Physical Pretreatment Methods.

Physical pretreatment refers to any pretreatment not involving chemicals. Gupta and Polach (1985) divide this into two parts; the removal of obvious contamination; and the reduction in size of the sample (Gupta and Polach, 1985;13-16). The most common methods of removal include separation of sample and rootlets using tweezers and magnification, and the scraping of the contaminated exterior layer using a surgical scalpel, dental drill and carborundum paper. The rationale behind this is that the exterior surfaces will show the effects of contamination, or isotopic exchange, first. Fragments of charcoal are scraped clean with a scalpel, larger bones have the outer layer excised with a drill and shells have the 'chalky' exterior removed with dental equipment. Physical pretreatment can be preferable to using chemicals because it enables the radiocarbon specialist to be selective in which parts of the sample are removed. The use of chemicals often results in the destruction of pure sample material.

The pretreatment of shell is a good example.The outer layers of shell may be removed using drills and carborundum paper, beneath a fume hood, designed to remove the dust from the laboratory environment. The aim of drilling being to isolate one fraction of the shell material. The section considered to be most reliable for dating is aragonite, the less stable crystal form of the two shell carbonates (the other being calcite). Aragonite is selected because it will show the effects of recrystallisation first. Recrystallisation describes an isotopic exchange. In the post-depositional environment, certain shells incorporate new calcite from other sources of carbonate. This shell may be of a different age and isotopic ratio and will affect radiocarbon determinations. This process is termed recrystallisation. It is difficult to test for recrystallisation on shell calcite, because it is hard to differentiate between sample and contaminant, but it is easily apparent on aragonite. A test for calcite peaks in the aragonite samples of shell by X-ray diffraction places limits on the probability that recrystallisation has occurred.

Usually, calcite that has recrystallised becomes chalky and white and this is the fraction targetted for removal by drilling. The other method commonly employed in shell pretreatment is 'acid-washing'. The acid most often used is hydrochloric acid (HCl). A dilute HCl solution (usually 5-10% conc.) is added to the shells, which are shaken for approximately ninety seconds while the acid dissolves a portion of the shell exterior.

After initial pretreatment, the samples are usually crushed and reduced in size to increase the surface area prior to further pretreatment, acid evolution or combustion. Shell, rock and bone samples are often crushed in a mortar and pestle. Wood is either splintered using a hammer and chisel, or ground into sawdust in a mill. Charcoal is usually ground in a petrie dish or mortar and pestle. Soil samples are slurried and wet seived prior to acid pretreatment. Distilled water and seives are used to separate soil, mud and peat samples into 'fine' and 'coarse' fractions. The coarse fraction is submitted to the archives as a reference sample, while the fine fraction is acid washed and eventually dated.

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