Preparing And Cleaning Fossils

Collecting is only half of the job. Few fossils are found so clean that they are fit to be placed in a collection without further work. Some need only a brushing; some require painstaking treatment to remove rock that obscures the details of the fossil. Proper cleaning is important. Almost every day, an amateur collector brings a fossil to a museum to be identified. Too often a rare specimen has been damaged because its owner brushed varnish on it or destroyed fine detail by plunging the fossil in acid to get rid of matrix.

Preparation begins in the field with use of proper tools. Each person in the collecting party should have a prospector's pick, a flat chisel, and a square-pointed chisel; and the party should share sledges, crowbars, and shovels. Without proper tools, the collector will be unable to remove fossils in an undamaged condition. He will also need a knapsack, a collecting bag, or an apron with pockets where he can stow away wrapped specimens. For some areas, a metal bucket or a basket is more convenient. Putty knives or old table knives are useful for splitting shale, and an old toothbrush will be useful for scrubbing dirty fossils in the nearest puddle or stream.

The shallow cardboard trays that hold four six-packs of beer (commonly called "beer flats" by collectors) can be fitted together if the corners of

Beer Can Plaster Paris
Pieces of large turtle shell are cemented together with plaster of paris and dextrin. The plaster fills in chipped edges and can be colored to match the fossil.

the bottom tray are bent in. This makes a shallow, extremely strong, covered box of convenient size to transport or store fossils.

Extremely soft matrix must be treated to harden it for the trip home. Some carbonized fossils, such as plants and fish, must be sprayed to keep the fossils on the matrix, or they will crumble to dust after a few miles of traveling. Vertebrate fossils require elaborate plaster casts before they can safely be moved from their resting places in the field. At home, the actual work of cleaning fossils will begin: clinging matrix can be removed or trimmed to size, and rock can be dissolved or otherwise eliminated to free its content of small fossils.


Most fossils found in the field need little care other than wrapping them in paper to prevent abrasive contact with companion specimens on the way home. Loose, sturdy fossils such as brachiopods that are collected from shale exposures can even be piled without wrapping in a tin can or a small box, if the container is packed full so that the contents do not rattle.

A cigar box is excellent for this purpose. When collecting at some sites where there are thousands of loose specimens, this will save much time. Most loose fossils have their own thin protective jacket of mud and shale that acts as a buffer.

A few time-saving hints will expedite packing fossils in the field. Matrix specimens can be wrapped loosely in newspaper taped shut at the ends. Fragile specimens can be wrapped individually in toilet paper by winding it around the specimen in loosely twisted rolls until the specimen is completely bandaged.

A faster way is to layer fragile specimens in a sturdy box or can, separating the layers with sawdust. Any sawmill has mountains of coarse sawdust for the asking. Small amounts accumulate at any lumber yard. The sawdust can be carried in a sack and added to the specimen-collecting box as needed. If the fossils have deep nooks and crannies, grains of the wood have an annoying tendency to lodge there, but they can be picked out.

An emergency method that works well if conditions are favorable is to enclose a fragile fossil in a gob of wet clay or mud. The mudball can then be wrapped in paper and tossed in with the sturdier fossils. This method is especially useful in keeping together the loose parts of a broken fossil. These mudpacks should be removed as soon as possible, before they dry out. Hardened mud becomes difficult to loosen, and the shrinkage during

How to wrap a fossil concretion in the field to preserve its fine detail. Place sheet of paper between halves, fold in sides, then roll lengthwise, and secure with rubber band or tape. (Drawing courtesy Illinois Geological Survey)

Broken fossils can be reassembled in aluminum foil. Folded gently over the specimen and pressed tightly, the foil keeps edges from chipping and pieces from becoming lost.

drying may break fragile specimens. Simple soaking at home will remove all the clay or mud.

A fossil found in several broken pieces can be reassembled roughly in a small square of aluminum foil. The foil should be folded over the fossil gently and squeezed to keep the pieces firmly together until the specimen is home. Broken fossils can be mended in the field, but this takes time and often results in a poor job. It is better to protect the pieces and work the puzzle at home. Fast-drying household cements (such as Duco) are suitable for repairing broken fossils.

Don't forget to include a label describing the specific collecting-location in detail. Labels should record the general geographical position of the dig, the assumed geological age of the rock matrix, and the name of the formation and associated formations. Sometimes it is possible to describe the formation by measuring its vertical distance from level ground or a distinctive rock layer. Experience teaches the collector that the best memory is not to be trusted with these technical details, but a good label is forever. Without such a record of the location, a fossil loses most of its cash value and all of its value to science.

Specimens taken from rock layers that are obviously different should be kept separate and should be labeled separately. Loose, weathered specimens taken from the bottom of a slope should be kept separate from those taken directly from an identifiable rock unit. The latter fossils are more valuable because their source is precisely known.

Hardening Matrix

Some shales or weakly cemented sandstones may be so fragile that they cannot be removed without disintegrating. Shales that enclose plant fossils seem to be particularly weak, though fragile invertebrate fossils that must have supporting matrix, such as trilobites, graptolites, and bryozoans, often occur in crumbling shales. These specimens must be hardened on the spot.

A professional concoction invented by the British Museum for protecting and hardening shale containing British Mesozoic fossils consists of two to five tablespoons of flake-form polyvinyl acetate dissolved in a pint of toluene. It may take a day or so for the flakes to dissolve. Polyvinyl acetate is not to be confused with other polyvinyl compounds; it must be the acetate. The mixture is stored in a jar with a tight-fitting lid, since the fumes are irritating and inflammable.

Acetone may be more readily available to amateur fossil collectors than toluene. It will also dissolve polyvinyl acetate. It is inflammable and will cause dizziness if breathed for prolonged periods. Cellulose acetate, in sheets or flakes, can substitute for polyvinyl acetate. These hardeners are similar to such household cements as Duco. In fact, fresh Duco cement dissolved in a few spoonsful of toluene or acetone will serve admirably for small hardening jobs.

The mixture can be brushed on, or the piece of shale can be immersed in the liquid for several seconds. Immersion will make the shale stronger

Shales such as this specimen are poorly consolidated and will turn into mud at the touch of water. Fragile, thin-shelled fossils such as this trilobite will often crumble unless the shale is hardened.

but will also coat the fossil unless the block is hardened by immersing only the backside. The block may need a second dip for super-strengthening. The pieces should be dried in a well-ventilated area; they will dry in a few minutes.

If the mixture contains too much plastic, the fossil will become shiny, particularly if the fossil itself is composed of smooth, nonabsorbent, chiti-nous material, as trilobites or cephalopods are. If the specimen is too shiny after it has dried, and the fossil can stand rubbing, it can be wiped with a cloth soaked in acetone or fingernail-polish remover (which is perfumed acetone).

These polyvinyl hardeners are also useful for gluing back bits of shelly material that pop loose from a fossil during preparation.

Some fossils, particularly carbonized plant fossils and some Eocene fossil fish of Wyoming, are beautiful when first removed from their stony graves, but as they dry the carbon particles flake off and blow away. What once was a fish skeleton disappears with the drying wind. As soon as the specimen is reasonably dry, it can be sprayed lightly with one of the clear plastic sprays. The specimen should not be sprayed when it is wet; the plastic layer will peel away along with the fossil. Krylon is particularly well suited for spraying fossils as it does not leave a very shiny surface.

Moist shale, which tends to disintegrate as it dries, can be preserved for several weeks if it is sealed inside large plastic bags until there is time to

Carbon Film Fossil Drawings
Loosely attached carbon films, such as fossils of many plants, fish, and graptolites, should be sprayed shortly after collecting with a non-glossy plastic.

work the fossils out of it. This is a method developed in recent years by professional collectors for museums.


The carefully unwrapped specimens will need a bath or more extensive cleaning before they can be properly studied or displayed. This is the most tedious part of fossil collecting. Many amateur collectors wonder why specimens in museum are so detailed and sharp, whereas their specimens remain muddy looking. The answer is in the preparation — or lack of it. Museums hire full-time preparators to remove the adhering matrix painstakingly by hand or with machinery.

Some specimens will need nothing more than to be soaked in warm water with a dash of detergent, followed by a scrubbing with an old toothbrush and a rinse of clear water. Specimens that have weathered free from soft shales fall into this category. So do Miocene or Eocene shark teeth and shells found along both coasts in soft, sandy matrix. Concretionary fossils, such as ammonites, bones, crabs, and fern fossils, break to a clean surface and often need no further preparation than washing or brushing to remove dust.

But most fossils, even those that at first glance appear free of matrix, need further cleaning. Brachiopods invariably have matrix wedged in the hinge line. Trilobites seem to have concrete packed in the furrows of their corrugated skeletons. Crinoids have thousands of feathery arms to prepare. Snail openings are obscured with rock. Bony fossils are encased in a rock jacket, but by the time the bones weather free they have become bone meal. In all these cases, hard matrix must be removed.

Soaking and Scrubbing

All hard fossils should first be washed with detergent and water. Hard fossils are durable specimens that are not on a matrix of soft shale or sandstone that is likely to disintegrate when wet, or are not thin delicate films that might loosen in water. Graptolites, carbonized plants, and thin-shelled arthropods are examples of these delicate specimens. When in doubt, experiment with a broken specimen. Some fossils, such as brachio-pods found in shale as single shells or valves, are so thin that when the adhering shale is loosened by the water they fall apart.

Small nylon brushes such as toothbrushes are ideal for scrubbing a fossil. Weathered limestone and shales can sometimes be entirely removed

Ceaning Dinosour Bones With Toothbrush
Trilobite Elrathia kingi from Wheeler amphitheater, Utah. Specimen in shale was cleaned with a toothbrush and water. Cambrian period.

by gentle but persistent scrubbing. Nylon is softer than the calcite substance of fossils and softer than the matrix, too. It removes only matrix grains that have weathered loose. Hard, fresh limestone and shale will not be touched by brushing. Sometimes soaking for several days in water will soften matrix.

If scrubbing does not remove matrix, set the specimen aside for mechanical preparation.

Stained specimens can sometimes be cleaned by soaking them in a sodium hypochlorite solution (such as Clorox) overnight. Always try this on a sample piece first. If specimens have been permeated with natural crude oil or asphalts (some Silurian fossils of northern Illinois and Indiana are found in this condition), immerse them outdoors for a day in gasoline, scrub them with a brush, then rinse them several times in clean gasoline. Avoid plastic-handled brushes, such as toothbrushes; they soften in gasoline. Allow the specimens to air for a day or two before bringing them inside. Do not pour the dirty gasoline down a sewer: the fumes are explosive. Kerosene or light oils and even the strongest detergents are not as satisfactory as gasoline, because gasoline will penetrate the specimen and remove the crude oil and then will completely evaporate.

Wheeler amphitheater, House range, Millard county, Utah, whose shale beds and limestone strata are highly fossiliferous. (Photo by Dwayne Stone)

Specimens on the surface of soft shales cannot be cleaned in water. The shale will swell and literally explode. These shales are best hardened from the back with the hardener mentioned above, and the fossil itself can be cleaned by gently wiping it with a cloth or paper towel soaked in alcohol. Try to keep the alcohol from soaking into the matrix. When the specimen is cleaned, an allover brushing with the hardener will protect the surface and waterproof the specimen.

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