page_title: Coal ball
section_title: Analytical methods
hierarchical_section_title: Coal ball / Analytical methods
caption_attribution_description: English: An unstained thin section of a coal ball from Ohio, United States of America. Calcite can be seen in the plant stem. The microscope magnification was at 50x with plain, polarised light.
context_page_description: A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab. Coal balls were formed in Carboniferous Period swamps and mires, when peat was prevented from being turned into coal by the high amount of calcite surrounding the peat; the calcite caused it to be turned into stone instead. As such, despite not actually being made of coal, the coal ball owes its name to its similar origins as well as its similar shape with actual coal.
  Coal balls often preserve a remarkable record of the microscopic tissue structure of Carboniferous swamp and mire plants, which would otherwise have been completely destroyed. Their unique preservation of Carboniferous plants makes them valuable to scientists, who cut and peel the coal balls to research the geological past.
  In 1855, two English scientists, Joseph Dalton Hooker and Edward William Binney, made the first scientific description of coal balls in England, and the initial research on coal balls was carried out in Europe. North American coal balls were discovered and identified in 1922. Coal balls have since been found in other countries, leading to the discovery of hundreds of species and genera.
context_section_description: Thin sectioning was an early procedure used to analyse fossilised material contained in coal balls. The process required cutting a coal ball with a diamond saw, then flattening and polishing the thin section with an abrasive. It would be glued to a slide and placed under a petrographic microscope for examination. Although the process could be done with a machine, the large amount of time needed and the poor quality of samples produced by thin sectioning gave way to a more convenient method.
  The thin section technique was superseded by the now-common liquid-peel technique in 1928. In this technique, peels are obtained by cutting the surface of a coal ball with a diamond saw, grinding the cut surface on a glass plate with silicon carbide to a smooth finish, and etching the cut and the surface with hydrochloric acid. The acid dissolves the mineral matter from the coal ball, leaving a projecting layer of plant cells. After applying acetone, a piece of cellulose acetate is placed on the coal ball. This embeds the cells preserved in the coal ball into the cellulose acetate. Upon drying, the cellulose acetate can be removed from the coal ball with a razor and the obtained peel can be stained with a low-acidity stain and observed under a microscope. Up to 50 peels can be extracted from 2 millimetres (0.079 in) of coal ball with this method.
  However, the peels will degrade over time if they contain any iron sulfide (pyrite or marcasite). Shya Chitaley addressed this problem by revising the liquid-peel technique to separate the organic material preserved by the coal ball from the inorganic minerals, including iron sulfide. This allows the peel to retain its quality for a longer time. Chitaley's revisions begin after grinding the surface of the coal ball to a smooth finish. Her process essentially entails heating and then making multiple applications of solutions of paraffin in xylene to the coal ball. Each subsequent application has a greater concentration of paraffin in xylene to allow the wax to completely pervade the coal ball. Nitric acid, and then acetone, are applied to the coal ball. Following that, the process merges back into the liquid peel technique.
  X-ray powder diffraction has also been used to analyse coal balls. The X-rays of a predetermined wavelength are sent through a sample to examine its structure. This reveals information about the crystallographic structure, chemical composition, and physical properties of the examined material. The scattered intensity of the X-ray pattern is observed and analysed, with the measurements consisting of incident and scattered angle, polarisation, and wavelength or energy.
caption: A thin section of a plant stem depicting calcite crystals.
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