Three other strong, cheap, and easily obtained acids are contained in this group)—hydrochloric, sulfuric, and nitric. The last two are dangerous to use and have no advantage in fossil use. Hydrochloric acid, also called muriatic acid, is an excellent solvent for any carbonate matrix, acting faster than organic acids though not as gently.
Hydrochloric acid should be diluted to about a 10 percent solution, or weaker, for fine work. Cold water should be used, because mixing acid and water creates enough heat to crack a glass dish. Always add the acid to the water. Remember the triple A: Always Add Acid. Acid is heavier than water and when poured into water will sink, mixing as it goes. It should be stored in a container made of glass, stone, or plastic—never one made of metal—and the tightly closed bottle should be kept away from metal objects. Vapors will escape from the tightest bottle and will rust nearby metal. Fumes from an opened bottle of acid are irritating, so the pouring should be done outdoors while standing upwind. Fossils placed in normal dilutions of acid can be fished out with the bare fingers; gloves or tongs are not necessary unless there are unhealed cuts on the fingers.
Hydrochloric acid is used to free fossils composed of quartz and pyrite. It is also used to free rare anhydrite fossils. Fossils replaced by silica, such as quartz, opal, agate, jasper, chert, or flint, are not uncommon, especially in some areas. If some fossils in a rock layer are silicified, most of them will usually be silicified. Some stone blocks contain enough calcium carbonate to cause them to dissolve or fall apart when treated with acid, and their delicately preserved fossils can be released in this way.
Natural weathering of limestones and limy shales that contain silica fossils will free the fossils, but it may take years to free a small brachio-pod. During this parturition the fossil is exposed to freezing, thawing, heating, cooling, miniature landslides, and perhaps the misplaced foot of an animal. When it is finally free it is little more than dust. This is one case in which rapid weathering by man is better than the patience of nature.
A classic locale for silicified fossils is in the Glass Mountains of Texas, where a layer of Permian limestone contains ornate brachiopods in perfect preservation. Many of these brachiopods possess delicate spines projecting several inches from their small shells. These are invariably broken off when the fossils weather from rock layers. Blocks of this Glass Mountain limestone, taken back to the laboratory, placed in glass aquariums, and covered with acid, will in some weeks or months be reduced to a mound of magnificent fossils, perfectly cleaned and so delicate that they often collapse of their own weight when the water is drained away. A 180-pound block of this stone, when dissolved, was found to contain 10,000 brachio-pods, an average of nearly 55 a pound, as well as other fossils. Large blocks are favored for this treatment, as they are more likely to contain large undamaged specimens. Such giant pieces are beyond the capacities of the amateur fossil collector, but pieces of a pound or so should produce fine results for him.
Another classic locale for silicified fossils is in the Ordovician of Virginia, where acid dissolution of blocks of limestone has released trilobites, particularly juvenile forms never before found.
An easy way to learn whether loose fossils found in an outcrop are silicified is to break off a corner of one and scrape this corner against a coin. If it digs into the metal easily and leaves a distinct gouge, it is silici-fied. If it doesn't make a good scratch, it is probably calcified. Often a weathered fossil, even if silicified, will wear a thin coat of limestone that will fizz and give a false signal in acid.
The block of fossil-bearing stone to be dissolved should be placed at the bottom of a glass or plastic bowl and covered with acid. If all the fossils seem to be on one side of the block, that side should be placed facedown, and only enough acid to cover the fossil layer should be added. The acid must be changed daily, as it quickly exhausts itself. The action goes on slowly after the violent bubbling stops. This violent bubbling can rip apart newly exposed, fragile fossils. If the fossils are particularly delicate, it may be wiser to use very weak acid for a longer time.
The mud that most limestones contain does not dissolve. It settles to the bottom of the container along with the fossils and makes their recovery from this murky mess difficult. If the specimen is placed on a piece of plastic screening supported above the bottom of the dish by glass marbles or quartz pebbles, the mud will settle through the screen, leaving the fossils behind.
After the block is gone, the fossils should be washed gently in water. If they are in mud, it should be washed away carefully, preferably not down the sink, as mud may clog the drain. The fossils should be dried on a piece of paper toweling. If they are very tiny, they should be rinsed in acetone or alcohol to remove water from them. Since most water is hard because of dissolved minerals, the small fossils will glue themselves to paper or glass or whatever they dry on as the water evaporates, leaving behind a layer of its once-dissolved minerals. This water glue is surprisingly tenacious; the tiny fossils may break before they snap loose. Fossils should never be dried over an open flame, since water still in them may form steam and burst them apart. Tiny fossils heated in a pan will dry to a certain point and then suddenly pop and splatter out of the pan.
Pyritized fossils may be exposed with hydrochloric acid. Most pyritized fossils are found in shales, however, and unless there is a high content of calcium carbonate in the shale it will not be touched by acid. Devonian fossils often become pyritized; the brachiopods of Sylvania, Ohio, and western New York State are examples of this process.
Pyritized fossils are often found in rock layers near layers of coal. Many so-called pyritized fossils are really replaced by marcasite, the unstable sister of pyrite. To expose the fresh surface of a marcasite fossil by acid action is to invite disaster. In a few months or years the marcasite may grow white whiskers, produce sulfuric acid, and crumble into a pile of corrosive dust and acid. This has happened to many pyritized snails found in the coal mines at Farmington, Illinois. The cause of this marcasite disease has recently been attributed to the appetites of iron-loving bacteria. Treatment to thwart them consists of soaking the specimens in a strong bactericide for a day or more and allowing them to dry without rinsing.
Thick-shelled fossils often are not completely pyritized and if they are cleaned in acid, they can be damaged. The acid will enter the inside cavity of whole brachiopods and eat out the limestone, leaving pyrite shells a few millimeters thick that fall to pieces. Cleaning pyritized specimens with acid will always remove some surface detail. While producing a golden fossil, such cleaning materially reduces its scientific value.
All acids except hydrofluoric can be flushed down the drain after use if
they are followed with plenty of water. Acids will not harm plumbing but will etch concrete basement washtubs. They will not damage porcelain.
Other acids, such as the dry powder acids, have limited use in fossil preparation. They are no better than the liquid acids and are usually expensive. If you are ordering acid from a catalog, you can choose the cheapest; it is not necessary to have chemically pure acid for dissolving rock. Some dilutions are cheaper than nearly pure acid, but sometimes the reverse is true. Remember that the two commonly used acids, acetic and hydrochloric, are easily available as vinegar (acetic) and muriatic (hydrochloric).
In using acid, remember:
1. Always Add Acid to water (AAA)
2. Always test a sample piece first.
3. Put a drop of acid on a spot of matrix before plunging the whole specimen into acid.
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