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| Tuesday, 15 October 1996 | ||
Quantifying Pottery in Pueblo III Assemblages from Southwestern Colorado
Christopher Pierce
The need to quantify pottery from archaeological contexts stems from the desire to compare different pottery assemblages or collections. Since a primary goal of the Sand Canyon Archaeological Project and the Site Testing Program involves quantitative and qualitative comparisons of assemblages to ascertain temporal, functional, and social relations, the methods used to quantify pottery for comparison must be considered. The task of quantifying pottery is complicated by the fact that most pottery recovered from archaeological contexts consists of broken pieces, or sherds, rather than the complete vessels--the unit of manufacture and use. This discussion begins with an assessment of the characteristics of sherds and vessels as units of comparison of archaeological assemblages and finds sherds to be the most appropriate unit in most cases. The two measures of sherd abundance used in the Crow Canyon analysis, count and weight, are then examined, and weight is found to be the best measure for most cases because of the effects that pottery technology, use, deposition, and postdepositional history (including excavation and lab treatment) have on sherd counts.
Sherds vs. Vessels
Although a variety of methods have been developed for estimating the numbers of vessels represented by a particular group of sherds (Orton 1993), the issue remains as to whether vessels or sherds offer the most valid and reliable units for quantification. A number of problems arise from the use of vessels as the unit of quantification. The most prominent problem concerns the practical issues involved in refitting sherds into vessels or estimating vessels from particular sherd characteristics. Most refitting efforts incorporate only a small proportion of the sherds recovered, and these sherds are usually from restricted contexts that introduce additional biases.
Methods for estimating numbers of vessels from sherds, usually minimum numbers, also introduce practical and quantitative problems. Vessel estimation methods either count unique parts of vessels such as bases, or measure some part of vessels for which a known quantity exists in a single vessel. For example, circular rims always cover 360 degrees. Thus, if you sum the degrees covered by sherds from vessels of the same size and divide this number by 360, you obtain an estimate of the minimum number of vessels represented by those sherds. The accuracy, precision, and validity of these estimation methods are affected by the degree of fragmentation of sherds and by approaches used for grouping or aggregating sherds for measurement and estimation. The smaller the sherd, the more difficult it is to identify specific vessel parts and make accurate and precise measurements.
Grayson (1984) explores the problem of aggregation in the context of estimating minimum numbers of individuals from faunal remains. For pottery, aggregation affects minimum vessel number estimates in two ways. First, in cases where different pieces of a specific vessel part--rims for instance--are used to calculate vessel estimates, these estimates will vary with the scale of the provenience units used to group sherds for estimation. If the entire assemblage of rim sherds is used, the smallest vessel estimate will result. As smaller and smaller provenience groupings are employed, the minimum vessel estimate for the whole assemblage will increase because in each provenience group the estimate is derived by rounding up to the nearest whole vessel (for example, sherds totaling 380 degrees of arc equals two vessels). The most extreme case would be provenience groups that contain individual rim sherds or single sherds from different vessel forms. In this case, the minimum vessel estimate equals the number of rim sherds.
A second aggregation issue arises when rim sherds are used to estimate numbers of vessels. The process of summing degrees of arc for sherds from vessels of the same size also involves aggregation. Grouping sherds from vessels of the same size usually involves estimating the diameter of the vessel opening and creating groups through arbitrary divisions of the total distribution of sizes, or using modes in the distribution of diameter values to make "natural" groups. Once again, the minimum vessel estimate will vary with the number of size groups employed.
Although the significant practical and quantitative problems inherent in estimating numbers of vessels from sherds support the use of sherds as the unit of quantification, using sherds has been criticized because their abundance is affected by both the number and size of vessels (Egloff 1973; Orton 1993). However, as Feathers (1990) argues, this criticism is valid only if vessels are the unit of comparison, and in most cases it is sherd assemblages and not vessels that are compared. When comparing assemblages, the relationship of sherd abundance to vessel count and size is no longer a bias, but a potential source of meaningful variation among sherd assemblages. However, in cases where vessels are the unit of comparison, there is little choice but to work with the less reliable vessel estimates. Sherd Count vs. Sherd Weight
Although sherd abundance is a more reliable unit of quantification, important differences exist between count and weight as measures of sherd abundance. The most significant difference between counts and weights is that counts vary dramatically with the degree of fragmentation whereas weights do not (Egloff 1973; Evans 1973; Solheim 1960). The degree of fragmentation of pottery in a given assemblage is a function of a variety of factors, including manufacturing technology, uses, discard processes, and postdepositional history. Patterns of variation between counts and weights of pottery can, therefore, yield important information about the pottery itself and potentially unique histories of individual assemblages that can affect comparisons. We examine the relationships between counts and weights of sherds along several dimensions to assess the relative importance of these factors on the tested-site assemblages and their potential impact on comparative analyses.
The effects of manufacturing technology and vessel use on sherd fragmentation can be determined by comparing counts and weights of sherds for different classes of pottery. The most dramatic difference between counts and weights occurs at the ware level, particularly between gray wares and white wares. At each of the tested sites, the abundance of gray wares relative to white wares increases when using counts and decreases when using weights. This pattern derives from a consistent difference in the average size of gray and white ware sherds in all assemblages (Pierce et al. 1999). This pattern almost certainly results from differences between gray wares and white wares in their manufacture and use.
White wares recovered from the tested sites are predominantly bowls for serving and jars for storage, whereas corrugated gray wares are primarily cooking vessels. White wares tend to have thicker walls and finer temper, and they may have been fired at higher temperatures than gray wares, all of which promotes their mechanical strength. Gray wares, on the other hand, have larger temper composing a greater proportion of the paste (Wilson and Blinman 1991), which decreases their mechanical strength but increases their resistance to thermal stress (Steponaitis 1983; West 1992). In addition, the exposure of gray wares to thermal stress during use further weakens the pottery through thermal fatigue. Consequently, the larger size of white ware sherds mostly reflects their greater mechanical strength and less rigorous use relative to gray wares. These conditions also account for the differences apparentin frequencies of bowl forms relative to other forms when using counts and weights (Pierce et al. 1999).
In addition to the differences between gray wares and white wares, there is also a consistent tendency for the more specific traditional types to compose a larger percentage of the site assemblage when using weights. This reflects the fact that sherds assigned to traditional types are larger because larger sherds are more likely to have the attributes necessary to be assigned to a traditional type. Examining the percentage of Mesa Verde Black-on-white in the site assemblages illustrates the difference between counts and weights (Pierce et al. 1999). At every site, Mesa Verde Black-on-white makes up a greater percentage of the assemblage when weights are used. The difference between counts and weights can be dramatic, as illustrated by Stanton's Site, where Mesa Verde Black-on-white makes up 1.8 percent of the site assemblage by count but 8.1 percent of the assemblage by weight. The opposite pattern often holds for grouped types because smaller sherds are more likely to be assigned to these types.
The degree of sherd fragmentation also varies between sites located on the mesa tops in upper Sand Canyon, where sherds are smaller, and sites located on the talus slopes or in lower Sand Canyon, where the sherds are larger (Pierce et al. 1999). This systematic difference in sherd size most likely results from differences in historic land use. The mesa-top sites have received greater impact in historic times than the sites on the talus slopes and the sites in lower Sand Canyon. The sites with the smallest sherds, Shorlene's (5MT3918), Kenzie Dawn Hamlet (5MT5152), and Roy's Ruin (5MT3930), have been repeatedly plowed. The reduction of artifact size as the result of plowing has been discussed in detail elsewhere (Dunnell and Simek 1995; Pierce 1994). The unplowed mesa-top sites (Troy's Tower, G and G Hamlet, and Lillian's Site) have all been chained and used for cattle grazing. The remaining tested sites, all of which have larger-than-average sherds, can be divided into those located on the talus slopes or in lower Sand Canyon; these sites have not been plowed or chained, although cattle have grazed these areas.
Finally, there is a consistent difference between counts and weights when the pottery from early components is considered. This is especially true for Basketmaker III pottery. This early pottery makes up a greater percentage of the site assemblage when counts are considered; the percentage decreases when weights are considered, which indicates that the average size of these early sherds is smaller.
This discussion indicates that a great deal can be learned about different kinds of pottery and entire pottery assemblages by comparing counts and weights. However, as a general measure of abundance, weight is superior because it is less affected by sherd size. This is particularly true for the tested-site assemblages because of the large degree to which postdepositional factors have differentially altered sherd size. Consequently, most of the pottery assemblage comparisons discussed in this volume are based on sherd weights, although data tables normally present both counts and weights. In a few cases, sherd counts or other units and measures are used, and the rationale for the decisions to use these alternative measures are discussed in those sections.
References Cited
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