Traceology: a summary

Introduction

Although artifact strictly refers to what is actually manufactured (e.g. by flaking or grinding), the term stone artifact commonly refers to any stones that are made, modified or used by humans, although other primates also make and use artefacts (e.g. Mercador et al. 2007). The by-products of manufacture are often also called artefacts in the sense that humans produced them. Stone tools are usually defined as those artefacts that actually have evidence of use. For reliable identification of archaeological stone tools and determination of specific functions, analysts rely on multiple lines of evidence that might include tool design, usewear, residues, breakage patterns, hafting traces and archaeological context. Ethnographic and experimental evidence is also important. Usewear and residue analysis has the potential to provide a reliable basis for reconstructing and evaluating the nature of prehistoric tasks, resource utilization and settlement history.

Definition

Residues in this context generally refer to materials that are transferred and adhere to implements in the course of use or preparation for use. We are particularly interested in residues that are transferred to stone tools (Briuer 1976; Hayden 1979; Loy 1994). Of particular interest is the transfer of residues linked with a specific task (e.g. harvesting cereals or hunting) or processing a particular material (e.g. woodworking or grinding seeds). However, some residues are unrelated to utilization, and may reflect incidental contact, burial processes, or even modern contaminants. Some tool residues can survive intact on artifacts for millions of years, while others may deteriorate rapidly and undergo chemical changes depending on their structure and specific taphonomic conditions. Consequently, specific methods of extraction, identification and analysis have been developed for particular conditions, particular artifact types (e.g. ceramics) and specific kinds of residues (Evershed et al. 1992 Pollard & Heron 1996). The principles of residue analysis are based on the identification of diagnostic microfossils, chemical signatures, atomic structure, genetic composition and other properties.

Usewear (or “use-wear”) refers to the wear on the edges and surfaces of an implement that are linked with its utilization (Odell 2004). Microwear sometimes refers to a particular approach that employs metallographic microscopes at high magnification, and especially (but not exclusively) to observe and interpret polishes on stone tools (see below). Traceology is a term that may refer to the study of any traces (whether residues or surface alterations), usually in the context of tool use, and can be synonymous with microwear. However, these terms are often used synonymously to refer to surface modifications that arise during use, hafting, handling, and storage. Some forms of usewear may incorporate or absorb residues within surface layers, providing a mixture of additive residue and usewear traces. The general principles of usewear analysis are experimentally based and derived from fracture mechanics, tribology and related sciences. While potentially applicable to all material classes (including artifacts made of wood, bone, stone, glass, shell and metal), specific methods and interpretive rules have been developed for particular tool materials (e.g. flint).

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The main forms of usewear on flaked stone tools include: scarring, striations, edge rounding, smoothing and polish and beveling (Hayden 1979; Kamminga 1982). Each form of usewear provides evidence of function but is rarely sufficient on its own to reconstruct particular tasks. Striations, for example, clearly indicate directionality or motion of tool use, but are not diagnostic on their own of particular processed materials such as wood, shell or skin. On the other hand, hafted flint flakes used for hide scraping sustain highly distinctive edge rounding in association with low incidence of edge scarring and abundant striations perpendicular to the utilized working edge.

The fracture mechanics of flaking indicate that particular properties of scars (e.g. initiation, termination type, orientation and size) are linked with force application, edge morphology, and the nature of materials worked. For example, low angled tool edges used to cut even soft tissue are particularly prone to scars with bending initiations with axial terminations. Bending scars on low angled edges like this are rarely diagnostic of particular raw materials. However, low angled tool edges used to saw bone sustain a highly distinctive bending scar pattern with crushed, rounded prominences and uniform scar spacing (Kamminga 1982). Particular functional tool types known ethnographically sometimes sustain diagnostic scarring in association with other forms of usewear. For example, stone points used as drill sustain characteristically angled scars oriented at right angles to the circular drilling motion. Stone arrow and spear tips sometimes sustain burin-like impact fractures that in conjunction with other features (e.g. microscopic striations/polish alignments) may be crucial in determining a projectile function.

Polish on flint tools has been studied extensively, and can be a diagnostic indicator of material worked (e.g. van Gijn 2010). Stone segments hafted as sickle blades sustain a highly characteristic gloss that indicates highly siliceous plants. However, particular forms of usewear are not studied not in isolation but in combination with other forms of usewear, other traces of use (e.g. residues) and other lines of evidence such tool design, breakage and archaeological context.

Grinding and pounding implements (upper and lower stones) sustain surface modifications that include smoothing, striations and pitting rather than the edge damage that is found on flaked stone tools. Crushing and grinding minerals (e.g. ochre) plants and animal tissue force residues (including liquids, chemical compounds and particles) into cracks and imperfections on the tool surfaces. Some grinding stones are selected for the composition of their cemented particles and their suitability for processing particular foods (e.g. grass seeds). Other stones are selected for their toughness and suitability for edge ground tools. Grinding stones and ground stone implements (like axes) with a porous surface or deep cracks provide, under suitable preservation conditions, a reservoir of deeply impacted and absorbed residues related to utilization.

Haft traces can be indicative of particular handles and tool function (Rots 2012). Experiments show that combinations of bindings, sockets and adhesives affect the type, location and abundance of scars, striations and polish.

Residues in archaeology are the minute remains that are transferred to an artifact. The presence of a residue follows Locard’s exchange principle which states “with contact between two items, there will be an exchange”.There is a range of residues currently studied by archaeologists, chemists and archeometrists. These can include microfossils, fibers, scales, particles, pigments, traces, amorphous residues and biomolecules. The microfossils studied are predominantly plant microfossils represented by pollen, phytoliths, starch grains and other inorganic crystals (eg. schlerieds, raphides and druzes). The fibers will include natural plant fibers (e.g. cellulose), animal fibers (e.g. collagen and hair), insect fibers (e.g. silk) and historically synthetic fibers. Scales can be observed from fish and reptiles in residues. Particles can include organic particles (e.g. Charcoal) or inorganic particles (e.g. metals) which can be found in a residue to indicate metal working. Pigments can also be inorganic or organic but are predominantly inorganic in the archaeological record (e.g. ochre and cinnabar) while traces can include tissues and cells from plants or animals.

Amorphous or absorbed residues can be very difficult to characterize or identify. These amorphous residues can be formed by degraded trace residues including fat from adipose tissues, dried fluids (e.g. blood, milk, egg) or plant exudates (e.g. resins, gums, oleoresins). Many biomolecules can be found in a residue, the most widely analyzed are fatty acids as these are slightly hydrophobic, well preserve and can be found in large amounts. Many other categories of biomolecules can also be studied including hydrocarbons, carbohydrates, proteins, nucleic acids, lipids, resin acids, drugs and hormones.

Residue analysis begins with low power incident light microscopy, continues with high powered incident light microscopy and culminates with the removal of the residue off the surface of the artifact for polarized light microscopy and further analysis. This archaeological microscopy approach to residue analysis is usually capable of identifying microfossils, fibers, scales, particles, pigments and some traces. Further analysis employing histology, immunology, simple biochemical test, absorbance spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, UV luminescence, Gas Chromatography mass Spectroscopy (GCMS), Liquid Chromatography Mass Spectroscopy (LCMS), elemental, genetic or protein analysis can characterize and identify archaeological residue even amorphous residues and residual biomolecules. Many of these techniques have been developed for specific residues under specific conditions on a definitive artefact type like absorbed residues on ceramic which can appear invisible. Residue analysis can be performed on artifacts made from the following materials, wood, bone, stone, textile, metal, shell, ceramic, glass, antler, horn and feather. Some key residue types include food, resin, antler, wood, bone, lipids, adhesives, sealants and blood.

Residue analysis characterizes a residue with sufficient detail as to interpret the function or process of a particular tool. This interpretation can include post-excavation contamination, environmental contamination, incidental contact and non-functionally related transfer. This will allow the archaeologist to define the tasks with which the tool has been associated. Most residue and usewear analysts will begin by first identifying if there is a residue present on an artifact. This is significant to identify if an artifact has been used. There are two steps; inorganic or organic and anthropogenic or environmental, that follows the confirmation of a residue which is critical to determining if the residue is the cause of environmental contamination. Determining if the residue is plant or animal and the tissue and taxa of origin is critical to establish the association of the residue and artifact.

Usewear/residue studies of stone, bone, shell, ceramic and other implements provide key evidence in the history of hunting technology, food processing, resource utilization and settlement history. Complex hafting of flaked stone tools (whether projectile tips or craft tools) may provide an archaeological indicator of technological sophistication and hence human intelligence with implications for tracking human evolution (e.g. Lombard & Haidle 2012). Similarly, the history of grinding stone functions may provide an indicator of sophisticated resource utilization and the complex processing of toxic foods and medicines; again with implications for the study of human evolution.