103 CURRENT PROBLEMS IN OBSIDIAN HYDRATION ANALYSIS Robert J. Jackson Abstract Obsidian hydration is alleged by some to have developed into a dating technique fully capable of yielding absolute chronometric dates. While signi- ficant advances in our understanding of the hydration process and methods of determining source-specific hydration rates have been made, there are many unresolved problems that may limit the accuracy and trustworthiness of absolute rate formulations. Extant analytic problems are reviewed and results of inter- laboratory comparisons of obsidian hydration measurements are discussed. Introduction Obsidian hydration has become a comrmTnly employed analytic technique, often to the extent that it is used without questioning the utility or accuracy of the method in specific archaeological applications. There is little doubt that obsidian hydration can be very useful for the analysis of many collections, but its potential value must be assessed on a site specific basis, with a full awareness of current problems and limitations of the method. Problems and limitations imposed on hydration studies fall into three basic categories: 1) the physical process of hydration and environmental variables that affect it; 2) problems in the measurement process; and 3) application and interpretation of hydration data for archaeological purposes. The Physical Process of Obsidian Hydration The history and nature of obsidian hydration has been well-documented (Friedman and Snith 1960; Evans and Meggers 1960; Clark 1961, 1964; Michel s 1965; Michels and Bebrich 1971; Friedman and Long 1976; Ericson 1977; Taylor 1976; Michels and Tsong 1980; and others). Very simply, all glasses (natural and artificial) are thermodynamically unstable and undergo progressive alteration through the gradual absorption of moisture fran the surrounding environment (soil and atmosphere). Absorbed water layers form gradients of concentration demarcated by diffusion fronts. The water content of the hydrated layers increa'ses greatly. This increased water content changes both the density and the volume of the hydrated layers. It was found that rhyolitic obsidian, the most abundant form of natural glass, contains about 0.1 to 0.9 percent water by weight, as derived from the parent magma. After cooling, molecular water is incorporated into the obsidian from its surface, increasing the water content tenfold to approximately 3.5 per- cent by weight for most obsidians. An increase in density raises the index of refraction of light passing through the obsidian, while increased volume produces mechanical strain at the interface between the layer of absorbed water and the nonhydrated interior of the obsidian, resulting in an optical effect 104 called birefringence (the power of double refraction). It is the strain- produced birefringence and the higher index of refraction that microscopically differentiate the hydrated from the unaltered obsidian. It was demonstrated that this hydration process is not only continuous, but the rate at which water enters the stone is relatively predictable and continued evenly despite the water content of surrounding environments, producing visible hydration rinds within a few hundred years of exposure (Friedman and Smith 1960: 482). It has been clear for some time that obsidian fram different geographic/ geologic sources hydrates at differing rates (Aiello 1969; Taylor 1976; Michels and Bebrich 1971; Ericson 1977; Kaufman 1980; and others). Source identification is but one hurdle to overcome in the delineation of source- specific obsidian hydration rates, and detailed chemical analyses have not yet been conducted for many obsidian source areas (cf. Hughes 1983; Kaufman 1980). In addition, while many major obsidian sources have been chemically differen- tiated, intra-source chemical variation and the potential effects on hydration have not been thoroughly explored (cf. Jack 1976; Taylor 1976; Ericson 1977; Hughes 1983). The physical process of obsidian hydration is still poorly understood, though many diffusion models have been proposed for the reaction and diffusion of water from glass surfaces. Charles' (1958) model, for instance, relied on base exchange of water and alkali, while Moulsen (n.d.) proposed two models, both relying on direct reactions of water and silicon networks. Ericson et al. (1976) have reviewed these and other theories of water diffusion into glass and structurally complex, rhyolitic obsidians. Drawing comparisons between simple glasses and obsidian, researchers have distinguished three classes of obsidian, characterized by molecular quantities of A1203, CaO, and NaO + K20; or excess alimina, calcalkaline, and excess alkali rhyolitic obsidians. The reaction of water and OH ions is said to vary in intensity and speed with increases and de- creases in the abundance of the three compounds (Ericson et al. 1976: 40). Friedman and Long (1976), however, found little relationship between alumina ratios, but did find that as SiO2 content increased, so did the hydra- tion rate. They also found that increased CaO and MgO content reduces the hydration rate. From these findings Friedman and Long (1976: 347) derived a 'chemical index' which they suggest monitors hydration rate more accurately than silica content alone. Michels and Tsong (1980) and Michels (1982, 1983) also use alumina, alkali, and silica indices to determine the effects of chemistry on hydration rate. However, few obsidian sources have been chemically analyzed to determine the extent and nature of the intra-source variability for chemical components critical to the hydration process. The effect of temperature on the rate of obsidian hydration has been more widely discussed, and is better understood than chemical composition (cf. Friedman and Smith 1960; Clark 1961; Michels and Berbrich 1971; Friedman and Long 1976; Ericson 1977; Taylor 1976; Michels and Tsong 1980; among others). It is recognized that hydration rate varies as a power function of temperature, so mean annual air temperature cannot be used to calculate hydration rates. In addition, it has been suggested that obsidian exposed on the ground surface will hydrate more rapidly than buried obsidian, a disparity that is greater at high elevations. Friedman (1983, personal comumnication) suggests that hydration 105 rates for some obsidians may as much as double with a 100 Centigrade increase in temperature. Similarly, changes in the effective temperature on the order of 2-30 Centigrade may affect rates by 20 percent. This degree of temperature change may approximate that imposed by climatic fluctuations over the millenia. Various methods of calculating annual effective or ambient air temperatures and ground temperature gradients have been developed and proposed (Friedman and Long 1976; Michels 1982, 1983; Trembour and Friedman, this volume). While temperature is recognized as a significant affective variable in the rate of obsidian hydration, regional temperature data are often not incorporated into hydration rate formulations. Obsidian is generally dark in color, readily absorbing heat like a small "solar collector." The temperature of surface exposed obsidian often far exceeds the surrounding air or ground temperature, and should be considered as a potentially important variable affecting the hydration rate. Layton (1973) was one of the few researchers to investigate differences in hydration between exposed and buried obsidian artifacts. He compared two artifact assemblages fram archaeological sites in northwestern Nevada, both consisting of similar, temporarily diagnostic projectile points. Artifacts from one of the sites occurred on the exposed desert surface, whereas the second assemblage was excavated from a moist midden site. Comparison of hydration measurements for similar projectile point types from the two sites revealed that the surface assemblage exhibited much greater hydration thicknesses than buried materials. This led Layton (1973: 131) to conclude that high surface temperatures accounted for a greatly accelerated hydration rate. There are several problems with Layton's study which make his conclusions less than convincing. First, the geologic source(s) of the obsidian were not determined, but were assumed to be homogenous. Northwestern Nevada contains numerous obsidian sources (Hughes 1982, 1983), many of which have not been investigated. Furthermore, unlike flaking debitage, formal tools experience a higher incidence of curation and are more likely to be transported greater distances. It cannot be assumed, a priori, that projectile points from sites in close proximity to an obsidian source derived from that source. Variation in chemical camposition, alone, might account for the disparity in hydration thicknesses between the two sites (cf. Hughes 1983). Secondly, Humboldt type bifaces were the most numerous point type in Layton's sample. Many Great Basin archaeologists consider Humboldt a temporally and functionally problematic type (Thomas 1981; Heizer and Hester 1978; Hughes 1983; and others). Excluding Humboldt points, Layton's sample of projectile points would be reduced to 12, which are further split into surface and subsurface projectile point lots between the two sites. Such a sample size may be too small to demonstrate consistent differences between surface and subsurface hydration. Other investigators (Origer and Wickstrom 1981; Origer 1982) have addressed the problem of surface versus subsurface hydration in the North Coast Ranges of California, with very different results. The arid soils of the Great Basin are generally shallow, pavemented, and support widely dispersed bushes and some grasses with bare spots between patches of vegetation. Archaeological specimens laying on 106 the ground surface were not insulated from solar radiation and when subjected to direct sunlight were described by Layton as too hot to hold in one's hand. In contrast, the soils of our study area are deep often churned by a variety of disturbing agents (i.e. gophers, worms, erosion, and discing), and generally support grasses, forbs, and occasional scattered trees that serve to insulate archaeological specimens from intense solar radiation. It is suggested that exposure toectreme temperatures of the Great Basin influenced the hydration rate in Layton's study (1973) while the temperatures of the Santa Rosa Plain had much less effect since insulated from less intensive solar radiation (Origer 1982: 81). Stability and integrity of archaeological deposits is perhaps the most overlooked issue regarding the surface-subsurface hydration problem. Many hydration analyses suggest that small archaeological materials experience considerable post-depositional, horizontal and vertical movement (Layton 1973; Jackson 1982, 1983; Hall 1983; Origer 1982; among others). Many archaeological materials were undoubtedly deposited on living surfaces that were subsequently buried, yet the temporal range of site occupa- tion is often reflected by the artifact types on the surfaces of deep archaeo- logical deposits. In many instances, there is no reason to assume that buried obsidian artifacts were not exposed on the surface as long as material observed at the time of archaeological recording and collection. Archaeologists may never be able to determine the depositional history of individual artifacts to the degree necessary for precise hydration temperature calculation. WYithout taking these issues into account, temperature calculation may also misrepre- sent the accuracy of the method for many archaeological applications. This is not to suggest that temperature should be ignored. Ambient air temperature data should be calculated and used for those archaeological materials that appear to evidence long-term surface exposure and intact depositional provenience for features such as burials. For many sites, however, the most reasonable approach might be the calculation of regional temperature calibrations based on both air and ground temperature data. Other variables potentially affecting the rate of hydration are soil chemistry, weathering, and burning. Kaufman (1980: 379) has suggested that both geothermal activity and soil pH may be important affective variables in the hydration process, but very little research in this area has been conducted. Friedman (1983, personal communi- cation) has found that alkali soils often remove or obliterate hydration rinds. He further notes that a whitish coating on artifacts can indicate prolonged contact with alkaline soils. Further research is also necessary to understand the process of patination as it relates to obsidian hydration. Empirically, heavily patinated artifacts often produce diffuse, ill-defined hydration rinds. It is also possible that exposure to alkaline soils or patination accounts for the obliteration of hydration on artifact surfaces. Weathering of obsidian can physically remove hydration rinds through mechanical processes. Water tumbling and erosion from wind-borne particles are 107 the two most carmnn sources of weathering (Friedman and Snith 1960: 485), thus measurements obtained fran weathered specimens may be erroneously snall or non-existent. Logically, it would seem that since older artifacts have been exposed for longer periods, they would therefore have had more opportunity to weather and erode. However, the specific microenvironments in which artifacts were deposited probably played the greatest role in determining the severity of such inpacts. It should be apparent that the possibility of anomalous or poor resolution in hydration data may result from current problems relating to the physical circumstances of the hydration process and the environment in which it occurs. More detailed technical discussion of these various topics appears in Friedman and Smith (1960), Clark (1964), Michels and Bebrich (1971), Taylor (1976), Ericson (1977), and Michels and Tsong (1980). Measurement of Obsidian Hydration More than one archaeologist has been discouraged from conducting obsidian hydration studies because of poor results obtained by inexDerienced technicians whose fundamental errors were not detected for several years. There are currently no agreed upon means of assessing such knowledge and ability. The basic procedures for the preparation and measurement of obsidian hydration specimens appear, from textbook descriptions, to be quite easy (Clark 1961; Michels 1965; and Mlichels and Bebrich 1971). Anyone with access to a polarizing microscope, lapidary saw, and a few miscellaneous, inexpensive supplies can obtain an appropriate text, learn the fundamental procedures, and prepare thin sections. There are a multitude of subtle problems and pitfalls in the technique which can deceive the self-taught or inexperienced analyst and produce inaccurate data. For instance, large hydration bands are relatively easy to observe, but small hydration bands can be quite difficult to measure. Thick hydration bands usually produce obvious birefringence, but hydration layers in the one micron range tend to be faint and difficult to discern, often requiring a more trained, experienced eye. Technical categories potentially producing variability or affecting the visual nature of the hydration band include: the location and nature of the hydration cut on the artifact; the method of grinding and mounting; the quality and magnification of the microscope; and the locations and method of measurement. Many technical descriptions of the preparation process are relatively detailed, but slight modifications in procedures have been and should be made according to the particular attributes, idiosyncracies and needs imposed on individual laboratories by different equipment and personnel. Ultimately, the accuracy of certain sets of techniques should be determined by the degree of replicability in measurements between laboratories and technicians. The concern for replicability of hydration rim measurement is by no means new. In the early 1960's Donovan Clark re-examined slides he had studied earlier and found a mean deviation of 0.13 microns (Michels 1965: 17). In the mid-1960's Joseph Michels conducted a series of experiments to determine the variance in hydration readings on the same specimen by two analysts using the 108 same equipment. He found an average variance of approximately 0.01 microns between the two readers involved in the experiment (Michels 1965: 18, 19). An error factor of 0.2 microns, imposed by optical limitations inherent in the magnification process, is generally accepted. Michels' test and others like it are valuable for gauging the accuracy of readings between technicians working with the same procedures and equipment. Inter-laboratory Data Comparisons To my knowledge, no published reports have appeared which discuss the comparability of hydration readings between separate laboratories, each using their own equipment and procedures. To address this problem, inter-laboratory comparisons of obsidian thin sections were conducted. Two sets of obsidian hydration slides were selected. The first set consisted of 30 slides from a site in Napa Valley, California, prepared at the U.C. Davis Obsidian Hydration Laboratory several years ago. These specimens were selected for two reasons: 1) the technician responsible for their preparation measured the specimens at 500X magnification, different than the magnification used for examination of the second data set at U.C. Davis; and 2) an independent preparator insured that both technicians involved in the experiment were unfamiliar with the specimens. Thus, bias resulting from preparation or knowledge of the archaeological assemblage was avoided. The second set of specimens (n=31) were prepared from obsidian samples obtained from Owens Valley archaeological sites. The U.C. Davis Obsidian Hydration Laboratory examined both sets of thin sections under 1250X magnification using an oil immersion objective on a Lietz polarizing microscope. Eight readings were recorded for each specimen, four on each of two sides of every thin section. A two-tailed, difference of means t-test was applied to the resulting data to determine the statistical similarity between different edges of each specimen. If mean values for both edges were found to represent the same hydration thickness, a grand mean was derived for all eight readings. If the two sides were found to be dissimilar, the mean hydration value for the readings from each side were calculated. The 61 specimens were then submitted to the Obsidian Hydration Laboratory at Sonoma State University. The Sonoma technician commonly examines specimens at 563X magnification using an American Optical microscope (Origer 1982). Six readings were taken on each specimen and a mean value derived. A very good correlation between laboratories was obtained for the Napa Valley specimens (Table 1A). The average difference between the Sonoma State and U.C. Davis readings was 0.15 microns, less than the inherent error factor of 0.2. Measurements were identical for 10 of the 20 specimens when rounded to the nearest tenth micron. While the range of differences was 0-0.6 microns, only three specimens differed more than 0.2 microns. However, the U.C. Davis and Sonoma State readings for Owens Valley speci- mens were quite dissimilar (Table 1B). In this case the Sonoma technician had great difficulty identifying and measuring hydration bands on the Owens Valley slides; he was unable to measure 12 of the 31 specimens, and coamented on additional measurements that the slides appeared "poorly prepared," or that 109 they exhibited "ragged edges." The Davis technician had little difficulty measuring these same Owens Valley specimens. Of the specimens that he found readable, the average difference between Davis and Sonoma measurements *was 0.7 microns, and the greatest difference was 1.4 microns. Such discrepancies can translate to several hundred years of elapsed time, depending on the obsi- dian source under examination. Fram results of the Napa specimens it appeared that both technicians obtained similar hydration measurements under certain circumstances, but that inter-laboratory difficulties arose when specimens were prepared for examin- ation under 1250X magnification. One possible explanation for the discrepancy between laboratories on the Owens Valley sample relates to the difference in specimen preparation according to the intended magnification. A higher power objective, such as the lOOX oil immersion lens, captures a smaller percentage of the transmitted light field so that illumination of the image is not as great as that achieved with lower magnification. Higher magnification produces larger images which may, in some instance, allow the analyst to discern and more precisely measure sqall hydration bands. To compensate for decreased light under high magnification, there has been a tendency to prepare hydration sections quite thin, thus exceeding the range of thickness optimal for measure- ment under lower magnification. Consequently under high magnification there is a greater tendency for partial obliteration of thin section edges resulting from thinner sample preparation. This particular example illustrates that such affective variables do intervene in the measurement process and that, in some cases, these can be attributed to differences in sanple preparation. Procedures for thin section preparation have beenr modified since this comparative study was completed in order to increase the potential for interlaboratory calibration. Another inter-laboratory comparison of obsidian hydration results was recently conducted, involving measurement of hydration by two laboratories, each cutting, preparing and examining separate thin sections on the same obsidian specimens. Artifacts from archaeological sites in Kern County, California (CA-Ker-317 and 878) were submitted to the Obsidian Hydration Laboratories at U.C.L.A. and Sonoa State University. Laboratory procedures follwed at U.C.L.A. were unavailable at the time of this writing, but Sonoma State lab methods were the same as those discussed in conjunction with the previous study. Measurements obtained by each laboratory are listed in Table 2. Data from several specimens which were in no way comparable, such as a "no visible hydration" determination from one laboratory compared with two hydration bands observed by the other laboratory (Table 2) were not included in statistics derived fram the comparative study. Of 31 compared specimens, the average difference between laboratory measurements was 0.56 microns, quite similar to the difference obtained in the Sonoma-Davis study, while the greatest difference was 1.8 microns. Sixteen (50%) of the compared specimens varied 0.3 microns or less. Considering the potential differences in preparation and measurement procedures, equipment, technicians, and location of hydration cuts, the differences in measurements between laboratories is not altogether surprising. One may question whether or not it is desirable or necessary that slides prepared for one laboratory be interchangeable or measureable by another. Under normal circumstances hydration specimens are prepared at a specific 110 laboratory with the expectation that thin sections will be measured at the sae lab. Apart fran the issue of inter-laboratory comparability, practical problems sametimes arise which argue for standard preparation procedures. For example, obsidian hydration examination of several hundred artifacts was recently conducted for a large archaeological project in the North Coast Range of California. The analysis was performed by both Sonoma State University and U.C. Davis laboratories, each examining part of the assemblage. Scheduling conflicts required that specimens prepared for analysis at one of the labs be sent to the other for measurement. Standardization of preparation proce- dures since the time of the previous comparative study enabled a technician fram one laboratory to prepare thin sections suitable for measurement at the other laboratory. The lack of correspondence in many hydration measurements between laboratories should not be dismissed as minor, but let us consider the implications of inter-laboratory differences in hydration data with respect to interpreting the hydration data fram the Two Eagles site in Owens Valley (Table 1A). While significant variatAon in individual hydration measurements existed between the two laboratories, the majority of measurement obtained by both labs fell between 3.0 and 4.0 microns. These data are relatively well clustered, suggesting major site occupation during a relatively restricted time period. A number of different obsidian artifacts from the same site were examined for obsidian hydration by the laboratory at U.C. Riverside with very similar results in terms of mean and range of hydration. Inter-laboratory comparative data suggest that obsidian hydration measurements obtained for individual artifacts may occasionally be in error for any number of reasons, so hydration measements for single specimens should not be neavily relied on for chronologic imformation. Inter-laboratory comparisons suggest that large samples may be most appropriate to insure that the ages (relative or absolute) of archaeological sites are accurately reflected. Further intra- anc inter-laboratory comparisons should be made, including examination of selected specmens by ditferent technicians using tne same equipment, the same technicians using different equipment, ana different tecnnicians using different equipment. In addition, multiple hydration cuts on the same artifacts by the same and different technicians shoula be made and measured. It is important that same system of monitoring tne consistency and compar- ability ot results between laboratories be developed so that technicians are adequately trained and there are means of checking work quality and precision. While each laboratory will develop procedures suited to the equipment and indi- viduals involved, broad guidelines should be established to encourage inter- laboratory comparisons of methods and data. Once major variables in the prepar- ation and measurement process are identified and controlled, it will be possible to isolate those which produce significant inter-laboratory varia- bility, and improve the precision of the hydration measurement process. Sunnary of data from inter-lab comarison of obsidian hydration measur.nents. Owens Valley Archaeological Sites UCD La # 891 890 889 888 887 886 885 884 883 882 881 880 879 878 877 U.C. Davis Reading TM Eagles Site 3.27 3.36 2.91 2.77 2.96 2.86 3.24 3.06 3.48 3.17 3.49 1.22 NVH 3.56 3.23 Sonoia State Reading 4.0 3.9 3.4 3.4 2.9 3.0 3.8 3.4 4.3 2.6/3.1 3.8 Readings fran aberrant, later feature at Two Eagles site 2.03 1.4 1.72 1.97 3.65 Crater Midden Site 1.8 1.88 2.01 2.26 Pinyon House Site 1.21 1.23 NVH 6.3 1.6 NVH 3.5 Mean Difference on mutually read specimens = (the "-" symbol represents specimens that could not be visible hydration). Measurements in microns (um). 0.7 microns read; NVH denotes no Table 1A: 111 896 895 894 893 892 1.6 2.5 4.0 900 899 898 897 876 875 874 873 872 871 870 2.5 2.9 1.1 5.3 2.2 NVH 112 Table 1B: Surmury of data from inter-lab comparison of obsidian hydration measurements. Napa Valley Archaeological Site CA-Nap-58 XI) Lab # 1191 1179 1186 1177 1155 1149 1148 1146 1145 1142 1139 1133 1129 1127 1124 1122 1118 1114 1240 1235 1230 1229 1226 1223 1225 1218 1214 1208 1207 1201 U.C. Davis Reading 2.74 4.29 2.23 1.93 3.12 2.38 1.16 3.0 3.4 2.86 2.01 3.13 2.71 1.54 1.95 1.74 3.91 1.95 2.4 2.66 2.86 2.36 1.72 3.23 1.64 1.5 Sonana State Reading 2.7 4.5 2.2 2.2 3.1 3.0 1.3 2.9 3.2 2.9 2.2 3.1 2.7 1.2 2.6 1.6 3.5 2.2 2.7 3.1 2.3 1.0 1.7 3.2 1.6 1.3 Mean Difference on mutually read specimens = 0.15 microns Measurements in microns (rn). 113 Table 2: SuTmary of data from inter-lab comparison of obsidian hydration measurements. Sonoma State Mleasurement CA-Ker-317 1.3 4.9 5.0 5.8 8.0 5.8 7.2 8.3 4.6 8.9 9.5 2.7/4.6 CA-Ker-878 0.9 2.3 1.9 2.2 4.4 3.3 2.3/5.2 1.4 3.5 2.2 NVH 2.2 4.3 3.3 7.3 8.0 3.9 2.6/6.2 7.1 7.1 6.0 8.0 7.0 7.3 2.6 U.C.L.A. Measurement 1.0 1.8/3.8 5.3 5.9 7.6 5.9 7.2 8.8 4.4 8.0/9.0 9.4/10.7 3.7 NVH 1.6 3.7 NVH 4.0 2.1 5.4 2.5 NVH 3.1 1.4 NVH 1.4 4.6 2.5 6.6 8.0 NVH NVH 6.9/7.3 6.8 5.4 7.9 6.0 7.0 19.4 * Hydration specimens not included in calculation of inter-laboratory differences in measurements. NVI indicates no visible hydration band. Measurements in microns (n). Specinn Cat. No. 317-091 317-108 317-109 317-111 317-122 317-182 317-187 317-191 317-049 317-062 317-073 317-018 78-001 78-025 78-027 78-030 * 78-037 78-069 78-074 78-088 78-092 78-093a 78-093b 78-110 78-120 78-123 78-140 78-160 78-181 78-193 * 78-210 * 78-217 78-220 78-226 78-243 78-251 78-257 * 78-263 * 114 References Clark, D. L. 1964 Archaeological chronology in California and the obsidian hydration method. University of CaZifornia Archaeological Survey Annual Report 1963-64: 139-230. Los Angeles. Ericson, J.E. 1977 Prehistoric exchange systems in California: the results of obsidian dating and tracing. Ph.D. dissertation, Department of Anthropology, University of California, Los Angeles. Ericson, J.E., J.D. Mackenzie and R. Berger 1977 Physics and chemistry of the hydration process in obsidian I: theoretical implications. In: Advances in Obsidian Glass Studies: ArchaeoZogical and GeochemicaZ Perspectives, edited by R.E. Taylor. Pp. 46-62. 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