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 This solution-based method was largely superseded by the use of pre-formed sheets of film, similar to those used in overhead transparencies; cellulose nitrate and cellulose acetate can be used, although the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:
Disadvantages
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.