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The film pull technique is a means of recovering carbonaceous compression fossils for study under transmitted light microscopy. An acid is applied to the surface of the rock to etch away the matrix from the surface, leaving carbonaceous tissue protruding. (Surfaces not to be etched can be coated in a wax (e.g. Vaseline) or grease.) This is usually accomplished by placing the rock upside-down in a weak, continually stirred acid, so that any debris can be washed away. Nitrocellulose is then painted on to the fossil-bearing surface, and once dry may be peeled from the rock, or the rock dissolved in hydrofluoric acid.
The method was pioneered by John Walton in 1928 as a method to derive serial thin-sections without the time, expense and lost material incurred by dissolving the rock. An improvement on the method, using gelatine (with glycerin and formalin) instead of cellulose, was reported in 1930, and is especially suitable for larger samples. This solution-based method was largely superseded by the use of pre-formed sheets of film, similar to those used in overhead transparencies; cellulose nitrate and cellulose acetate can be used, although the latter is preferable. By wetting the reverse surface of the film with acetate, the film becomes more labile and makes a better contact with the material. The peel can be washed in acid to remove any remaining matrix before mounting onto a slide with resin for further study. The method is somewhat destructive, as the acid etching used to remove the rock matrix can also destroy some finer detail; the fizzing caused by the reaction of the acid with the matrix breaks up less-robust cellular material. A second peel without further etching, a "rip peel", will remove any cell walls that are parallel to the surface, and would otherwise be destroyed when subjected to acid.
Details of the modern application of the method can be found in reference (). Even the latest technique does have some disadvantages; most notably, smaller fossils that may lie between cell walls will be washed away with the acid etch, and can only be recovered by a thin section preparation.
In order to mount the slides for microscopy, a series of steps are necessary:
- A glass slide is wetted with acetone, and a fresh layer of acetate is placed upon it. The acetone allows the acetate to 'suck' itself onto the slide, maintaining a good contact by suction. This will later be dissolved, allowing sections of the resin-mounted peel to be cut for transmission electron microscopy.
- A thin layer of epoxy resin is applied, covering the acetate and spreading onto the slide. This will serve to attach the preparation to the slide once the underlying acetate has been dissolved.
- A glass plate is greased, and the smooth side of a peel pressed down onto it.
- The rough side of this peel is covered with warmed (55 °C) epoxy resin, and pressed down onto the previously-prepared slide. After about 45 minutes the glass plate is removed, and the resin is left, warm, to cure.
- The preparation is washed in acetone and acid to remove any residues, which would otherwise produce optical artefacts when imaged.
Specimens recovered by film pull are prone to wrinkling, especially if the surface to be peeled is not perfectly smoothed—if acetone pools, it can cause the acetate to wrinkle.
- Hernick, L.; Landing, E.; Bartowski, K. (2008). "Earth's oldest liverworts—Metzgeriothallus sharonae sp. Nov. From the Middle Devonian (Givetian) of eastern New York, USA". Review of Palaeobotany and Palynology. 148: 154. doi:10.1016/j.revpalbo.2007.09.002.
- Walton, J. (1928). "A Method of Preparing Sections of Fossil Plants contained in Coal Balls or in other Types of Petrifaction". Nature. 122 (3076): 571–571. Bibcode:1928Natur.122..571W. doi:10.1038/122571a0.
- Walton, J. (1930). "Improvements in the Peel-Method of Preparing Sections of Fossil Plants". Nature. 125 (3150): 413–414. Bibcode:1930Natur.125..413W. doi:10.1038/125413b0.
- "A Rapid Cellulose Peel Technique in Palaeobotany".
- Holmes, J.; Lopez, J. (1986). "The disappearing peel technique: an improved method for studying permineralized plant tissues". Palaeontology 29: 787–808. http://palaeontology.palass-pubs.org/pdf/Vol%2029/Pages%20787-808.pdf.
- GALTIER, J.; PHILLIPS, T.L. (1999). "The acetate peel technique". In Jones, T.P., and Rowe, N.P. Fossil Plants and Spores: Modern Techniques. The Geological Society, London. pp. 67–70. ISBN 978-1-86239-035-5.
- Taylor, T. N.; Krings, M.; Dotzler, N.; Galtier, J. (2011). "The Advantage of Thin Section Preparations over Acetate Peels in the Study of Late Paleozoic Fungi and Other Microorganisms". PALAIOS. 26 (4): 239–244. doi:10.2110/palo.2010.p10-131r.