The acetate peel technique is used make a topographic replica of an etched rock surface on transparent plastic film. The peel method was first described in the 1940s (see McCrone, 1963, Journal of Sedimentary Petrology, v. 33, p. 228-230). The benefit of this technique is that the plastic replica reveals considerable textural detail under low-power magnification but is far cheaper and quicker to produce than a thin section. Another advantage of peels is that they can be much larger in area than standard thin sections. Peels were popular in the late 1960s through the 1970s, but are rarely used today, except in paleontology, where the advantages for examination of wall structure in large skeletons is appreciated. Making peels could be a sort of fun 'craft' project for students. Use a well-ventilated area, or, ideally, a fume hood because of the acetone fumes. Acetate sheets come in different weights (thicknesses) and in clear and frosted varieties. Below is a general list of the steps to get you started. If you'll be making a lot of peels, it may be worthwhile to experiment a bit to determine the materials and methodology that work best for your rocks.
Making an Acetate Peel of a Carbonate Rock:
1. Cut and polish (600 grit should be sufficient) a rock slab and carefully clean it in water. Use detergent if the rock is even a bit greasy and rinse well.
2. Etch the polished surface with dilute (1-2%) hydrochloric acid or acetic acid for several seconds. The sample need not be dry before doing this, but if it is wet it should be uniformly wet. The appropriate acid, acid strength, and etching time will vary with rock composition and texture. If the sample contains dolomite, a longer etching time is required. The etched surface is fragile and must not be touched so that microrelief on the surface is preserved. It is possible to stain the slab for mineral ID following the etch and have the stain colors pass onto the peel.
3. Rinse acid gently from the slab surface (use distilled water if you have it, but tap water will work) and allow the sample to air dry.
4. Support the sample in a manner that allows the polished surface to remain horizontal. A small sand box makes a nice support and also serves to catch the excess acetone. Place a pool of acetone on the surface. A squirt bottle to dispense the acetone is helpful. Flex a sheet of acetate that is cut slightly larger than the dimensions of the sample and apply to the slab surface with a continuous rolling motion. Do NOT touch or press the acetate while it is still wet as this will leave topographic artifacts. Allow the sample to dry (minutes to hours depending on the size of the specimen) and then peel the acetate from the surface.
5. Mount the peel between glass plates or within a cardboard frame
and bind the edges with strong tape:
At left, completed acetate peels,each about
10 cm across.
6. Peels can be scanned and also examined in transmitted plane-light.
Left:
Low-magnification image of an acetate peel. Red box indicates area of image at right.
Right:
Higher-magnification transmitted light image showing the nice textural detail in the cement crystals.