By the numbers, here are some guidelines that you can can use in order to select obsidian and basalt samples for trace element and hydration analysis. Be sure to let us know if you have other questions or feel that we've missed a topic that you would like to have included.
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First off, you will need to select which specimens you wish to send to the lab for trace element studies or for obsidian hydration measurements. Please bear with the length of this discussion of sample selection - other than the actual analysis, this is the most critical part of the analytical process and we will cover it some detail. Given that budgets often do not allow for the ideal 100 percent sample, we recommend that you tailor your choices of artifacts (1) to maximize the quality of analytical results, and (2) to meet explicit research objectives. A guided effort is likely to produce better results than a fishing expedition.
Size matters! In XRF analysis, larger samples produce better trace element results and higher-quality results mean a greater number of successful source assignments. Samples must be a minimum of 10 mm in diameter and should be at least 1.5 mm in thickness. Larger sizes are preferred for analysis and should be selected whenever possible - the best results occur when artifacts are at least 15 mm in diameter and 2 mm in thickness. There are no minimum specimen size requirements for obsidian hydration measurements. However, we generally recommend that samples be no smaller than approximately 5 mm in diameter. Specimens smaller than this are difficult to prepare for analysis and are largely destroyed during slide preparation. The occasional recuts that are sometimes needed will not be possible in this case and a higher number of unsuccessful rim measurements may result. Samples should have at least one relatively flat surface available for analysis. The flatter the X-ray target surface, the better the analytical results. Normal flake removal scars and some irregularities are fine but avoid any seriously irregular surfaces if possible. Samples should be relatively clean and free of heavy surface encrustations. XRF spectroscopy analyzes the surface layer of the artifact. The cleaner the surface, the more likely that the analyzed trace element values will reflect the true composition of the obsidian or basalt. Washing the artifacts with a toothbrush and tap water will take care of most residues. Ideally, the specimens should be free of sample numbers or residues (e.g., adhesives). Painted sample numbers are very high in some trace elements (in particular - Ti, Mn, Fe, or Zn) that may be useful or essential for the successful characterization and source assignment of an artifact. Typically, these numbers are also written on the flattest artifact surface, the one that would make the best target for XRF analysis! If you can avoid it, we'd appreciate it if you would wait until after we've analyzed your artifact before you paint on any numbers. If the artifacts already have numbers painted on them, do not try to clean them off with a solvent! It won't work because detectable traces will almost always be left behind. It's better for us to be able to see the numbers (or other residues such as glue) so that we can attempt to avoid them completely. For hydration analysis, please do not mark specific hydration cut locations with something like typing correction fluid or small pieces of masking tape. These are both high in undesirable elements and may interfere with hydration rim visibility. If you have any special requests regarding the location of hydration measurements, please attach a separate sheet with tracings or photocopies of the samples and well-marked measurement locations.
Different classes of artifacts will tend to reflect different procurement behaviors and patterns. In general, formed tools such as projectile points will tend to come from a wider variety of sources (higher source diversity) than unformed, utilitarian artifacts or debitage (lower source diversity). If you're looking at local raw material use, select debitage. To look at quarry behavior, choose cores. If you're looking for evidence of longer-distance procurement or interaction, you'll probably want to pick formed tools. Similarly, the analysis of temporally diagnostic projectile points may add a valuable dimension of time to the analysis. More samples = greater source diversity. In other words, larger numbers of analyzed artifacts tend to yield larger numbers of different individual sources. The number of different geochemical sources of obsidian or basalt that are identified at a given site are affected by numerous environmental and cultural variables, most notably the number of available sources, their relative distance to the site, and the number of artifact samples that are characterized. In areas of low source diversity (e.g., southern California), few sources were used and a modest number of samples will provide a good overall idea of the range and proportion of utilized sources. In areas of high source diversity (e.g., the Fort Rock Basin of Oregon), many sources of obsidian were utilized and it will take a proportionately larger number of samples to reconstruct an accurate scenario of overall procurement patterns.
Larger numbers of analyzed artifacts will produce results that are statistically more significant. The factors that influence the formation of a hydration rim over time are complex and even specimens manufactured at the same time are unlikely to all develop hydration rims of identical dimensions. This is one of the reasons that we recommend that rim values be used primarily as a relative, not an absolute dating method. Larger numbers of measurements are more likely to reveal accurate temporal trends and are less likely to be influenced by outliers. In other words, it's usually better to analyze more artifacts from fewer units than fewer artifacts from more units. The placement of the hydration rim sample cut. Different artifact surfaces may be created at different periods of time through breakage, rejuvenation, and scavenging. A technological analysis of the artifact to be analyzed may prove to be a useful and productive part of the research strategy. The careful placement of multiple cuts at surfaces of different potential ages, for example, can provide evidence not only of the original manufacturing event but of later periods of reuse. We hypothesize that stratigraphically, artifacts tend to migrate upwards over time as archaeological deposits accumulate. During site formation activities, some older artifacts will become mixed with contemporary materials, a cycle that will repeat itself throughout the history of the formation of the site. Because of this, the hydration rim values of artifacts from a buried stratum will span not only the range of the deposition of that particular stratum, but all other underlying units as well. Similarly, the artifacts collected in the uppermost deposits (or on the surface) of a site tend to provide a temporal sampling of the entire period of occupation at the site. In practice, we have observed that a carefully collected sample of artifacts from apparently deep and undisturbed deposits will generally show a surprisingly weak relationship between depth and hydration rim width. The hydration analysis of a simple surface collection of artifacts may prove to be an effective and simple way in which to assay the occupational history of the entire underlying site.
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Next, you might as well fill out the lab paperwork. To make sure that we get all the information that we need, we provide a sample submission sheet for this purpose. You can print off the webpage version or you can print out the nicer-looking Adobe Acrobat copy. On request, we'll also be glad to send you a copy. What we would like to know about you and your project is:
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This is a pretty straightforward step. Dirt and grunge should be removed from the artifacts with tap water and a toothbrush or with a damp washrag. Persistent surface residues may be removed with an X-acto knife although you'll want to let us take care of this step because of the potential for hydration rim damage. If the artifacts are placed in individual ziploc bags, it will simplify the login process at the lab. In any case, you will receive them in individually labeled bags so that the there can be no confusion about which results go with which artifacts.
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All that's left to do at this point is to pack your samples in a box and ship it to the lab for analysis. We recommend that you sandwich the artifacts in a solid cardboard box with bubble wrap or some other type of packing material and securely tape the box. UPS, FedEx, and the U.S. Postal Service all provide parcel tracking should the package fail to arrive in a timely fashion. We usually ship via UPS and have never had a project fail to arrive safely. Address all projects to:
Northwest Research Obsidian Studies Laboratory If you provide us with an e-mail address or request a phone call, we'll let you know when your project has safely arrived at the lab. |
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