Functional Studies of Prehistoric Grindingstones (2024)

3The aim of this paper is to give an overview of the functional approaches proposed in recent years, and focuses on the methodological aspects. The potential of Use-Wear analysis in particular will be illustrated through experimental results obtained by the author within the framework of ongoing research on Natufian grinding stones.

4Before focusing on the methodologies themselves, we should first detail the questions functional analyses need to define as well as the data that can be used to determine the function of a tool.

What does it mean to diagnose a function?

6Determining how the tool worked: one of the particularities of grinding tools is the morphology of the working parts, which are in general broad surfaces. These artifacts are assigned, according to Leroi-Gourhan’s classification (op.cit.) to the category of tools that operate by “diffuse” percussion (i.e.,the working part is a surface). The way the tool was used can be characterized by: –the active or passive nature of the tool; the mode of percussion: pounding (lancée), grinding (posée), or a combination of both, –the direction of motion and incidence on the material: back-and-forth, circular, random, oblique, perpendicular or parallel. In addition, the way the tool is held (one hand or two hands), the position of the body, possible use of complementary instruments affecting means of grasping, holding of the lower tool, and the gathering of the finished product are also critical factors.

7Determining the type of material processed: apart from identifying the material itself, and the specific or multifunctional use of the tool, the preparatory work on the material before processing needs to be defined, and whether an additive was used.

8Determining the function of the tool: according to Sigaut (op. cit) this not only requires defining its action and the material processed but also the socioeconomic context of use (when, by whom, for whom and why). We also need an understanding of the full cycle of processing of the material, and to determine at which point in this process the grinding tool was used.

9Functional analyses applied to grinding tools have focused primarily on determining their motion and the type of material worked.

Archaeological data and methodologies developed

10Various types of data relating to external and internal tool characteristics can be used to diagnose its function:

11External characteristics: spatial distribution of archeological remains can provide information on the context of use (position of a tool during its use, use of complementary instruments, social context) as well as the history of the object (reuse and recycling).

12Internal characteristics: raw material, morphology, use wear and residue are generally studied to identify tool motion and the material worked.

13Recent methodological research emphasizes the need to take all these data into account to formulate a functional hypothesis. I turn first to a study of the latter characteristics by presenting a rapid overview of approaches proposed. Use-wear analysis will be explored more fully in the second section.

The Raw Material

Morphological studies

18The approach proposed here consists of characterizing objects by morphological criteria, reconstructing their taphonomic history and then determining which morphological variations are relevant with regard to the way the object was used. This analysis serves to formulate hypotheses as to the presence of tools with different functions, which can then be tested through use-wear or residue analyses.

Current research

19Only a few use-wear studies have been carried out for grinding stone material. They benefit from previous methodologies and techniques developed for use-wear analyses of cutting edges. Like them, they are based on ‘actualism’, using natural or experimental reference collections, and on observation at various magnifications.

21To summarize these findings, the traces produced in the grinding process can be characterized as follows. Use leads to the formation of flat areas or plateaus on the asperities of the surface (Procopiou et al, op.cit) resulting from the leveling of areas of higher elevation. According to Adam’s experiments, use may also lead to the flattening of the entire surface. These two studies view this surface modification mainly as the result of a mechanical process. This is congruent with Mansur et alii’s work (op.cit) on polishing. Researchers using low magnification tend to describe the observed use-wear according to the alteration of the grains that compose the stone. This procedure, used in particular by Adams, indicates that the most relevant criteria to describe the observed differences are the type of grain alterations (crushing, leveling, rounded), their spread and distribution on the surface. Adams shows that low power magnification observation on sandstone can differentiate polishing / abrasion from the grinding process as well as the types of worked material as a function of their general properties (abrasive, presence of fatty substances). Several researchers have pointed out that the mineralogical composition of the stone influences the development of use-wear. Some minerals, in particular quartz, develop more pronounced use-wear than others. Mansur and Fullagar and Field (op.cit) note that traces are less visible or absent on certain raw materials. Their studies also indicate that micro-wear polish can be identified. To date, there are few quantitative analyses of wear. Gray levels and luminosity graphs produced by Mansur and Serhnisky (op. cit) can, in my opinion, complement photographic illustrations of use wear in a compelling manner. At present, they do not really serve to quantify wear. Rugosimetry studies carried out by H.Procopiou and her team are more advanced in this field. For the time being, they have not been applied for identifying worked materials; however the author intends to test laser rugosimetry for the study of micropolish. A chemical characterization of microwear, as developed for flint tools, has not yet been attempted. However two studies associate use-wear and residue analyses .

Reference collection developed for Natufian grinding tool use-wear analysis

22Three Natufian assemblages were studied in order to test the hypothesis of preferential use of grindstones to process vegetable matter. This study combines several approaches cited above and mainly focuses on use wear analysis. For this purpose, I developed an experimental reference collection. The clarification of experimental protocols is often lacking and I would like to stress its importance for the development of complementary studies and future critical analysis.

Creation of a reference collection

23Various parameters can affect the formation of use-wear, the main ones being: the raw material of the tools, the presence or absence of shaping and the type of shaping, the kind of material processed, the preparatory work on the material before processing and the eventual use of an additive, the motion, pressure, and the duration of use.

24In addition, the environmental conditions; i.e. the place (laboratory or field), the temperature and relative humidity, can also affect development of wear.

25A rigorous approach would require a comparative analysis of the impact of each of these factors by varying them separately. Such an exhaustive reference collection represents several years of team-work. Choices had to be made, and I opted to test the impact of processed material on use-wear. I worked exclusively on basalt, since this rock is one of the best represented in the assemblages studied. I mainly used “flat grindingstones” such as handstones and lower grindingstones. Despite these restrictions, the cases to be tested remain numerous. I will discuss each of the factors mentioned above to justify the variables that have been taken into account.

26Raw material: there is high variability in basalt rocks. Study of archaeological series indicates use of crypto-crystalline basalt composed of crystals that can be observed under low magnification. Within this class, the variability remains high in terms of mineral composition, cohesion of crystals and porosity. I defined different categories of basalt within archaeological assemblages and oriented the experimental protocol partially to test variations of use wear as a function of these categories.

27Shaping: the archeological material presents high morphological variability. I did not try to reproduce these shapes but rather the preparatory work of the surface before use (hammering).

28Motion: most experiments deal with paired tools (handstone and grindingstone) and keep motion and mode of grasping constant (i.e. grinding back and forth, preceded by a few pounding movements where the active tool is held in one hand, the passive part held on the lap). In several cases the impact of a change in the direction of the motion was tested. In addition, work with a single tool (i.e. abrasion/polishing with back and forth motion) was also performed.

29Worked material: the aim was to test a broad range of mineral, vegetable and animal matter. A list of potential materials was drawn up from archaeological, paleo-environmental, and ethnological data.

30Duration of use: grinding stones are often used over a long period of time, and handed down from generation to generation. If the focus is on wear surfaces, resharpening can be defined as a return to the original surface. Ideally, the experiment should attempt to reach complete wear- out of the working surface, requiring resharpening. This stage can be defined according to observations made on archaeological artifacts and experiments. Even after the longest performed working time, 51/2 hours, this stage was not reached. Archaeological collections are not entirely composed of worn-out tools or resharpened ones. Use-wear was therefore documented at different stages.

31The experiments carried out can be divided into four different groups. Their purpose and protocols are presented below.

Experiment1: stone on stone contact

32Purpose: characterize traces resulting from stone on stone contact without an intermediary substance.

33Experimental material and protocol: a lower block was divided into parallel strips corresponding to different zones. On each of these zones, a different upper block of basalt was used in grinding in a back-and-forth motion. There was no pre-shaping of the blocks. I also used an upper block of sandstone to compare use wear produced by this material with that made by basalt. The upper and lower surfaces were observed at different stages of work: 10min, 20 min, 40min and 1hour.

Experiment2: shaping surfaces

34Purpose: describe surface resulting from the pecking and hammering.

35Experimental material and protocol: the working surface of tools used to grind various materials was first shaped - for the lower grindingstones, the neocortex was removed, a concave part was shaped to maintain the ground material and eliminate surface irregularities;

36–for the upper tools, the neo-cortex was removed, the surface was regularized and the working part was shaped to provide maximal contact with the passive tool. In addition, a motion which combines pecking and abrasion in varying order was also tested. Hammerstones of different rocks including basalt, quartz and flint were used. A total of 17 blocks or zones were examined in detail and submitted to a comparative analysis. Fifteen others were studied more rapidly in order to verify the results.

Experiment3: grinding various types of material with handstone and grinding stone

37Purpose: test the impact of various worked materials on use wear.

38Experimental material and protocol: experiment 1 showed a low variability of use wear depending on the type of basalt; therefore the raw material parameter was not systematically taken into account. (Table 1)

Experiment4: work with a single tool (Table 2)

39Purpose: extend the range of material processed and test whether there are significant differences in use wear between paired tools and a single tool.

40Experimental material and protocol: most of the blocks were used as a lower tool, the material was rubbed by a back and forth movement. Only work on hide was carried out using an upper tool, the hide was held on a wooden plank.

Main results

Surface analysis procedure

41I mainly worked using a low power microscope (magnification up to 80x). Examination of archaeological artifacts and preliminary observations of experimental materials prompted me to use the criteria proposed by J.Adams for the surface description which I completed by data descriptions from other studies (in particular H.Procopiou).

42The principles and the descriptive terms used in this study are presented below.

43Macroscopic observation serves to describe the overall topography of the working surface: this refers to the presence or absence of a contrast between interstices and “leveledoff” asperities (i.e regularized areas known as plateaus when they have a flat morphology in profile, “rounded out wear areas” when they are not entirely flat). The surface can also present an all-over regularization without being organized into peaks/hollows. Striations and shiny surfaces visible to the naked eye are also described.

44With the low power microscope, the description of the micro-topography includes identification of altered zones, their morphology and distribution. These altered zones are characterized on the basis of grain surface modifications described by Adams: leveling of grain, rounded or sharp edges, chipping or crushing marks. Another criterion used by Adams to describe wear is the presence or absence of interstices between grains. In this paper, I will use the term “homogeneous zones” when the grains do not show interstices and form uniform surfaces. Areas of leveling without interstices refer to cases where grains are still recognizable. In the opposite case, the grains are said to be “dug out” or “in relief”. For each type of alteration, its extent on the grain itself is defined (for instance, a rounded part can be overall or only developed on the top of the grains) as well as its distribution on the surface in general. These observations also aim to describe the types of striations and distribution of sheens. The description of striations refers basically to the length, depth, orientation, presence singly or in groups, and distribution.

Results of low power microscope analyses

45The experimental protocols serves to document several points concerning the various factors involved in the wear formation process:

• impact of the raw material

• impact of shaping techniques

• tool use: paired or singly

• impact of the worked material

• motion

46The fourth point was investigated more fully and will be developed in greater detail.

Tools working as a pair

47Examination of the natural surfaces, experiments conducted on rubbing stone against stone and study of pecked surfaces serve as a reference to assess the role of the worked material in the use-wear formation. Although conducted for relatively short periods of time, these experiments indicate that direct contact of stone on stone is highly abrasive and creates crushing and breaking off of the grains as well as the formation of small wear zones on the asperities, sometimes visible to the naked eye, although better under the low power microscope. They form elevated homogeneous zones, which are generally not smooth but rather are irregular, dark in color, and bearing striations (Plate 1, a and b). These alterations are recurrent and appear to be diagnostic of stone-stone contact as they were only produced in this specific context.

48When material is placed between the two stones, the use-wears are different both at the macroscopic level as well as under low power magnification. The areas of leveling on the asperities are in general more pronounced and larger, the grains show a greater range of alterations including, among others but not systematically, the development of homogeneous zones different from the ones produced by stone-to-stone contact.

49Turning now to the different worked materials, several groups can be distinguished.

50The clearest differences can be seen between work on minerals and the other ground materials. The abrasive properties of ochre cause an overall leveling of the relief visible at a macroscopic level and the formation of specific wear zones visible at low magnification (small homogeneous zones). The entire surface is affected by chipping and micro crushing of the grains (Plate 1, c and d).

51As for the other grounded materials, major differences emerge between those containing oil or fat, and those which do not. Cereals and leguminous plants can be contrasted with animal matter, nuts, mustard seeds and fresh acorns (the features were less apparent for dry acorns). For the latter, wear is much more pronounced and leads to the formation of flat areas of various size. In these areas, wear is characterized by a leveling of the grain and rounding of its edges. These alterations are associated with the development of highly reflective sheens and deep striations, the former often develop in the interstices. For cereals, regardless of the type of work, the leveled areas are less pronounced, and have a generally rounded profile. The alterations on these areas cover a broad spectrum, the surface has less sheen, the striations are associated with the development of homogeneous zones (Plate 1 g and h). Leguminous plants appear to form a separate category, with traces of grain removals and micro crushing dominating the leveled asperities.

52In the category of material containing fat or oil, a difference can be seen between animal and vegetable matter. Grinding of fish and meat leads to use wear which appears to be recurrent and can be defined as follows. Although some grains stay in relief and are rounded, most of them are truncated, present no interstice but remain identifiable. These zones are covered by a highly reflective sheen and striations, intersected with small interstices that correspond to grain removal. For vegetable matter although grains are highly worn down locally, most remain slightly in relief and rounded. The reflective sheen appears much more opaque and dark (Plate 1, e and f).

53Differences were also observed between the various types of vegetable matter containing oil. As for wheat and wild barley, despite the variety of processes tested (husking, grinding with and without water in the case of barley), grinding leads to the formation of quite similar use-wear.

Work with a single stone

54The range of experiments carried out was less extensive (ochre, shells, hides, bones, wood). Each type of material produces specific use wear. Differences are more apparent than on paired tools. There could be two explanations for this. First of all, is the more limited range and higher disparity of worked materials in comparison to experiments on paired tools. Second, the abrasive contact between the stones in paired work probably creates “interferences” and makes the use wear traces more uniform.

55Hence each type of worked material in this series of experiments produced specific traces. Wood, bone, shell and ochre can be grouped together because they all produce surface alterations by micro crushing and grain truncation (more pronounced for ochre and shells). Sheen areas correspond to dark homogeneous zones for bone, shell and ochre, and to irregular zones where grains still can be individualized for wood. Work on shell shows two forms of homogeneous zones. For ochre, homogeneous zones show a striated morphology, are orange, have a high sheen, and some grains still appear in relief. During work on hide, non-leveled, discontinuous shiny surfaces developed (intersected with small interstices) and correspond to opaque, black homogeneous zones.

56For each series of experiments, casts of small zones of the working surface were made at different stages in order to sample the development of wear and to enable analysis of the surface using other techniques.

Analysis under high power magnification

58The size of archaeological objects often precludes direct observation of the grinding tools under the microscope. Several types of silicon as well as casts in acetate have been tested to reproduce samples of the working surfaces. Generally casts are made of dental elastomere (Provil L) and the replica of resin. I tested and compared observations on the cast, semi-transparent and opaque (the resin is tinted with powdered or liquid dyes) replicas, and for few cases only, on the items themselves. Three microscopes were used, Leica, Wild M20, and Olympus BH3 for the transmitted light observations. This analysis is currently in progress and I will only mention the results of the study using the transmission light microscope.

59First of all, out of all the techniques tested, the casts in dental elastomere (ProvilL) and acetate film yielded the best quality, and enabled, as was shown for flint and other materials, an accurate reproduction of the surface. In both cases however part of the amplitude of the relief is lost, since the products do not penetrate into the deepest interstices. Observations on different types of casts and replicas show that the coalescence zones appear most clearly in transmitted light (non-polarized) on semi-transparent replicas. This procedure could be less relevant for characterizing micro-polish such as carried out in use wear analysis, since observations are hindered by a loss of depth of field and a less accurate rendering of their texture. The contrasts between the coalescent zone and micro-relief unaffected by this alteration thus appear particularly visible in transmitted light. Further study will help refine the following conclusions. The development of these zones is most pronounced on tools used singly (see Plate 2 g and h: bone abrasion). On paired tools, there is a kind of “noise” created by the abrasive contact of the two stone surfaces. Most of the surface shows regularized but granulated areas and are only slightly reflective. This characterizes a very abrasive contact such as in the case of stone-to-stone abrasion (for instance compare Plate 2 a et b: surface of an upper bloc shaped by pecking / abrasion; Plate 2 c and d: grinding fenugreek). A significant development of coalescence zones was however found for work with materials containing lubricants, such as nuts, meat, fish and fresh acorns (for instance Plate 2 e and f grinding nuts). In this category, differences in the distribution and the texture of the coalescence zones were observed. Further studies aim to characterize the micro-polish as a function of the worked material.

60Recent advances in functional studies on grinding stone material have led to the development of analysis procedures which today appear to be well established at least on the theoretical level. The application to archaeological material remains restricted but will probably help refine the methodologies proposed. In general, these approaches encourage the integration of different foci of analysis to build a functional diagnosis for the study of raw material, morphologies, use wear and residues. My study basically consisted of exploring the potential of use wear analyses. In this field, some studies have demonstrated the relevancy of this approach and proposed observation and descriptive systems for use-wear. The hypothesis of a significant variation in use wear as a function of materials processed with basalt tools remained to be tested. The experimental referential testifies to the possibilities for differentiating large categories of worked materials. It confirms in addition the relevance of low power observations and its complementarity with higher magnification studies. In addition, this referential enabled me to contribute to our understanding of the development of traces, and the influence of various factors such as raw material, the type of shaping, and the way the tool was used. At this stage of the research, it seems crucial to complete the experimental work as different points emerge as priorities; namely, increase the duration of use, develop processes of a given material prepared in different ways, grind with various additives, and more generally integrate these into the complete chaîne opératoire of processing, to test for multifunctional uses, and the preservation of use wear after exposure to mechanical or chemical agents. This latter point is particularly important to enable the interpretation of archeological collections.

61The experimental collection also provides a robust basis for testing different techniques for observation and surface analysis. In particular, a quantitative approach should be attempted. I believe that it is above all through the development of experiments, team-work and comparing studies carried out by different researchers that we can give a solid foundation to interpretations.