HYDRATION ANALYSIS OF OBSIDIAN FLAKES PRODUCED BY ISHI DURING THE HISTORIC PERIOD Thomas M. Origer A LTHOUGH OBSIDIAN HYDRATION HAS BEEN employed as a dating method in archaeology for nearly 30 years, there remain fundamental questions regarding the glass hydration process. Researchers do not agree on how hydration proceeds, whether by a diffusion or a reaction process, and many models have been proposed to describe the rate of hydration over time. Researchers have employed diffusion (e.g., Ericson 1977; Michels 1982), linear (e.g., Meighan 1981; Garfinkel 1980; Basgall 1983) or power function (e.g., Hall 1983) rate constant formulas in an effort to interpret hydration band thickness as elapsed time since the initiation of hydration. Given the differences in these approaches, a single obsidian hydration band of a certain thickness may be assigned a calendar date within a span of some 12,000 years, depending on the conversion formula used (Bouey and Basgall 1984: 157). Many of the hydration rate formulas which have been proposed are clearly inadequate in some or all respects. For example, Bouey and Basgall (1984: Tables 1 and 2) have shown that some linear rates proposed for Casa Diablo obsidian yield dates for certain hydration band measurements which are in the future. These findings are supported by additional research carried out by Robert Jackson (1984: 180). Problems associated with obtaining a proper "fit" between hydration band thickness and calendar dates stem, in part, from inadequate numbers of hydration samples associated with materials with assigned, reliable, absolute chronometic dates (see Hall and Jackson, this volume). While this data base is gradually improving, it is clear that there is an exceptional paucity of absolute/hydration date correlations for very old and for very young samples. Many proposed hydration rate formulations are inaccurate, in part, because they lack data reference points at the recent end of the temporal scale. Why is there a paucity of obsidian samples with thin hydration bands? Thomas Jackson commented on the scarcity of archaeological specimens from the southern Sierra Nevada with hydration bands measuring less than one micron. He concluded: "either there is a real uniformity in the rate at which the initial micron of hydration is created on the obsidian specimens, regardless of the source of the glass, or that the technical aspect of detection and measurement is in some respect lacking. Perhaps the archaeologists working in the Sierra Nevada might endeavor to resolve this matter by addressing a large- size sample of obsidian arfifacts recovered from very late period prehistoric or early historic period sites" (T. Jackson 1984: 121). The clear implication of Jackson's remark is that the archaeological data base lacks samples from sites of Contributions of the Archaeological Research Faciity Number 48, December 1989 TABLE 1 VISUAL ATTRIBUTES OF OBSIDIAN SOURCE GROUPS IDENTIFIED IN THE ISHI COLLECTION. Transparency/color 2 Opaque/gray Opaque/rd black Opaque/gray Semi-translucent/gray-brown Semi-translucent/gray Translucent/gray Semi-translucent/gray Inclusions 3 None None Phenocrysts Bands Bands None None Texture' Glassy Glassy Glassy Glassy Glassy Glassy Grainy Source Assignment Napa Valley (Group 1) Napa Valley (Group 2) Mt. Konocti (Group 1) Mt. Konocti (Group 2) GF/LIW/RS s Borax Lake Tuscan Legend: I This indicates the color of reflected light;2 indicates the color of light observed when specimens are held against a light source;3 indicates the presence of attributes that detract from the specimen having a homogenous appearance;' this describes specimens having smooth, shiny surfaces (glassy) or specimens with coarse, dull surfaces (grainy); 5 Grasshopper FlaVtst Iron Well/Red Switchback Chemical Group (see Hughes 1986: Appendix 1). recent age. The observation that samples with hydration measurements smaller than one micron are truly un- common in the archaeological record is supported by examination of the data files of the Obsidian Hydration Laboratory at Sonoma State University. Since its founding in 1978, the laboratory has processed more than 10,000 obsidian artifacts from archaeological contexts in the United States and abroad, yet only a handful of specimens (a few dozen) have hydration bands measuring less dtan one micron. These observations prompted several questions. Why are thin (less than one micron) hydration bands so rarely encountered among the archaeological samples? Are hydration bands that measure less than one micron not detected because of limitations associated with standard analytic procedures? How long does it take a measurable hydration band to form under normal conditions? This paper addresses these questions, along with others relating to processes of hydration band formation and destruction, rate of hydration, and the effects of chemical composition and environmental temperature. In addition, this study generates data on the length of time necessary to develop a discemible and measurable hydration band, relationships among coeval source- specific hydration band measurements, and the effect of temperature on hydration development. Donovan Clark (1961: 11) suggested that hydration bands measuring at least one micron should form on obsidian specimens after ca. 100 years of uninterrupted hydration. Clark's hypothesis effecively defmes "recent' (less than 100 years) for purposes of this study and the logical source of such 'rcent" specimens should be archaeological sites. However, this assump- tion is complicated by the fact that the disruption of most California Indian cultues occurred more than 150 years ago. After Euro-American colonization most native peoples quickly abandoned the manufacture of flaked stone tools in favor of metal items, long-distance exchange of obsidian was foreclosed, and archaeologi- cal investigations of early contact sites are very rare. Color1 Black Black Black Gray Gray Gray/black Gray/black 70 Hydraton Analysis of Obsidian Flakes Produced by Ishi7 HISTORIC PERIOD HYDRATION SAMPLES: THE ISHI COLLECTION In 1980 Paul Amaroli and the author conceived a project to analyze obsidian samples from very recent historic period contexts and decided to examine obsidian artifacts made by Ishi, the Yahi Indian made famous by the writings of Theodora Kroeber (1961). Ishi was brought to the University of California by Alfred Kroeber, and lived in San Francisco and Berkeley from August 1911 until his death in March 1916 (Pope 1918; Waterman 1918). During this period Ishi knapped a variety of tools from obsidian and synthetic glass which are now housed at the Lowie Museum of Anthropology, University of Califomia at Berkeley. Despite their appropriateness to obsidian hydration study, the prospect of damaging artifacts made by Ishi was unacceptable. However, in 1984 Dr. David Fredrickson learned from Dr. Frank Norick of the Lowie Museum that some far-sighted individual, some seven decades ago, had collected and saved a sample of Ishi's chipping debris. This material, accessioned in 1915, is now curated with other Ishi artifacts at the Lowie Museum. Although it is not known for certain whether the Ishi chipping debris was made in 1915, this date will be used in this analysis. Forty-two samples drawn from Ishi's chip- ping debitage were selected for hydration band meas- urement. TIME, TEMPERATURE, AND SOURCE One critical factor in this study, the precise time that the samples were manufactured, is now fixed. In 1986, when hydration thin sections reported here were prepared, Ishi's flakes were about 71 years old. A second critical variable, temperature history, is also known. According to Dr. Norick the specimens have been stored at a stable temperature regime between 66 and 68 degrees F. (18.9 to 20.0 degrees C.). The remaining variable, obsidian source, needed to be addressed. An initial examination of the Ishi flakes was made in order to characterize them on the basis of their physical (visual) attributes. Attributes such as color, clarity, texture, and presence/absence of inclusions are vmiables used to assign some obsidian specimens to their geologic sources in certain parts of California (Bettinger et al. 1984; Origer and Waechter 1987). Visual examination of the Ishi chipping debris indicated that several types of obsidian were present: two varieties from Napa Valley, two varieties from Mt. Konocti in Lake County, and one from the Medicine Lakle Highland in Siskiyou County. Another source, Borax Lake (Lake County) was tentatively identified based on the observation of only a few small specimens in the collection. Table 1 lists visual characteristics of the samples selected for hydration dating. ITese visual sourcing results raised an interesting, albeit unresolved, question: why was obsidian from so many sources, some more than 320 km apart, repre- sented in an afternoon's chipping demonstation by Ishi? Napa Valley obsidian may have been ubiquitous in the Ishi collection because it was in close proximity to San Francisco and it provided a ready source of raw material. Although published literature (Nelson 1916; Pope 1918) sheds little light on the question of why so many sources are present, it is known that among the obsidians found archaeologically in Yahi territory are Tuscan and varieties from the Medicine Lake Highland. While speculative, it is interesting to conjecture that while Ishi lived in San Francisco he was at least provided with some obsidians that he was familiar with. X-ray fluorescence (XRF) analysis, summarized in Table 2 and reported in concentration values (i.e., parts- per-million by weight) confirmed the visual source assignments for all but one specimen. One flake, which based on visual criteria was tentatively assigned to the Borax Lake source, was assigned to the Tuscan chemical group (cf. Hughes 1986: Appendix 2) on the basis of XRF data. XRF trace-element values for flakes visually assigned to the Medicine Lake Highland were attributed to the Grasshopper Flat/Lost Iron Well/ Red Switchback chemical group (GF/LIW/RS; see Hughes 1986: Appendix 2). OBSIDIAN HYDRATION ANALYSIS Once the three variables of manufacture date, temperature history, and glass geochemistry (source) were resolved, hydration analysis was undertaken. All hydration bands were measured using a 100 power oil emersion objective and a 10 power filar micrometer eyepicece for a total magnification of 1000X. At this magnification, the measurements have an error factor of ? 0.2 micron due to normal limitations of the optical equipment (see Scheetz and Stevenson 1988; Stevenson and Scheetz [this volume]). Each thin section was measured at a minimum of three and at as many as six loci along the hydration band. Hydration band meas- urements are averaged to two decimal places and reported with other pertinent data in Table 3, while Table 4 summarizes these by source. Inspection of Tables 3 and 4 shows that while the hydration band mean values for Napa Valley and Mt. Konocti obsidians are statistically identical (given the ? 71 Contibutions of the Archaeological Research Facility Number 48, December 1989 TABLE 2 SOURCE DATA FOR 42 OBSIDIAN SPECIMENS TAKEN FROM THE ISHI COLLECTION Cat.No. CHSQ PB TH RB SR Y ZR NB Source 3.7 12.6 +-2.1 4.8 39.4 +-2.1 5.3 33.0 +-1.9 4.5 35.4 +-2.3 5.1 34.4 +-2.0 4.7 38.6 +-2.2 6.1 39.6 +-1.9 5.1 37.3 +-2.0 6.3 41.5 +-1.9 5.3 34.0 +_1.9 4.4 34.9 +-2.3 5.4 36.8 +-1.9 5.4 34.6 +-1.8 5.2 36.5 +-2.0 4.5 39.1 +-2.2 5.6 39.1 +-2.0 5.8 35.0 +-1.9 4.7 37.8 +-2.1 5.0 34.6 +-1.9 4.7 39.8 +-2.2 4.2 13.4 +-1.8 5.7 34.5 +-1.8 0.0 18.8 +-3.7 17.0 +-3.2 17.3 +-3.8 18.1 +-3.4 22.3 +-3.7 19.7 +-3.1 19.2 +-3.3 192. +-3.2 23.7 +-3.3 22.5 +-3.8 19.2 +-3.3 17.3 +-3.2 23.2 +-3.5 19.6 +-3.9 15.6 +-3.4 25.4 +-3.2 17.1 +-3.5 20.0 +-3.3 +-3.6 0.0 +-0.0 21.3 +-3.1 47.3 +-2.6 210.0 +-3.5 189.2 +-3.1 178.3 +-3.6 178.2 +-3.3 20S.1 +-3.5 188.7 +-3.0 201.0 +-3.3 191.0 +-3.1 187.3 +-3.3 173.3 +-3.6 196.3 +-3.1 179.6 +-3.1 207.1 +-3.4 200.4 +-3.6 193.0 +-3.3 190.8 +-3.1 184.4 +-3.4 181.1 +-3.2 216.2 +-3.6 48.4 +-2.5 182.7 +-3.0 325.2 +4.7 8.7 +-1.8 9.5 +-1.6 6.7 +-1.8 8.6 +-1.6 9.4 +-1.8 8.9 +-1.6 7.1 +-1.6 7.0 +-1.6 6.7 +-1.7 5.3 +-1.9 8.6 +-1.6 8.1 +-1.5 6.1 +-1.7 8.4 +-1.9 7.7 +-1.7 6.1 +-1.6 6.4 +-1.7 6.5 +-1.6 8.3 +-1.8 340.6 +-4.6 8.3 +-1.5 36.1 +-3.2 54.7 +-3.1 45.1 +-2.8 42.9 +-3.3 45.6 +-3.0 56.1 +-3.2 45.3 +-2.7 51.3 +-2.9 48.7 +-2.8 49.2 +-2.9 43.3 +-3.3 47.7 +-2.8 49.1 +-2.8 52.5 +-3.1 51.0 +-3.3 4&8 +-3.0 48.3 +-2.8 45.4 +-3.0 47.9 +-2.9 50.0 +-3.2 39.7 +-3.1 50.7 +-2.8 182.6 +4.4 253.9 +4.0 2272 +-3-5 216.2 +-4.2 213.4 +-3.8 247.6 +-4.1 219.3 +-3.5 233.6 +-3.7 245.5 +-3.6 233.5 +-3.8 214.5 +4.2 225.1 +-3.6 240.8 +-3.7 240.3 +-3.9 234.1 +-4.2 225.3 +-3.8 235.3 +-3.7 225.4 +-3.9 217.8 +-3.7 251.5 +4.1 179.8 +-4.3 225.5 +-3.5 10.2 +-3.7 8.8 +-3.2 13.6 +-2.9 8.4 +-3.4 9.5 +-3.1 6.3 +-33 6.9 +-2.9 10.4 +-3.0 5.1 +-2.9 13.3 +-3.1 11.5 +-3.5 13.2 +-2.9 11.3 +-2.9 13.1 +-3.1 7.7 +-3.4 10.0 +-3.1 IQ0 +-3.0 10.0 +-3.1 4.8 +-3.0 13.5 +-3.3 9.5 +-3.6 11.8 +-2.9 NapaVallcy NapaVale-y NapsValley BCR-1 = US. GeoDoglm Survey lnternatdonal rock tandard. See Hugh. 1986 for further detals pertaining to thes analysu BCR-1 19874-M-A -B -c -D -E -F -G -H -I -J 19874A-N-A -B -c -D -E -F -H -I BCR-1 19874-N-J 72 u Hydration Analysis of Obsidian Flakes Produced by Ishi 73 TABLE 2, CONTINUED. Cat.No. CHSQ PB TH RB SR Y ZR NB Source 19874-K-A 5.1 27.8 20.5 205.5 77.9 36.1 188.3 14.0 MLKonocti +-1.9 +-3.4 +-3.3 +-2.4 +-2.9 +-3.6 +-3.1 -B 4.3 31.4 21.9 202.1 76.9 37.2 190.6 11.2 +-2.1 +-3.7 +-3.6 +-2.6 +-3.1 +-3.9 +-3.3 -C 4.1 36.0 18.6 204.0 87.4 36.8 186.5 14.7 +-2.1 +-3.9 +-3.7 +-2.8 +-3.2 +-4.0 +-3.4 -D 3.6 31.3 30.1 205.1 71.0 32.6 185.4 10.6 +-2.4 +-4.2 +-4.1 +-2.8 +-3.5 +4.4 +-3.7 -E 5.3 34.6 23.2 228.1 84.1 41.4 208.0 9.0 +-1.9 +-3.5 +-3.5 +-2.4 +-2.9 +-3.7 +-3.1 -F 3.3 33.4 28.7 200.6 83.0 37.3 185.4 11.4 +-2.4 +-4.4 +-4.1 +-3.0 +-3.6 +-4.5 +-3.8 -G 4.5 36.8 29.1 230.3 88.6 42.5 212.0 9.5 +-2.2 +4.0 +-3.9 +-2.8 +-3.4 +4.2 +-3.6 -H 4.2 33.8 23.5 209.2 82.9 34.7 197.0 9.7 +-2.0 +-3.7 +-3.6 +-2.6 +-3.1 +-3.9 +-3.3 -I 4.4 34.3 29.2 214.7 81.0 36.9 194.7 10.6 MLKonocti +-2.1 +-3.8 +-3.7 +-2.6 +-3.1 +-3.9 +-3.3 -J 4.3 33.3 27.6 212.5 84.5 40.7 191.7 5.5 +-2.2 +4.1 +-3.9 +-2.8 +-3.4 +4.2 +-3.6 19874-TJ-A 4.3 30.8 0.0 79.4 93.7 19.8 61.3 7.4 Tuscan +-2.1 +-0.0 +-2.8 +-2.8 +-2.9 +-3.2 +-3.4 TJ-B 2.6 30.2 17.2 202.0 10.2 38.4 74.6 7.2 BomxLake +-3.2 +-5.6 +-5.4 +-2.8 +-4.7 +-4.6 +-4.9 19874-Z-A 5.0 27.3 20.5 146.5 73.4 32.5 181.5 15.2 GFALlWIRS +-1.8 +-3.2 +-2.9 +-2.3 +-2.7 +-3.5 +-3.0 -B 4.3 27.4 12.5 140.1 73.4 28.4 167.9 11.0 +-1.9 +-3.4 +-3.0 +-2.4 +-2.9 +-3.7 +-3.2 -C 3.7 33.9 21.3 145.4 72.4 27.0 174.8 8.3 +-2.4 +4.3 +-3.7 +-3.0 +-3.5 +-4.4 +-3.9 -D 5.3 29.0 18.4 142.3 73.3 27.7 170.9 9.3 +-1.7 +-3.1 +-2.8 +-2.2 +-2.6 +-3.4 +-2.9 -E 5.5 28.0 16.7 140.6 75.6 29.1 175.2 7.4 +-1.7 +-3.1 +-2.8 +-2.2 +-2.6 +-3.4 +-2.9 BCR-1 4.7 12.0 0.0 48.2 343.3 33.5 173.9 13.0 +-1.9 +-0.0 +-2.4 +-4.6 +-3.1 +4.3 +-3.6 19874-Z-F 3.6 26.4 14.8 153.3 78.4 30.2 185.7 8.5 +-2.1 +-3.9 +-3.6 +-2.8 +-3.3 +4.3 +-3.7 -G 4.8 25.9 15.0 154.2 78.4 30.8 176.3 13.2 +-1.9 +-3.5 +-3.1 +-2.5 +-2.9 +-3.7 +-3.2 -H 4.2 28.4 16.2 137.2 73.7 29.2 168.8 10.0 +-2.1 +-3.6 +-3.2 +-2.6 +-3.0 +-3.9 +-3.4 -I 5.0 26.3 16.5 131.3 70.9 27.0 167.2 5.5 +-1.7 +-3.0 +-2.7 +-2.2 +-2.6 +-3.3 +-2.9 -J 4.5 26.6 15.9 158.4 77.7 31.6 191.0 9.0 +-2.0 +-3.6 +-3.2 +-2.6 +-3.0 +-3.9 +-3.3 Contributions of the Archaeological Research Facility Number 48, December 1989 TABLE 3: HYDRATION DATA FOR THE ISHI SAMPLE CA-ISHI STUDY Submitted by Thomas Origer - SSU Dec 84-Mar 86 Lab# Catalog# Description Provenience MuseumRemarks Readings Mean Source 01 19874-N-A 02 19874-N-B 03 19874-N-C 04 19874-K-A 05 19874-K-B 06 19874-N-E 07 19874-N-D 08 19874-N-E 09 10984-N-E 10 19874-N-G 11 19874-N-H 12 19874-N-I 13 19874-N-J 14 19874-K-D 15 19874-K-E 16 19874-K-F 16 19874-K-F 17 19874-K-G 18 19874-K-H 19 19874-K-I 20 19874-K-J 21 19874-M-A 22 19874-M-B 23 19874-M-C 24 19874-M-D 25 19874-M-E 26 19874-M-F 27 19874-M-G 28 19874-M-H 29 19874-M-I 30 19874-M-J 31 19874-TJ-A 32 19874-TJ-B 33 19874-Z-A 34 19874-Z-B 35 19874-Z-C 36 19874-Z-D 37 19874-Z-E 38 19874-Z-F 39 19874-Z-G 40 19874-Z-H 41 19874-Z-I 42 19874-Z-J flake Lowie Museun none flake Lowie Museum none flake Lowie Museun none flake Lowie Musemn none flake Lowie Museun none flake Lowie Musemn none flake Lowie Museumn none flake Lowie Museum none flake Lowie Museum none flake Lowie Museun none flake Lowie Museum none flake Lowie Museum none flake Lowie Musewn none flake Lowie Museum none flake Lowie Museum fb flake Lowie Museum 1st b flake Lowie Museum 2nd flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Museum none Biface frag. Lowie Museum none flake Lowie Museum none flake Lowie Museum none flake Lowie Musewn none flake Lowie Museum none flake Lowie Museumn none flake Lowie Museum none flake Lowie Museumn none flake Lowie Museum none flake Lowie Musemn none flake Lowie Museun none 0.6 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.7 0.7 0.6 0.6 0.6 0.6 0.6 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.8 0.7 0.7 0.7 0.7 0.7 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.8 0.7 0.8 0.8 0.8 0.8 0.8 0.6 0.6 0.6 0.6 0.7 0.7 0.5 0.5 0.6 0.6 0.6 0.6 05 0.6 0.6 0.6 0.6 0.7 3.03.0 3.1 3.13.1 3.2 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.6 0.5 0.5 0.6 0.6 0.6 0.7 0.6 0.6 0.6 0.6 0.6 0.6 0.7 0.7 0.7 0.7 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.7 0.7 0.8 0.8 0.6 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.8 0.8 0.7 0.7 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.7 0.7 0.7 0.9 1.0 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.9 1.01.0 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.9 0.9 0.9 1.0 0.8 0.8 0.8 0.8 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.9 0.9 0.9 0.7 0.8 0.8 0.8 0.8 0.9 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.8 0.8 0.9 0.8 0.8 0.8 0.9 0.9 1.0 Lab Accession No.: 84-H363 Technician: Thomas Origer NV G1 = Napa Valley (Group 1) BL = Borax Lake NV G2 = Napa Valley (Group 2) T = Tuscan K Gl = Mt. Konocti (Group 1) GF = Grasshopper FlaVLost Iron Well/Red Switchback K G2 = Mt Konocti (Group 2) 0.68 0.70 0.70 0.63 0.62 0.60 0.72 0.72 0.70 0.77 0.70 0.72 0.78 0.63 0.57 0.60 3.08 0.60 0.52 0.58 0.60 0.73 0.70 0.73 0.68 0.67 0.73 0.77 0.70 0.77 0.70 0.98 0.93 0.80 0.80 0.88 0.83 0.90 0.85 0.80 0.80 0.77 0.87 NV G1(x) NV Gl(x) NV Gl(x) KGI(x) K G2(x) K Gl(x) NV G1(x) NV Gl(x) NV GI(x) NV Gl(x) NV Gl(x) NV G1(x) NV Gl(x) K Gl(x) K GI(x) K G2(x) K G2(x) K Gl(x) KGl(x) K Gl(x) K Gl(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) NV G2(x) T(x) BL(x) GF(x) GF(x) GF(x) GF(x) GF(x) GP(x) GF(x) GF(x) GF(x) GF(x) 74 Hydradon Analysis of Obsidian Flakes Produced by Ishi 0.2 measurement error factor), the mean values for GF/ LIW/RS, Borax Lake, and Tuscan are larger, although they overlap. DISCUSSION The Ishi collection provided a unique opportunity to examine hydration band development on obsidian. In this case, time of manufacture (age), temperature history, and geochemistry are all known. The results of this study show that hydration bands do develop on very recent glass surfaces and that the bands are detectable and measurable using standard laboratory techniques. To some degree the data support the proposition that hydration band width development varies according to obsidian chemistry under conditions of relatively constant environmental temperature. Clark's (1961) suggestion that the initial micron of hydration develops within 100 years is supported by the results of the present study. Optically discernible hdyration bands form rapidly - within approximately 70 years - on varieties of obsidian represented in this study. The rarity of thin hydration bands on archaeo- logical samples is attributed to a lack of archaeological investigation of very recent Native American sites. Thomas Jackson's (1984: 121) thought that initial hydration development might be uniform is not supported by the results of this study, although it is acknowledged that mean hydration values for some obsidians (e.g., North Coast Ranges obsidians) in the Ishi collection overlap when a measurement error of? 0.2 micron is considered. Because temperature is a critical factor in hydration band formtion it has been advocated that corrections be made when comparing hydration measurements of specimens with different thermal histories (Friedman and Smith 1960). For example, a hydration rate constant for Napa Valley obsidian has been calculated to be 153.4 at an effective hydration temperature (EHT) of 16.1 degrees C. (Origer 1982). Using this rate constant in the diffusion formula (T=kx2) the Napa Valley hydration mean converts into a date of 70 ? 7.9 years before present (B.?). The range of dates for manufacture of the Ishi materials, based on the hydration rate constant for Napa Valley obsidian, is 71.1 to 86.9 years B.P. The more recent age conforms well with the age of the Ishi materials as estimated from information at the Lowie Museum; however, the mean date and the range generally are slightly older than anticipated. This could reflect the fact that the EHT of the area where the rate constant was developed is different from the Lowie Museum environment: 16.1 degrees for Santa Rosa, Califomia, versus 19.4 degrees for the museum. Given this, a correction factor of approximately 10% rate adjustment per degree C. difference (as recommended by Trembour and Friedman 1984: 79) was applied. With the 10% correction, an age of 58 years B.. was obtained for the Ishi flakes. This is an incorrect age; calculations based on the Ishi data suggest a tempera- TABLE 4 SUMMARY OF ISHI COLLECTION HYDRATION MEASUREMENTS BY SOURCE Source Measurements (in microns) Mean S.D. Napa Valley (Group 1) .68 .70.70.70.70.72.72.72.77 .78 0.72 0.030 Napa Valley (Group 2) .67.68 .70.70.70.73 .73 .73 .77.77 0.72 0.032 Mt. Konocti (Group 1) .52.57.58.60.60.60.63.63 0.59 0.033 Mt. Konocti (Group 2) .60.62 0.61 0.010 GF/LIW/RS1 .77 .80.80.80.80.83 .85 .87 .88 .90 0.83 0.041 Borax Lake .93 0.93 Tuscan .98 0.98 Grasshopper Flat/Lost Iron Well/Red Switchback chemical group (see Hughes 1986:300-301) 75 Contributions of the Archaeological Research Facility Number 48, December 1989 ture correction factor of 4-6%. A correction factor of this magnitude (i.e., 4-6%) is also supported by previous work comparing Napa Valley obsidian artifacts from coastal and interior settings (Origer 1982: 78). CONCLUSION Detectable and measurable hydration bands form on obsidian artifacts in less than 100 years. Differential hydration development (rates) for geochemically different obsidians is suggested, even at this early stage (ca. 70 years) of hydration development, by the slighdy different mean measurements. The paucity of artifacts from archaeological sites with hydration bands measur- ing less than 1.0 micron probably is a function of the relative lack of archaeological investigation of recent historic-era Native American sites, or at least a lack of samples from such sites submitted for hydration dating. Analysis of the Ishi material indicates that a hydration rate temperature correction factor of approximately 4- 6% is in order when comparing hydration results of artifacts with different temperature histories. ACKNOWLEDGMENTS I wish to acknowledge the assistance of Dr. Thomas L. Jackson for conducting x-ray fluorescence trace-element analysis of the Ishi obsidian flakes, and for generously providing editorial assistance. Support for this research was provided by Dr. David A. Fre- drickson, Department of Anthropology, Sonoma State University, who reviewed and commented on various drafts of this paper. A version of this paper also benefited by the critical editing of Ms. Sharon Waech- ter. I extend special thanks to Dr. Frank Norick and the staff of the Lowie Museum of Anthropology, Univer- sity of California at Berkeley, for help and interest in this project. This study was partly conceived by Paul Amaroli who is ackmowledged for his role in develop- ing the idea of analyzing examples of Ishi's work. Finally, I thank Dr. Richard E. Hughes for his help in producing this paper. REFERENCES CITED Basgall, ME. 1983. Archaeology of the Forest Service Forty Site (CA-MNO-529), Mono County, California. Ms. on file, Inyo National Forest, Bishop, California. Bettinger, R.L., M.G. Delacorte, and RJ. Jackson 1984. Visual sourcing of cental eastern Califor- nia obsidians. IN: Obsidian Studies in the Great Basin, edited by RE. Hughes. 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