The effects of water on processes of ceramic abrasion

Journal of Archaeological Science 1987, 14, 83-96

The Effects of Water on Processes of Ceramic Abrasion James M . S k i b o a a n d M i c h a e l B. Schiffer a

(Received 6 February 1986, accepted 21 April 1986) The effect of wet and dry abrasive processes were studied experimentally in order to provide information relevant to studies of ceramic technofunction, use-wear, and noncultural formation processes. Briquettes of different paste compositions (untempered, fine-sand tempered, coarse-sand tempered) were subjected to tumbling tests using sand, pea gravel, and large gravel abraders in wet and dry modes. Abrasion resistance was indicated by weight loss after 4 h of tumbling; micro- and macroscopic differences in the traces of various abrasive processes were also documented. In every case, wet conditions cause an increase in the rate of abrasion. Explanations are offered for the observed patterns and some implications are drawn for archaeological studies.

Keywords: EXPERIMENTAL ARCHAEOLOGY, CERAMICS, ABRASION, USE-WEAR, FORMATION PROCESSES, MATERIALS SCIENCE.

Introduction The breakdown of materials, in use and as they interact with their environment, is a concern today as it undoubtedly was in the past to users of material items. An enormous scientific literature that crosscuts m a n y disciplines is dedicated solely to investigating how, under various conditions, natural and h u m a n - m a d e materials break down. Materials are not static entities and from the m o m e n t of production they are being transformed into simpler forms by chemical, biological, and physical processes. This paper is concerned with the deterioration and wear of porous ceramics. Deterioration is regarded here as breakdown by non-cultural processes, whereas wear refers to artifact modifications resulting from h u m a n use. Specifically, we examine the effect of free-standing water on abrasion processes of porous ceramics. In conformity with the well-known destructive role of water in all weathering processes, thoroughly documented in geology and materials science (e.g. Washburn, 1963; Winkler, 1975; Wallbridge et al., 1983; Birkeland, 1984; Scott, 1985), our preliminary investigations had suggested that the rate of ceramic material loss is greater in a wet environment under various forms of laboratory-produced abrasion. Nevertheless, the specific effects that water has on the deterioration and wear of archaeological ceramics, and the magnitude of those effects, have not received adequate attention. The objectives of this research are to quantify the aLaboratory of Traditional Technology,Department of Anthropology, University of Arizona, Tucson, Arizona 85721, U.S.A. 83 0305-4403/87/010083 + 14 $03.00/0

© 1987AcademicPress Inc. (London) Limited

84

J.M. SKIBO AND M. B. SCHIFFER

wet-dry abrasion differences under controlled laboratory conditions and to explore some implications of these differences for various studies of archaeological ceramics. Three archaeological interests have motivated this series of experiments. The first is the effect of water on the natural destruction of ceramics. Archaeologists are just beginning to appreciate that ceramic sherds, long thought to be a virtually indestructable portion of the archaeological record (cf. Dowman, 1970: 5), do undergo deterioration by environmental processes (e.g. Murphy, 1981; Reid, 1984a, b). Destructive processes, in which moisture plays a key role, might selectively destroy certain ceramic types or assemblages, leaving behind an incomplete inventory on which to base inferences. A second area of research that calls for this type of experiment is ceramic use-wear studies. Analysis of abrasive use-wear (e.g. Griffiths, 1978; Bray, 1982; Hally, 1983), although still in its infancy, must take into consideration effects of water on abrasion, especially to the extent that wear would have been amplified had the vessel been used with liquid contents or in a wet environment. Thus, to infer the type and intensity of use from vessel abrasion patterns, the effects of moisture on the production of abrasive use-wear must be ascertained. A final area of research that may benefit from this experiment is technofunctional studies of ceramics. These studies are beginning to investigate how technological decisions, such as choice of temper, influence the mechanical properties of a completed vessel (for a recent review see Bronitsky, 1986). One important concern has been the relationship of vessel function to thermal shock resistance capabilities (e.g. Braun, 1983; Steponaitis, 1983; Bronitsky, 1986; Bronitsky & Hamer, 1986). It remains to be investigated, however, if potters were aware of the destructive effects of water and employed various strategies to improve the durability of their vessels. For example, if a vessel were to be used with liquid contents, appropriate technological adjustments might have been made in production to avoid premature failure through moisture-induced deterioration and more pronounced use-wear abrasion.

Ceramic weathering The breakdown of ceramics owing to natural processes falls within the broad subject of "weathering". The geological study of weathering includes the physical and chemical alteration of materials near the earth's surface. Although the main concern of weathering is the breakdown of rocks and minerals, similar processes are responsible for the breakdown of archaeological ceramics. Water is vitally involved in most weathering processes. In porous ceramics, which include most open-fired prehistoric wares, water acts as a transporting agent for chemicals that react with the clay minerals and as a solvent in which many reactions take place (Winkler, 1975). For example, salts carried into a porous material crystallize as the moisture evaporates, creating stresses in the pore spaces (Winkler, 1975:119 122). The stresses are relieved by exfoliation--"salt e r o s i o n " - - o f material from exterior surfaces. A similar type of destruction is produced when a saturated porous material is subjected to temperatures below freezing. Pressures as high as several thousand pounds per square inch are exerted in confined pore spaces as water expands during freezing (Ollier, 1975: 11). Although these chemical and physical weathering processes cause ceramic breakdown and should be studied experimentally, the first step is to investigate how water at a constant temperature and free of long-term chemical reactions contributes to ceramic weathering. In the presence of water, several different processes occur within a porous ceramic body that make it more susceptible to abrasion. First, the ceramic bonds begin to dissolve, particularly in low-fired wares, and minerals leach out (Murphy, 1981; Ware & Rayl, 1981). Vitrification temperatures, at which clay minerals melt and new minerals are

WET AND DRY CERAMIC ABRASION

85

formed, are rarely attained by most North American open-firing techniques (Shepard, 1965: 83). Below this point, in a process known as sintering, the edges of the clay minerals soften and adhere (Davidge, 1979: 5-8; Rye, 1981: 106), producing a material that contains incompletely bonded clay constituents. Although the bonding that takes place during sintering is sufficient to produce a useful ceramic container, the resultant material may be particularly susceptible to breakdown in the presence of water. In a moist environment, water would penetrate the pores and cleavages, loosening the soluble portions ofehe ceramic bonds. For example, pottery fired at 600 °C and immersed in water for a year disintegrated almost completely (Ware & Rayl, 1981: 3-24). Secondly, during wetting and drying cycles pressures within pores can contribute to ceramic distintegration (cf. Murphy, 1981). The absorption and then evaporation of water has an effect similar to that of the freezing and thawing of a saturated porous material (Hudec, 1978: 5). Stress is created as the rigid pore space fills with water; upon evaporation the stress is released, but through repeated cycles the material is weakened. Associated with the sorption-desorption process is an actual volume increase during wetting (Winkler, 1975; Hudec, 1978). Quartz sandstones, for example, expand between 0.01% and 0.044% (Hockman & Kessler, 1950) during water absorption, undergoing stresses similar to those exerted during ice formation. Both the dissolution of incompletely fused clay bonds and the stresses exerted by the sorption~tesorption process and changes in volume combine to reduce the strength of the ceramic fabric. Ware & Rayl (1981) found a significant reduction of tensile strength in ceramic briquettes after only a relatively short period of immersion. In view of this decrease in ceramic strength, one would expect water also to render porous ceramics more susceptible to abrasive processes.

Experimental Methods This experiment is designed to measure the rate of material loss of saturated ceramic briquettes relative to identical dry briquettes under controlled conditions of abrasion. To monitor variability in the effects of wet abrasion on various types of ceramics, the test was carried out on three kinds of briquettes: untempered, fine-sand tempered, and coarse-sand tempered. In addition, to simulate a variety of abrasive environments, three grades of abrasives were used in the experiment. Each tempered briquette contained by volume 20% fine sand (0-5-1-0 mm) or coarse sand (1.0-2-0 ram). The sand was composed primarily of quartz and feldspar and was obtained from a Tucson area wash. A commercial pre-moistened clay (Westwood EM-21), Composed of two ball clays, 5.5% talc and silica, was used in the experiment to ensure the highest degree of consistency in the composition of the test briquettes. (Although Westwood clays are of excellent quality, their precise formulas are proprietary information; thus, their use should be avoided in experiments where composition must be known in detail.) A square clay slab, 22 cm on a side and 9.5 mm thick, was produced by rolling out the clay between two 9.5 mm thick metal bars. After air drying for at least 3 days, the clay slabs were fired in a Neycraft Fiber Furnace (Model J F F 2000) for 1 h at 900 °C. The furnace was carefully monitored during firing and the temperature was maintained within ± 5 °C. After cooling, the fired slabs were cut with a diamond-blade lapidary saw into test briquettes that were 3.0 cm on a side. Previous laboratory studies had demonstrated that surface texture, firing temperature and overall edge area are critical factors in quantifying abrasion resistance (Skibo, n.d.; I. Vaz Pinto, M. B. Schiffer, S. Smith & J. M. Skibo, unpubl, obs.). Therefore, every effort was made to fabricate briquettes as uniformly as possible to limit the observed variation

86

J.M. SKIBO AND M. B. SCHIFFER

in material loss during testing to variables we were studying rather than to variables introduced by inconsistent specimen preparation. All briquette surfaces were ground on an Astro-Met polisher using water-lubricated 200 grit silicon carbide abrasive paper to create uniform surface textures. The polisher removed saw marks and edge anomalies and made consistent width and length measurements possible (0.20 mm maximum variation). Polishing also removed the thin slip that was produced when the clay was rolled into briquettes. Polishing was stopped when the temper particles were exposed on the upper and lower surfaces or, in the case of the untempered briquettes, when approximately equivalent polishing times had been reached. Thickness of briquettes was not as consistent as the width-length dimensions owing to differential shrinkage of tempered and untempered specimens. Nonetheless, the briquettes were still quite uniform in thickness (1.27 mm maximum variation). Three Beacon Star tumbling units, each with two tumbling barrels, served as our standardized test of abrasion resistance. Tumbling barrels have been used to study fluvial abrasion of stone (Wentworth, 1919; Kuenen, 1956) and are routinely employed in materials science to quantify the durability of industrial products such as coal, iron ore, and fibre insulation (American Society for Testing and Materials, 1983: C 421~483; D 441445; E 279-269). In addition, tumbling tests have been employed in archaeological experiments for studying abrasion of bone (Shipman & Rose, 1983), flint implements (Shackley, 1974), and sherds (Skibo, n.d.). The versatility of tumbling barrels, which can be used with different materials and with different abraders made them the ideal choice for this experiment. In addition, other apparatus for testing abrasion resistance, such as the Dietert-Fargo abrasion tester, do not simulate archaeologically relevant abrasion processes (cf. Schiffer & Skibo, unpubl, obs.). The abraders consisted of sand (1.0-2-0 mm), pea gravel (mean diameter, 7.79 mm), and large gravel (mean diameter, 22.81 mm). Each type of briquette was tested with both a wet and dry mixture of each abrader. All tests employed 500 ml of abrader; 500 ml of tap water was added to the wet test cases. All briquettes were submerged in tap water for 24 h in order to equalize the amount of hydration that took place during the cutting and polishing process and were then air-dried for 24 h to allow the absorbed water to evaporate. Briquettes were tumbled one per barrel for 4 h, and then air-dried for at least another 24 h before measurements were made. Weights before and after tumbling were recorded to the nearest 0.001 g (the mean of three consecutive weighings). In addition, the dimensions of the briquettes were remeasured after tumbling. Results

The briquettes were abraded in test units consisting of six identical specimens. One briquette was abraded wet and one abraded dry in each of the three types of abraders. The procedure was replicated for the three briquette types, producing a total of 18 cells in the data matrix (three temper types, three abraders, two wet-dry conditions). The entire procedure was repeated five times, for a total of 90 briquettes. Two limitations of the present testing procedure should be noted at the outset. First, the temper particles have a density higher than the clay matrix; thus, briquettes differ in total weights. To standardize for these differences in briquette weight, percentage weight loss is computed. Even so, some potential problems remain because the abrasion process may not proceed uniformly across all test specimens. For example, we suspect that in the earliest stages of abrasion the ratios of clay to temper lost may vary with different abraders and different tempers. In view of this possible source of variability, overall percentage weight losses might not be completely comparable in all test situations unless the abrasion process has proceeded for many hours beyond the tests carried out here. Thus, at this

WET AND DRY CERAMIC ABRASION

87

Figure 1. Greater degreeof edgeroundingunder wet abrasion conditions.Briquette on left was abraded wet. point only very large differences should be accorded attention when the results are interpreted. A second problem arose during the course of the experiment with the large gravel abraders. On some test specimens a marked asymmetry was observed in the degree of abrasion on upper and lower surfaces. The mechanism responsible for this abrasion pattern remained mysterious until, by chance, a briquette was discovered adhering to the flat side of the barrel. Apparently, with the large gravel abrader, it is possible for a briquette to become positioned in such a way that it avoids tumbling. Although the surface is still abraded, the process differs slightly. We suspect that this modified abrasive process leads to less weight loss than would otherwise be the case because one side of the sherd is in effect protected. These additional sources of variability do not compromise the basic findings of the experiment--that wet abrasion causes greater material loss under all test conditions--but they do limit our ability to offer more refined statements about the effects of wet abrasion on ceramics of different types and with different abraders. Differences in wet versus dry abrasion are visually apparent. The most obvious difference is a greater degree of edge rounding on briquettes abraded wet (Figure 1). Surface abrasion differences, although less apparent to the naked eye, are visible microscopically. The surface texture of the dry-abraded briquettes is, in all cases, noticeably rougher (Figure 2). The difference is greatest with the gravel abrader but is visible microscopically in all wet-dry cases. Although the process responsible for this pattern has not been positively identified, one potential source of the variation was discovered as the briquettes were removed after tumbling. After each tumbling period, the briquettes were gently cleansed under tap water. The briquettes abraded wet required little if any cleaning but the specimens abraded dry had a thin layer of sediment adhering to their surface. Apparently water, in the wet test cases, flushes away the material removed from the briquette; however, the clay and temper material removed from the dry-abraded specimens is not flushed away and some of it adheres to the surface. Potentially, this creates a slightly more abrasive environment that could account for the rougher surface of the dry-abraded briquettes. When the wet~try abrasion difference is expressed as a simple ratio, several interesting patterns emerge (Table 1). As would be expected, abrasion is greater under wet conditions.

88

J.M. SKIBO AND M. B. SCHIFFER

Figure 2. Differences in surface texture of untempered briquettes. The briquette in (a) was abraded with dry pea gravel and is slightly rougher than that in (b), which was abraded with wet pea gravel ( x 7.75).

Table 1. The wet versus dry abrasion ratio

Temper Untempered

Fine sand Coarse sand

Abrader

Wet/dry ratio

Sand Pea gravel

6-52 1.84

Large gravel Sand Pea gravel Large gravel Sand Pea gravel Large gravel

1.83 2.65 1.95 1-62 4.40 2-10 1-76

The highest ratio, regardless of temper, occurs with a sand abrader: briquettes tumbled in wet sand lost an average of 6.52 times more material than identical briquettes tumbled in dry sand. A somewhat more intriguing result is that all briquettes tumbled in pea gravel and large gravel have nearly identical ratios (mean of 1.85, with a range of only 1.62~2-10). Thus, briquettes of the types we constructed (except those tumbled in the mildest abrasive) will lose approximately 1.85 times more of their original weight when abraded wet. It should be noted that the two lowest wet~lry ratios occur with the gravel abraders. It was predicted that these ratios would be low as a result of the problems discussed earlier. If these two slightly anomalous figures are considered as underestimates of the wet-dry relationship, the similarities in wet~zlry ratios across temper types and abraders become even more striking. Briquettes abraded in sand had a proportionally higher wet-dry ratio than those abraded under the other conditions because the dry sand causes minimal abrasion on ceramics as durable as the specimens in this experiment. The briquettes abraded in dry sand lost an average of only 0.018 g and no material loss is visible to the naked eye. Tempering material and the type of abrader have an effect on abrasion resistance. A previous experiment demonstrated that fine-sand tempered briquettes are more resitant to abrasion than coarse-sand tempered specimens when abraded by pea gravel (Vaz Pinto et al., unpubl, obs.). Our experiment confirms this finding and demonstrates further that

WET AND DRY CERAMIC ABRASION

89

Table 2. The effect o f temper on abrasion resistance illustrated by mean percentage weight loss after 4 h tumbling

Abrader Wet sand Dry sand Wet pea gravel Dry pea gravel Wet large gravel Dry large gravel

Temper Untempered Fine sand Coarse sand Untempered Fine sand Coarse sand Untempered Fine sand Coarse sand Untempered Fine sand Coarse sand Untempered Fine sand Coarse sand Untempered Fine sand Coarse sand

Mean percentage weight loss 0.73 0.82 0.66 0.12 0.31 0.15 3.63 3.37 4.27 1-97 1.73 2.03 10.02 11.38 10.67 5.49 7.06 6.07

different types of abraders can change the relationship between briquette temper and abrasion resistance. Although the differences are small, the coarse rather than fine-sand tempered briquettes are most resistant to abrasion when abraded with sand or large gravel (Table 2). This dramatic shift in abrasion resistance can be attributed to a relatively complex relationship between several factors, including the size, quantity and distribution of temper, and the size, shape and mass of the abrader (Schiffer & Skibo, unpubl, obs.; Vaz Pinto et al., unpubl, obs.). In the case of the large gravel abrader, the most severe method of weight reduction is likely to be the actual dislodging of temper particles; abrasion of the clay itself is secondary (Figure 3). Small, fine-sand tempered particles are more likely to be dislodged than the coarse-sand temper providing, in part, the improved abrasion resistance of the coarsesand tempered briquettes. The pea gravel and sand abraders are capable of dislodging temper; not by impact but by eroding the material adjacent to a particle until it is removed. However, pea gravel is less able to abrade the exposed clay between temper particles. Unlike sand, the mean diameter of pea gravel is too great to effectively abrade material between the closely spaced fine-sand tempered particles. This accounts, in large part, for the increase in abrasion resistance of the fine-sand tempered briquettes abraded with pea-gravel. On the basis of these findings, one might suppose that the most resistant briquettes would be those consisting ofdensely packed large particles. Not only would such temper be less easily displaced, but the susceptible clay matrix between temper particles would be exposed only to very fine abrasives. One does find that many prehistoric pots, especially cooking pots, contain a high density of large temper particles, seemingly more temper than would have been needed to confer adequate thermal shock resistance. Moreover, after a certain point, the addition of more temper reduces the workability of the paste. Perhaps abrasion resistance is one factor that influenced the technological decision to use great amounts of large temper particles. Cooking pots are used wet, often with stirrers that contribute to abrasion, and frequently they are set down on an abrasive surface. In such a

90

J.M. SKIBO AND M. B. SCHIFFER

Figure 3. Displaced fine-sand temper particle.

Table 3. Height of pedestalled temper particles in uncalibrated units. Computed by first focusing on the apex of a temper particle under 100 x magnification and then counting the units on the fine focusing control necessary to resolve the base of each particle (each figure is the mean of 20 separate readings)

Wet

Abrader Sand Pea gravel Large gravel

Dry

Fine-sand

Coarse-sand

Fine-sand

Coarse-sand

temper

temper

temper

temper

33.00 53-76 84.45

40.40 73.45 74.75

32.10 44.15 51.80

27.50" 43.30 72.15

use environment, a cooking pot lacking good abrasion resistance might wear out very quickly. Another potentially useful finding of the study was the discovery that different abraders have a marked effect on abrasion and also create test briquettes with distinctive surface topographies. The most obvious difference is that material loss increases with an increase in abrasion intensity, that is, as one goes from fine to coarse abrader (Table 2). This difference is also illustrated by an increase in the height of the pedestalled temper particles (Table 3). In either wet or dry conditions, temper is pedestalled most when abraded with large gravel and least when abraded with sand. The latter case is probably a function of the stage of abrasion; much more tumbling would lead to higher pedestals. The sand and large gravel abraders also create a much rougher surface texture than the pea gravel (Figure 4). Two processes may be responsible for this effect. First, the large gravel abrader causes much more surface microchipping and temper displacement, resulting in a rougher surface topography. Secondly, there is a relatively large amount of temper

WET AND DRY CERAMIC ABRASION

91

Figure 4. Differences in surface texture caused by different abraders: (a) was abraded with stand, (b) with pea gravel, and (c) with large gravel ( x 7.75). and clay material removed from the briquette abraded with gravel (up to 11.38% by weight); this material, like sand, acts as a fine abrasive. Therefore, either sand or the material removed from the briquette abraded in large gravel creates a rougher microsurface texture. This may account for the surface texture similarities of the briquettes abraded in sand and large gravel. Although many factors in addition to abrader size contribute to the microtopography of abraded specimens, including strength of the bonds between temper and clay, the above observations suggest that microtopographic attributes may help to elucidate the nature of the abrasive process on archaeological specimens. This could be very useful in ceramic use-wear studies. Discussion Our experiments have demonstrated that the presence of water substantially accelerates ceramic abrasion processes, even after short-term immersion. Briquettes abraded wet lost almost twice as much material as those abraded dry (except the sand-abraded specimens), regardless of the size of the abrader or composition of the briquette. In almost all archaeological situations, the differential between wet and dry abrasion will be greater because wet~dry cycles and long-term immersion weaken ceramics (Murphy, 1981; Ware & Rayl, 1981) and so should render them much more susceptible to abrasion. A forceful example of the destructive effects of long-term water immersion on ceramic abrasion resistance came to the attention of the junior author during the 1985 field season at Kourion, a Roman site in Cyprus (Soren, 1985). Drinking water for the crew is kept in kuzis, ceramic water jars of modern Cypriot manufacture (Figure 5). When dry, kuzis appear to be well-fired with a reasonably hard paste. Throughout the field season, kuzis

92

J.M. SKIBO AND M. B. SCHIFFER

Figure 5. A modern Cypriot waterjar (kuzi).

Figure 6. Patch of abrasive wear on the base ofa kuzi, amplified by wet conditions. experience almost constant wet conditions and develop a distinctive patch of abrasive wear on the base from tipping (Figure 6). After 4 weeks of continuous use, the patch became so friable that one could scratch away temper and clay particles with a fingernail. One vessel was so weakened that when it was placed on a rock after filling it developed a leak. Nevertheless, this pot apparently regained some of its former strength after drying for several weeks, as it could no longer be scratched. Currently, archaeologists are beginning to examine, experimentally, the effects of various technological features on specific mechanical performance properties (Bronitsky,

WET AND DRY CERAMIC ABRASION

93

1986), following avenues of research first mapped by Shepard (1965). These studies are beginning to suggest that certain features once thought to be entirely stylistic, such as temper type, do contribute significantly to vessel performance. In view of the sizeable differential of wet and dry abrasion processes, one might expect some potters in traditional societies to have developed ways to improve the abrasion resistance of pots subjected to use under wet conditions, such as cooking, food serving and water storage. Indeed, differences in the composition of pastes used in vessels for cooking, or noncooking activities are known ethnographically (e.g. Thompson, 1958; DeBoer & Lathrap, 1979; Rye, 1981) and have also been documented in archaeological assemblages (e.g. Steponaitis, 1983). We suggest that there is an urgent need to discern the effects of manufacturing techniques on ceramic abrasion resistance. For example, surface treatments like polishing, slipping, painting, and application of resins might reduce the permeability of a vessel, thereby conferring greater protection against abrasion under conditions of short-term immersion. Similarly, treatments like polishing, which create smoother surfaces, should increase a vessel's resistance to abrasion under all conditions of use. In addition, as the experiments summarized here suggest, temper variation affects abrasion (also Vaz Pinto et al., unpubl, obs.). This family of questions is now being addressed experimentally in the Laboratory of Traditional Technology. So far, our results do indicate that surface treatment and temper types affect ceramic abrasion resistance (Schiffer & Skibo, unpubl. obs.). As the corpus of experimental findings from various laboratories grows, it may become possible to assess how the potter compromised particular technological features in order to obtain a pot with acceptable performance characteristics (Braun, 1983). As the study of ceramic use-wear expands in the years ahead, analysts must develop an appreciation for the diverse conditions under which particular kinds of Wear are produced. Abrasive wear is one of the most ubiquitous traces of use that we have observed on ceramic vessels from the American Southwest (e.g. Bray, 1982). The present study suggests that great differences in the degree of abrasive wear can be expected, depending on whether pots were used under wet or dry conditions. Clearly, this source of variability needs to be considered when interpreting abrasive wear. On the other hand, if abrasion is not found on pots known from other lines of evidence to have been used under wet conditions, the analyst may be able to posit the use of a protective coating. The rigorous study of use-wear will also require experimental work on distinguishing abrasive wear from salt erosion, thermal fatigue, freez~thaw damage, and other processes that can erode ceramics. We are optimistic that abrasive processes create distinctive types of surface modification. Finally, our findings have implications for explaining variability in the deterioration of ceramics in archaeological context. Archaeologists generally pay little attention to postuse deterioration of pottery (Schiffer, 1987; Bronitsky, 1986). One significant exception is Reid's (1984a, b) discussion of the poor preservation of fibre-tempered pottery in the Midwestern U.S. Reid argues that the infrequent occurrence and poor condition of Late Archaic pottery in northern latitudes results from the eventual destruction of the porous sherds by three processes: trampling by humans and animals, erosion of the buried sherds during frost heaving, and frost-wedging within the pore spaces created by the burning out of the fibre temper. Although, as Reid suggests, the most destructive processes may be frost-wedging, our data support the claim that abrasion either through cultural or natural processes results in significant sherd attrition. When the Nebo Hill sherds pictured by Reid (1984a: 44) are compared visually to fibre-tempered briquettes produced in our laboratory and subjected to freeze-thaw cycling, one can observe profound differences in their shapes and surface characteristics. The laboratory briquettes, although extremely exfoliated and friable after 15 freeze-thaw cycles (Figure 7), do not exhibit smoothed surfaces or the

94

J.M. SKIBO AND M. B. SCHIFFER

Figure7. Fine-sandtemperedbriquettesubjectedto freeze-thawcycling. same degree of edge rounding as the Nebo Hill sherds. The Nebo Hill sherds, in fact, more closely resemble sherds observed in the desert Southwest that have been subjected to fluvial abrasion (Skibo, n.d.). Recent experiments in our laboratory (Vaz Pinto et al., unpubl, obs.) have shown that fibre-tempered sherds have very low wet-abrasion resistance relative to sherds tempered with sand, grog and basalt. Clearly, additional experiments will be needed to determine the relative importance of abrasion and frostwedging in the deterioration of porous Midwestern ceramics. In any event, our experiment strengthens Reid's overall argument by providing data that demonstrate the increased abradability ofsherds, particularly fibre-tempered sherds, in a wet environment.

Conclusions This paper has shown by experiment that the presence of water accelerates various abrasion processes and that the w e t ~ r y differential is maintained regardless of abrader (sand, pea gravel, large gravel). Results were obtained in tumbling tests of untempered briquettes as well as those tempered with fine and coarse sand. The present experiments used relatively well-made and durable briquettes and were based on short-term immersion of sherds. In real archaeological situations, long-term deterioration and less durable ceramics may contribute to an even greater wet-dry differential in abrasion resistance. Based on the findings of this experiment, several implications were drawn for studies of ceramic technofunction, use-wear, and formation processes. First, it was suggested that potters perhaps found ways to improve the abrasion resistance of pottery that was habitually used wet. Secondly, it was recommended that use-wear analysts consider the accelerated abrasive wear that is produced when vessels are used wet. Thirdly, we proposed that sherds in wet depositional environments will abrade at high rates, contributing to the destruction of sherds of the kind noted by Reid (1984a, b). The present study represents just one small experiment among many that are needed to improve our appreciation of the processes that cause modification of ceramic artifacts. In the future, it is hoped that such experiments will establish a basis for extraction of more behavioural information from those problematic traces on sherds.

Acknowledgements The research was supported in part by grants from the National Science Foundation (BNS-83-10609 and BNS-84-19935) to M. B. Schiffer and by gifts from Harcourt Brace

WET AND DRY CERAMIC ABRASION

95

Jovanovich and the Stephen Tyler Fund (of the University of Arizona) to the L a b o r a t o r y of Traditional Technology. We also thank Mettler Instruments, Setra Systems, LabLine Incorporated, T e k m a r Company, GCA/Precision Scientific, Tenney Engineering, Discount Agate House, IBM, and Burr-Brown Research for donations of equipment used in the experiment. Special thanks must go to M a r y Van Buren for the m a n y comments and suggestions throughout the project and to Nancy Kowalski for providing insightful comments on an earlier draft of the paper. David Soren of the Kourion Project donated the worn-out kuzi and Sara Orzach transported it to Tucson; we are grateful to both. Finally, we thank Jean Sakwa for her help in preparing the manuscript.

References American Society for Testing Materials (1983). 1983 Annual Book of A S T M Standards. Philadelphia: American Society for Testing Materials. Birkeland, P. W. (1984). Soils and Geomorphology. New York: Oxford Unviersity Press. Braun, D. (1983). Pots as tools. In (A. Keene & J. Moore, Eds) Archaeological Hammers and Theories. New York: Academic Press, pp. 107-134. Bray, A. (1982). Mimbres Black-on-White, Melamine or Wedgwood? A ceramic use-wear analysis. The Kiva 47, 133-151. Bronitsky, G. (1986). The use of materials science techniques in the study of pottery construction and use. In (M. B. Schiffer, Ed.) Advances in Archaeological Method and Theory, Vol. 9. New York: Academic Press, pp. 209-276. Bronitsky, G. & Hamer, R. (1986). Experiments in ceramic technology: The effects of various tempering material on impact and thermal shock resistance. American Antiquity 51, 89-101. Davidge, R. W. (1979). Mechanical Behaviour of Ceramics. Cambridge: Cambridge University Press. DeBoer, W. R. & Lathrap, D. W. (1979). The making and breaking of Shipiho-Conibo ceramics. In (C. Kramer, Ed.) Ethnoarchaeology: Implications of Ethnography for Archaeology. New York: Columbia University Press, pp. 102-138. Dowman, E. A. (1970). Conservation in Field Archaeology. London: Methuen. Griffiths, D. M. (1978). Use marks on historic ceramics: A preliminary study. Historical Archaeology 12, 68-81. Hally, D. J. (1983). Use alteration of pottery vessel surfaces: An important source of evidence for the identification of vessel function. North American Archaeologist 4, 3-26. Hockman, A. & Kessler, D. W. (1950). Thermal and moisture expansion of some domestic granites. U.S. National Bureau of Standards 44. Research Paper 2087, pp. 395-410. Hudec, P. P. (1978). Rock weathering on the molecular level. In (E. M. Winkler, Ed.) Decay and Preservation of Stone, Engineering Geology Case Histories 11. Boulder, Colorado: Geological Society of America, pp. 47-5 l. Kuenen, W. J. (1956). Experimental abrasion of pebbles. Journal of Geology 64, 336-368. Murphy, L. (1981). An experiment to determine the effects of wet/dry cycling on certain common cultural materials. In (D. J. Lenihan, Ed.) The Final Report of the National Reservoir Inundation Study, Vol. 2. Santa Fe, New Mexico: United States Department of Interior, National Park Service, pp. 8-i-8-17. Ollier, C. D. (1975). Weathering. London: Longman. Reid, K. C. (1984a). Nebo Hill: Late Archaic prehistory on the Southern Prairie Peninsula. Publications in Anthropology 15. Lawrence: University of Kansas. Reid, K. C. (1984b). Fire and ice: New evidence for the production and preservation of Late Archaic fiber-tempered pottery in the mid-latitude lowlands. American Antiquity 48, 675-706. Rye, O. S. (1981). Pottery Technology: Principles and Reconstruction. Washington, D.C.: Taraxaeum. Schiffer, M. B. (1987). Formation Processes of the Archaeological Record. Albuquerque: University of New Mexico Press.

96

J.M. SKIBO AND M. B. SCHIFFER

Schiffer, M. B. & Skibo, J. M. (n.d.). A provisional theory of ceramic abrasion. Manuscript on file, Laboratory of Traditional Technology, University of Arizona, Tucson. Scott, H. G. (1985). Friction and wear of zirconia at very low sliding speeds. In (K. C. Ludema, Ed.) Wear of Materials. New York: American Society of Mechanical Engineers, pp. 8-12. Shackley, M. L. (1974). Stream abrasion of flint implements. Nature 248, 501-502. Shepard, A. O. (1965). Ceramics for the Archaeologist. Washington, D.C.: Carnegie Institution of Washington, Publ. 609. Shipman, P. & Rose, J. (1983). Early hominid hunting, butchering, and carcass-processing behaviors: Approaches to the fossil record. Journal of Anthropological Archaeology 2, 57 98. Skibo, J. M. (n.d.). Fluvial sherd abrasion and the interpretation of surface remains on southwestern bajadas. North American Archaeologist (in press). Soren, D. (1985). An earthquake on Cyprus: New discoveries from Kourion. Archaeology 38(2), 52-59. Steponaitis, V. P. (1983). Ceramics, Chronology and Community Patterns. New York: Academic Press. Thompson, R. H. (1958). Modern Yucatecan Maya pottery making. Memoirs of the Society for American Archaeology 15. Vaz Pinto, I., Schiffer, M. B., Smith, S. & Skibo, J. M. (n.d.). Effects of temper on ceramic abrasion resistance. Manuscript submitted for publication. Wallbridge, N. D., Dowson, D. & Powers, E. W. (1983). The wear characteristics of sliding pairs of high density polycrystalline oxide under both wet and dry conditions. In (K. C. Ludema, Ed.) Wear of Materials. New York: American Society of Mechanical Engineers, pp. 202-211. Ware, J. & Rayl, S. (1981). Laboratory studies of differential preservation in freshwater environments. In (D. J. Lenihan, Ed.) The Final Report of the National Reservoir Inundation Study, Vol. 2. Santa Fe: United States Department of Interior, National Park Service, pp. 3-i-3-108. Washburn A. L. (1963). Geocryology. New York: John Wiley & Sons. Wentworth, C. K. (1919). A laboratory and field study of cobble abrasion. Journal of Geology 27, 507-521. Winkler, E. M. (1975). Stone: Properties, Durability in Man's Environment. New York: Springer-Verlag.