Fossil Preparation

Excerpt from Introduction to Fossil Collecting

(C) 1994-2004, Glen Kuban,

Part of Kuban's K-Paleo Place home page

The process of cleaning, repairing, and removing excess rock from a fossil is known as preparation. Generally, only preliminary cleaning or trimming is done in the field, so that more careful "prep work" may be done later in the laboratory (which may be one's basement, garage, or kitchen table).

Initial work in the field may involve merely brushing off loose sediment and/or rinsing the fossil in water. Even this should be done with care, since some fossils are very fragile. If the fossils occur as thin carbon films, as is often the case with many plant and insects, you may want to apply a light, protective coat of non- glossy plastic spray while at the site. However, it is best not to coat fossils with heavy or glossy layers that can obscure the fossil's natural appearance.

If a fossil breaks or cracks in the field, one can make initial repairs with white hobby glue, which can be softened with water and removed later if necessary. Broken or friable (flaking) fossils may be hardened or consolidated with synthetic glues such as Duco Cement, Butvar, or clear nail polish dissolved in acetone or other petroleum solvents. Quick repairs can be made with cyanoacrylate "super glues," if the specimen is dry.

After carefully unpacking fossils at their destination, more detailed preparations may be advisable. First, some fossils may benefit from further brushing, rinsing, or washing with soap and water. Be careful not to scrub fragile specimens vigorously, which can damage or destroy them.

Next you may wish to remove some of the excess rock or matrix surrounding the fossil. Generally, most of the matrix should be removed, but it is often wise to leave some of the matrix around the fossil, to increase its stability, and to preserve the geologic context. In many cases fossils are considered less valuable if entirely removed from the matrix. Excess matrix can often be trimmed from soft rocks or relatively thin slabs with pliers or carefully chipped away with a hammer. However, using a saw reduces the chances of damaging the fossil. If you have patience and the rock is not very hard, a hack saw may suffice. More efficient tools include a carbide saw blade on a drill, or rock saws (discussed below). When removing matrix immediately around the fossil itself, proceed with caution, carefully chipping the matrix away with a sharp tool (such as a sharp nail, awl, hobby knife, or sharpened center punch). Fortunately, in many cases the matrix will tend to separate from the fossil at the point of contact. However, patience is often required to avoid gouging or cracking the fossil itself. Short and careful strokes are better than quick, haphazard ones.

Sometimes a shale matrix may be softened by soaking or boiling it in detergent and water. However, some soft shales will absorb water, swell up, and then disintegrate --destroying any fossils in them. Other fossils occur in very hard, often oval-shaped nodules called concretions. These may be split with careful hammer hits along the long axis, or by alternately freezing the specimen and then dropping it in boiling water.

When in doubt about the suitability of any preparation technique, always experiment with unimportant specimens. Most collectors have at least one horror story to tell when this advice was not followed. I myself destroyed a beautiful brittle star when I hastily began to clean the specimen with a stiff brush, and promptly brushed the delicate fossil clean off the rock. In short, follow the paleontologist's version of the hippocratic oath: "Above all, do no harm."

One of the more risky (but sometimes useful) techniques is to use acid in removing matrix from fossils. This should only be considered when the matrix is limestone, and the fossil itself is composed of silica or other acid resistant mineral. Use only weak acids such as acetic acid (vinegar), or heavily diluted hydrochloric (muriatic) acid, available at hardware stores. Acids must be used with extreme caution, as they may eat away the fossil itself, as well as your clothing and skin. Again, be sure to experiment on unimportant specimens before plunging a good fossil into acid. When diluting acids, remember the "AAA" adage: Always Add Acid to water, not vise versa.

To repair or fill major cracks, gaps, or holes in a specimen, one may use Plaster of Paris, urethane foam, water putty, or epoxy. Plaster and water putty may be mixed with powdered cement pigments (before adding water) to create fillers that match the color of the host rock. Epoxies (available in liquid and putty consistencies), are especially good for permanent, high- strength repairs. Some of the sources listed in the bibliography further describe these materials. Caution: many glues, solvents, and sprays give off hazardous fumes; use them outdoors or with good ventilation.

Once you have been fossil collecting a while and decide you like it, you may want to invest in some mechanical tools that can save preparation time and effort. Among these are small variable speed drills, such as a Dremel (R) tool or dental drill, to which various cutting and brushing bits can be attached; vibrating engravers or "Vibro- Tools," which work like tiny jack- hammers; and ultrasonic cleaning machines, which use sound waves and cleaning solutions to loosen dirt and matrix. Also useful at times are rock saws, grinding wheels, and air abrasives (small sand blasters)--all of which should be used carefully to avoid damage to your fossils and your body. These machines are best used outdoors or in a well ventilated area, since they can generate large amounts of rock dust, which may be harmful if inhaled. Always wear goggles and a dust mask when chipping, abrading, or cutting rocks.

Final preparation is largely a matter of judgement. Specimens that are very hard, or that are pyritized (replaced with an iron compound called pyrite), may be brushed with a soft brass brush to help remove any superficial matrix and bring a metalic luster to the specimen, but will sacrifice some surface detail. Although some workers frown on applying any unnecessary material to a fossil, when fossils lack contrast with the surrounding matrix (making them difficult to see in an exhibit), the contrast can be increased by applying a thin coat of non-glossy acrylic spray, egg white, slate dressing (available at hardware stores), or yellow dextrin (available from drugstores) diluted with water. Contrast may also be increased with a light application (buffing up any excess) of petroleum jelly, paste wax, or sun-screen--all of which may fade somewhat with time, but which leave a nice matte (non-glossy) finish. In any case, avoid thick coatings of plastic, shellac, varnish, or paint, which will detract from the natural appearance of a specimen. A label should be affixed to the specimen or otherwise associated with it.