The gentle organic acids —acetic and formic —are used to clean fossils by slowly dissolving calcite (limestone). Acetic acid gives vinegar its tart taste and smell. White vinegar is diluted acetic acid, formed naturally by fer mentation of alcohol. Formic acid, which causes the excruciating sting of an ant bite, is also found in some stinging plants.
The two acids are quite similar in action but not in price. Acetic acid is far cheaper. Formic acid may be a bit gentler on some fossils, but not enough to justify the difference in cost.
Plain white vinegar is a satisfactory, though slow-acting, source of acetic acid. Glacial acetic acid, a strong concentration, can be bought at a photo-supply store or chemical-supply house. Glacial acetic acid should be cut with five to ten parts of water to make a solution usable on fossils.
After a fossil has been thoroughly prepared, a thin layer of limestone or limy shale may remain. Mechanical removal would undoubtedly damage the fossil. This layer should be as thin as possible before acid removal is considered, and the acid should be the last resort. The specimen is placed in the weak acid in a glass or plastic dish (not metal) and left for a few seconds to observe progress of the reaction. The acid will eat through the matrix and will also eat the fossil, though much more slowly.
Fossils prepared with acid have an unnatural, polished appearance which may be objectionable. Some detail will be lost, and this may counterbalance any benefit from removing the last vestiges of matrix. The acid may be placed with a small brush on spots of matrix, little by little, until the spot
A pyritized brachiopod Mucrospirifer looked like this before acid treatment. Matrix is shale, but contains enough calcium carbonate to allow disintegration with acid.
disappears. When bubbling stops, the spot should be brushed to remove loosened stone and to check progress. This is a slow method but it permits exact control of the preparation.
Some delicate silicified fossils can be removed from carbonate rocks — for instance, Miocene insects from the small California nodules —by the use of acetic acid. These nodules take weeks to dissolve in a plastic container of acetic acid, but the gentle action leaves the insects intact. Such delicate features as legs and antennae are undamaged.
Acetic acid will not dissolve fossils composed of calcium phosphate, such as conodonts and a few species of brachiopods. These can be removed from carbonate matrix by submerging the blocks of limestone in vats of acid until they are dissolved. The mud at the bottom of the tank can be washed away gently with many changes of water. The remaining fossils when dried will be ready for mounting.
To test a fossil to see whether it is silicified or has been replaced by cal-cite, put a drop of strong acetic or hydrochloric acid on it and watch through a magnifying glass for signs of bubbling or fizzing. If the acid causes fizzing, the fossil is calcite.
Some weathered fossils will be coated with white calcium carbonate
deposited by ground water. A short dip in weak acetic acid will remove this coating without harming the fossil. This is commonly done with concretionary fern fossils from Illinois or Indiana that are found exposed as a result of weathering. Invariably the plant fossil is obscured by a white mask, which is probably derived from a chemical reaction between the concretion and water. After acid treatment, the loosened film will brush away with soap and water.
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