The manufacture of particulate samples for coal petrographic analysis destroys the original petrographic fabric of the coal, so that fine-scale (millimeter to centimeter) variations in petrographic composition through the seam cannot be identified. The use of oriented block samples prepared from pillar samples allows the physical integrity of the coal seam to be preserved and thus studied. While the preparation of polished block samples does not generally present any problems for coals of sub-bituminous rank or higher, the high moisture content of lignites means that the polished surface is prone to desiccation. This leads to the formation of shrinkage cracks, which cause distortion of the surface and render microscopic analysis under high-power magnification (×500) virtually impossible. In order to overcome this problem, samples of lignite were dehydrated in acetone and then impregnated with a low-viscosity epoxy resin. A thin film of immersion oil during analysis further inhibits the affects of any desiccation on the polished surface. Long-term storage under nitrogen has been effective in preserving the high quality of the polished surface for more than four years.

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