires or post-depositional burning; see e.g., Gregg and Grybush 1976). However, applying the surface luster and roughness method to archaeological assemblages is not always straightforward. Surface luster can be caused by other phenomena like abrasion in post-depositional environments or “sand blasting” of artifacts in surface scatters in arid environments (for an example of luster that had been mistaken for a heating proxy, see Olausson and Larsson 1982), and surface roughness on silcrete is governed by grain size. If no luster or roughness contrast between pre- and post-heating surfaces can be observed at the separating line of two adjacent removal negatives, the observation of unusual overall luster or smooth surfaces must be corroborated by supplementary techniques to demonstrate heat treatment. Such supplementary techniques are, for example, the evaluation of color change (see, e.g., Schindler et al. 1982; Ahler 1983; Domanski et al. 2009) or methods based on physical and chemical phenomena. For example, past heating events can be detected with thermoluminescence (TL), based on the same principle used for TL dating; that is, the accumulation of energy at crystal defects and its releases upon heating (Aitken 1985). Several authors (Melcher and Zimmerman 1977; Göksu et al. 1974) proposed TL applications to heat-treated stone assemblages, highlighting that heating events >300°C can be detected. A similar approach is based on non-destructive measurements of accumulated energy in samples with electron spin resonance (ESR) spectroscopy. Several approaches based on the ESR signals of amorphous carbon in organic inclusions (Robins et al. 1978), manganese (Robins et al. 1981), methyl (Griffiths et al. 1982), or several other organic radicals (Griffiths, Seeley, and Symons 1986) were proposed. Taking into account several of these signals together, it is possible to detect past heating from ~200°C upward. Potential problems with this method are that not all of these impurities can readily be found in different silica rocks. More problematic, reaction kinetics of the water-related thermal transformations of silica rocks may be very different than reaction kinetics of the phenomena detected by ESR (cf. Griffiths, Seeley, and Symons 1986; Fukuda and Nakashima 2008; Schmidt, Paris, and Bellot-Gurlet 2016). The latter implies that holding stone at relatively low temperatures for a short period of time may be sufficient to induce the wanted transformations but not to affect the impurities’ ESR signal, or vice versa, so that studying heat treatment through ESR signals is somewhat disconnected from the intentions of the heat-treating individuals. Another often used method is archaeomagnetism. Based on the principle that past heating events affect the remnant magnetization of iron-rich inclusions, several authors (Borradaile et al. 1993; Rowney and White 1997; Rowney 1994) proposed applications on chert and silcrete artifacts. As for TL and ESR spectroscopy, these magnetic approaches are based on phenomena that are independent of the transformations past heat treatment instigators were attempting to accomplish. An alternative method (Schmidt, Léa, et al. 2013) is based on the bonding behavior of silanol on intergranular pore walls within the chalcedony of chert. As the H-bonding behavior of SiOH with H2O is a function of the quantity of water held in pores, it is altered by the heat-induced loss of intergranular pore space. Measurements of the near infrared silanol signal can be used to detect past heating by quantifying the closure of pores (applications of this technique to archaeological contexts can be seen in Schmidt, Bellot-Gurlet, and Floss 2018; Santaniello et al. 2016; and Schmidt and Morala 2018). A similar method for detecting past heating was proposed independently (Weiner et al. 2015; Goder-Goldberger et al. 2017). This method is based on the results of a Si-O band assignment in the mid-infrared spectrum of chalcedony (Schmidt and Fröhlich 2011) and allows to effectively quantify the formation of new Si-O-Si bonds upon heat treatment. These two methods, both very similar with regard to the phenomena that are measured to detect past heating, provide very promising perspectives for future studies on possible new contexts in which stone heat treatment was practiced.
The Techniques Used for Heat Treatment in Different Contexts
Proposing a detailed exposé on the techniques used for heat treatment in different parts of the world is not straightforward because of the lack of detailed archaeological data. There are descriptions of industrialized heat treatment of agate for pearl production in India (Kenoyer, Vidale, and Bhan 1991), and there are several ethnographic records of heat treatment documenting the techniques used in different parts of the world (for a comprehensive overview over subrecent ethnographic observations, e.g., Hester 1972; Mandeville 1973), but they appear of very limited applicability for understanding archaeological heat treatment. Only very few direct archaeological finds are known. The most detailed description of a structure is from North America. Shippee (1963) described an undated pit containing an infill of flint and ashes. The pit (≈ 45 cm deep) contained a bed of ashes at its base and chert cores and flakes on top of the ashes. The top of the pit (on top of the layer of chert) was filled with sediment and limestone boulders. Shippee interpreted this structure as a firepit for heat treatment of chert. Another structure attributed to heat treatment was found in the Neolithic site of Khunjhun II (India): Clark and Khana (1989) described a pit with reddened walls that they interpreted as an oven for heat-treating chert. A third structure, a Holocene heat treatment pit found in eastern Australia, was interpreted by the authors (McDonald and Rich 1994) as in situ sand bath used for heat treatment of silcrete. The authors describe a pit within hardened clay that was filled with silt. Artifacts were discovered below a lump of charcoal. The structure was interpreted as a partially, but not totally pulled apart heat treatment pit. These three descriptions of direct archaeological in situ finds point in the direction of underground heating, but it must be kept in mind that alternative techniques, like using the active above-ground part of campfires, are not likely to produce archaeological structures, especially if they are not specifically looked for by archaeologists. There are also a few inferences made by archaeologists and a few archaeometric studies that can be mentioned here. For example, the earliest examples of heat treatment in the South African Middle Stone Age (MSA) yielded data on the techniques used to treat silcrete. In their first article on MSA heat treatment, Brown et al. (2009) conducted experiments aiming to replicate archaeological heat treatment with the sand bath technique (stone placed in sand below a burning fire). Other studies (Wadley 2013; Wadley and Prinsloo 2014) readily adopted the assumption of a sand bath in the MSA and argued for complex cognitive requirements for its set-up. To admit sand bath heating for the earliest context from which heat treatment is known perfectly fits the scenario of modern behaviors emerging in the MSA of southern Africa (see, e.g., Sealy 2009; Marean 2010; Henshilwood and Dubreuil 2011). However, it must be kept in mind that there were no direct archaeological data that actually documented sand bath or underground heating from the MSA. Indeed, the first direct archaeological data on the MSA heating technique documented the use of the above-ground part of fires: a previously unknown residue (organic wood tar), or tempering residue, resulting from the contact of silcrete and glowing embers (Schmidt et al. 2015; Schmidt et al. 2016) was found on artifact surfaces corresponding to the initial outer surface of the silcrete blocks during their heat treatment. Tempering residue was found to only form when freshly cut greed wood is burned in the fires used for heat treatment, providing information on the resource management associated with heat treatment. Above-ground heat treatment was corroborated by the finding that many silcrete blocks heat fractured during the treatment and were still knapped after, leaving behind identifiable remnants of heat-induced non-conchoidal (HINC) fracture surfaces. Heat fracturing occurs in silcrete only when heated to high temperatures and with fast heating rates as they are produced in open fires. These heating proxies have been identified at several MSA and Later Stone Age (LSA) sites in South Africa (Delagnes et al. 2016; Porraz et al. 2016; Schmidt and Mackay 2016; Schmidt 2016) since their discovery, highlighting an interregional continuum in terms of heating technique in this region.
Another younger context where heat treatment played a role is the European Upper Paleolithc Solutrean (see, e.g., Walter and Aubry 2001). One of the few explicit mentions of the heating conditions (Tiffagom 1998) proposed that slow heating rates, long dwell times, and slow cooling rates were used for heat treatment of chert on the Iberian peninsula. Others (Salomon et al. 2013) supposed that fireplaces used for chert heat treatment were arranged in a way to produce temperatures close to 250°C–350°C in chert. A similar range of temperatures was measured from heat-treated Solutrean bifacials (laurel-leaf points) from the Site Laugerie-Haute (Schmidt and Morala 2018), confirming these hypotheses. Based on these data, it is clear that the heating technique used in the Solutrean was already well established and mastered, allowing for the reproduction of similar temperatures during different heating events. Open fires could not have been used because of the high variation and relatively higher absolute temperatures produced.
Following the European Paleolithc, there are mentions of the heating technique used during the Mesolithic of southwestern Germany (in the Beuronian technocomplex). It was proposed that chert treated in the Beuronian was particularly heat resistant (Eriksen 1997; Eriksen 2006) and the hypothesis of a technique relying on the active part of above-ground fires was proposed for this period. A recent study (Schmidt, Spinelli Sanchez, and Kind 2017) confirmed this hypothesis by showing that artifacts were subjected to temperatures between 350°C and 500°C. These temperatures lie significantly above the temperatures determined for other heat-treated archaeological assemblages, namely, the Solutrean, and the degree of standardization was considerably lower. Standardized heating techniques, such as sand baths or earth ovens, are unlikely to produce such great scattering of heating temperatures. The observed pattern can be reasonably well explained by the hypothesis that Beuronian chert was heat-treated in the above-ground part of campfires.
There are also some references to heating conditions possibly used in the Neolithic Chassey culture. Several authors (Léa 2004; Léa 2005; Binder and Gassin 1988; Binder et al. 1990; Roqué-Rosell et al. 2010) supposed that slow heating and cooling rates and long heating durations were necessary to heat-treat the Lower Cretaceous chert used during this period. The reason for this was that this type of chert requires such conditions. The only available information about the Chassey process so far is the range of used heating temperatures. Chert was heated to a narrow interval of temperatures between 200°C and 250°C (Schmidt, Léa, et al. 2013; Milot et al. 2017). Similar to the Upper Paleolithc Solutrean, this result strongly indicates that Neolithic Chassey heat treatment relied on a dedicated heating environment similar to an oven, allowing for reliably reproducing such low and standardized temperatures during different heating events.
Yet another context that yielded prominent examples of stone heat treatment is the North American Prehistory. Except for the firepit mentioned by Shippee (1963), there are no explicit data on the technique used for heat treatment in this context. Following this, several authors (Mandeville and Flenniken 1974; Crabtree and Butler 1964; Wilke, Flenniken, and Ozbun 1991) relied on similar underground techniques for their own experimentations and ultimately for interpreting Paleo-Indian heat treatment.
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