Analytical Methods:Microscopy

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Questions
A microscope enables the examination of the microscopic components of archaeological deposits. This can provide information regarding the mineral composition of the deposit that can be used to help identify parent materials and source areas for sediment. Microscopic analysis can also be used to assess the presence and preservation of palaeoenvironmental remains such as pollen, shells, etc. Microscopic analysis of sand grains from deposits may also be used to help identify their mode of transportation (water or wind erosion) and to confirm the presence or absence of organic and iron sand coatings that are indicative of soil forming processes such as podzolisation.

Sampling
The appropriate sampling strategy and sample preparation depends on the nature of the material being looked at and the questions being asked.

For identifying source areas and parent materials it is important that not only is a representative sample taken from the context in question, but that samples for comparison are also taken from the range of potential source areas. To ensure that the samples are representative of the context or the source area, a number of samples should be taken from different locations within each area of interest. Individual samples may be bulked for this assessment as long as the total sample is thoroughly homogenised before sub-sampling for the analysis. However, may not be desirable if the samples are required for other analyses.

In general, for microscopic examination the clay and fine silt component are not of interest so the sample may require wet seiving at 500 microns before analysis.

The identification of palaeoenvironmental potential usually requires the sampling of areas where preservation is expected to be high, usually waterlogged deposits. It is also important that the sample is of a known volume so that the concentration of remains can be estimated (for pollen this is usually 1 cm3). Some remains, such as shells, may be easily visible in the bulk soil, however, others will require that the sample is pretreated first to remove other material and concentrate the remains of interest. Sample storage and preparation for the different types of palaeoenvironmental material varies. Specialist guidance should be sought and also to interpret the significance of the remains identified.

Analysis
Samples are mounted on glass microscope slides and examined using a microscope.

A low powered stereo microscope is sufficient for many purposes, such as the analysis of rounding of sand grains or the identification of coatings. For examination of silt sized particles and small environmental remains (pollen etc.) higher magnifications in the order of x400 (Moore et al., 1991) will be required. If mineral identification is required a petrological microscope and specialist knowledge are required.

Sand grain analysis can provide a rough estimation of mineral types and their proportions in the deposit, rounding and smoothing of individual grains and the presence of coatings. Counts of particles are the usual means of recording data, ensure that sufficient grains are counted to account for the variability in the deposit and to recover most of the rare mineral types.

Data and interpretation
Some specialist knowledge is required to interpret the data, and certainly to interpret the significance of palaeoenvironmental remains. However, having a microscope onsight can provide a quick, early indication of potential or source, which can inform the excavators decision to proceed further with specialist laboratory analysis. An assessment of this type for palaeoenvironmental remains unless carried out by a skilled practitioner does not replace an evaluation of the deposit by an expert who can comment on preservation, taphanomic effects, and the environmental significance of the sample.

Related Techniques

 * Micromorphology
 * Heavy mineral analysis
 * Element microanalysis

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