Lithic Thin Sectioning Methods 

Sample Selection

Petrographic thin sectioning is a process used to determine the physical makeup of artifacts and rock samples. If an artifact is used in this process it will be destroyed during the procedure. This is important to remember when determining which artifacts or lithics will be used. Samples used in this study are primarily identified as "unknown lithic" on inventory forms based on hand inspection in the lab or field. Others are selected for special qualities or rarity. These include some jaspers and cherts. The sample from site 72-170 was selected primarily because it representes difficult to identify, weathered stone types variously classed as hornfels, slate, or chert. Artifact samples are also selected by appropriate size and shape. Flakes are preferably greater than 4mm in maximum thickness and 2cm x 2cm across. However, samples deemed important to study are at times as thin as 2mm and as small as 1cm across. Samples can not exceed the 3cm diameter of the standard sample cups used as a mold. Samples are brushed free of any evident dirt.

Resin Preparation

Buehler Epo-thin low-viscosity epoxy resin is used as a casting agent. Epoxy is mixed as recommended 100 parts to 36 parts hardener by weight. 100 grams of resin and 36 grams of hardener are sufficient to produce 12 casts. Resin and hardener are poured into a paper cup and well mixed with a wooden rod slowly to prevent excessive introduction of bubbles into the mixture. The resin is allowed to sit for 5-10 minutes while sample cups are brushed with release agent and labeled with artifact information. If excessive bubbles are noted, the mixture is placed in a hand pumped vacuum (no more than 20cm Hg) for 5-10 minutes to draw out remaining gasses. By this time the mixture will begin to warm and is ready to pour. Sample cups are each filled to 1cm with resin to prevent bubble formation beneath the lithic samples. Samples are carefully dropped into the resin at an angle and pushed to the bottom of the cup with the stirring rod. Once all samples are thus set, the remaining resin is poured over the sample to a depth of at least 2cm or 3/4 of the capacity of the cup. The samples are then placed into the vacuum chamber at 20cm Hg for 10-20 minutes. The vacuum is then released gradually. If gases are still being released, the samples are returned to the vacuum for an additional 10-20 minutes. Cherts and rhyolites rarely vent gases from pores, though siltstones and related porous materials often require longer vacuum preparation. Samples are allowed to sit for at least 24 hours and preferably 48 hours to cure.

Sample Preparation

Cured samples are removed from the sample cups and stored in small ziplock bags with their provenience. Samples themselves are also labeled with a sharpie to prevent mis-identification. The rough upper side of the sample "plug" is cut with an 8" diamond saw on an isomet cutting machine. The cut is made at a low speed (100 rpm) to prevent heating of the epoxy which is found to slow cutting. Pressure on the cutting arm is also kept relatively low at 350gm, to produce as clean a cut as possible. This provides a flat, perpendicular surface to insert into the chuck. The sample material is then cut. An attempt should be made to slice the sample through its thinnest section to provide the greatest surface for examination. Cutting the sample usually results in a 2mm slice, which is saved as a backup. The cut sample plug is then rinsed of cutting solution, dried, and returned to its bag.

Grinding and polishing of plug

Grinding and polishing the sample surface is the most time consuming portion of the preparation procedure. In order to grind down and polish the cut surface it is best to use a figure eight motion. We found grinding most effective when a slurry of water and 14.5 micron aluminum oxide powder (equivalent to 600-grit paper) is added to the wet-dry sand paper paper. 220-grit paper is required if excessive grooves are left on the cut surface from the saw. Usually, 320-grit paper is suitable to begin the grinding process. Usually, the 320-grit paper is used for approximately 10 minutes, followed by 600-grit paper for approximately 15 minutes. After the 600-grit paper the artifact surface is polished in the final step of the grinding process. One way to accomplish polishing the block is to use a mixture of 3.0 micron aluminum oxide powder (equivalent to 1200-grit paper) and water on a piece of clean glass for approximately 10 minutes. Another method for polishing blocks is to use a polishing wheel. This is basically a metal circle with a piece of cloth pulled tightly over it that spins at a selected speed. A waterspout on the machine can add water to the cloth. An amount just covering the surface of the cloth should suffice with 3.0 micron aluminum oxide powder (equivalent to 1200-grit paper) added in. The block should be held on the wheel for approximately 5 minutes.

The cut surface is, most likely, finished being polished when it reflects light. The surface should be examined under the microscope to be certain, though. The appearance of the epoxy under the microscope is similar to glass when it is finished being polished. This is different from the blurry appearance that the epoxy has in the earlier stage of the grinding process. It is important that there are no scratches that could hinder the analysis during the sourcing process.

Fixing Sample to Petrographic Slide

Once sample plugs are ground and rough-polished they are ready to be affixed to petrographic slides. Resin is mixed as previously, though in much smaller quantities. Only two drops are required for each sample. Petrographic slides are cleaned and scuffed with a 3.0 micron powder slurry or on 1500-grit wet paper. They are dried and separate labels are affixed to each slide. One drop of resin is dropped onto the slide. Another drop is carefully spread across the surface of the sample. The sample is then slowly placed onto the slide starting at one edge. It is then pressed and gently turned a small amount to help release any possible trapped gasses. The slide and sample are then put onto a press (see dimentions below). The slide press is first wrapped in saran wrap to prevent the slide from sticking. The slide and sample are then pressed securely to the flat cork surface of the press and secured with a double elastic band (2 1/2" X 1/4"). Two samples are set on each press. Samples are then left to cure for 24-48 hours.

Sample Cutting from Slide After curing, samples are removed from the press and a utility knife is used to remove remnant epoxy from the edges and bottom of the slide. This is done to ensure a clean fit in the petrographic slide jig on the cutting machine. Residual epoxy resin could easily result in a non-parallel cut on the machine. Samples are cut from the machine as above. With practice, it was found that sample plugs could be cut to within 0.3mm of the slide face, greatly reducing grinding time. Once cut, the slide and remaining sample plug are washed. The plug is kept as a second backup. On very thin samples (1-2mm thick) it is often found that little material remains after cutting, and the plug can be discarded.

Grinding and Polishing the Slide

The grinding process described earlier is now employed a second time. The process begins with 220-grit or 320-grit paper and ends when the block on the slide is ground and polished down to a thickness of 30 microns. Once this polished thickness is achieved the lithic/artifact can be looked at under a polarized microscope, analyzed, and identified. This is far more accurate than viewing the sample with a plain eye by its exterior surfaces. This procedure has been found to work effectively in identifying, analyzing, and sourcing lithics such as artifacts and rock samples.

References

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