Acids

Nature's way of cleaning fossils is with acid, but she has more patience than the fossil collector. A gentle wash with carbonic acid so weak that we drink it daily without realizing it will clean fossils locked in a carbonate matrix. This is how nature does it, and it takes a few years. We can dip a fossil in dilute hydrochloric or acetic acid and do in ten seconds what nature would take ten years to do. But we also run the risk of ruining the fossil in the attempt.

Carbonic acid is carbon dioxide dissolved in water. It forms naturally in bubbling hot springs and in rainfall as the drops absorb carbon dioxide from the air. Ground water also picks up carbon dioxide released from decaying and living organic material. When any of these carbonated waters touch limestone, they slowly dissolve it. For some reason the calcium carbonate of the limestone is dissolved much more readily than the calcium carbonate of a fossil brachiopod.

All carbonated beverages from beer to cola are acid enough to dissolve rock. A brachiopod plunged into a can of cola will be cleaned if it is allowed to remain long enough in the can and if the carbonation is not allowed to escape. However, it is better to drink the beverage and clean the fossil some other way.

Stronger acids, when used properly, can expose and loosen fossils; but when used on the wrong types of fossils or in the wrong way, they can destroy them. A fossil should never be treated with even the weakest acid without first trying the acid on a broken specimen of the same type. Let the broken piece dry, then examine it, for much damage can be masked by a film of water.

Three types of acids are used in cleaning fossils. One type dissolves a matrix consisting of quartz, such as sandstone, without harming calcified fossils that are in it, such as brachiopods or crinoids. The second type includes the weak organic acids used for gently removing carbonate matrix from carbonate fossils. The third type has the strong acids, used for removal of carbonate matrix from silicified, pyritized, bony, or tough carbonate fossils.

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