Special Techniques

Every collector needs to know something about techniques that are more complex than the methods of preparation we have described so far. These are now-and-then matters —the preparation of certain unusual fossils that require specialized treatment. As the collector becomes more skilled in his work with fossils he will wish to go beyond the scraping, chipping, and other methods that had formerly sufficed.

Such techniques as the making of peels, thin-section work, and plastic embedment are not beyond the ability of the advanced collector, and they will add new satisfactions to his hobby and better specimens to his cabinet. Turning a dull-brown coal ball into a series of montages of the cellular structures of ancient trees, leaves, and seeds only 1/1000 inch thick gives him not only personal satisfaction but also a closer look at the life of ages otherwise hidden by the mists of time.


Thin sections, used in paleontology as in biology primarily for identification of plants and animals, are extremely fine slices of material prepared by grinding or other means. They are sheer enough to allow light to pass through them, making cell walls, pores, and other details visible. They must be viewed through a microscope.

Another type of thin section, known as a serial section, is used to determine the shape of a fossil embedded in matrix from which it cannot be removed, such as a seed in a coal ball or a brachiopod in marble. The thin serial sections are removed one after another, progressing right through the fossil. The outer margins of the fossil, visible in the thin sections, are measured and plotted on paper until the three-dimensional shape of the fossil is reconstructed. This is time-consuming. The same result can be accomplished by grinding away a measured amount of the specimen in stages and drawing the fossil outline at each stage on paper.

Thin sections can themselves be things of interest and beauty. A peel of a coal ball may show in fascinating detail a cross section of the intricate cell structure of a cone or a root. Thin sections necessary for identification of petrified wood are also attractive for their colors. They can be mounted like pieces of color film in glass projection slides and projected on a screen. Slides are particularly well suited to sections of petrified wood and coal balls, though interesting sections have also been made of coral.

Grinding Thin Sections

Thin sections can be made by two techniques — grinding and the peel. The first is like grinding a gemstone. The second uses acid to etch away a thin surface layer of matrix, leaving behind the actual cell walls of the plant or animal. These are then covered with a layer of liquid plastic. When it is dry, the plastic layer is pulled off, tearing loose the cells but keeping them in exactly the same position in the plastic.

Grinding of thin sections is commonly done with silicified wood which, like modern unfossilized wood, requires a cross section and sometimes a longitudinal section for identification. This is true also of fossil corals and bryozoans, which are identified largely by the position, shape, and size of the inner chambers. Sometimes these animals can be identified without making a thin section, but the fossils must still be cut and polished in a flat section both across and lengthwise to expose the chambers.

Most books used for identification of fossil corals (particularly horn corals) and bryozoans show sections of the individual species, sometimes without a picture or drawing of the entire animal, since so many species are quite similar on the outside.

Before grinding a thin section (or preparing a flat area on a whole specimen) it is usually necessary to cut the specimen with a diamond saw. A thin blade should be used, particularly with tiny fossils. Normally, a sec tion should be taken across the specimen (like cutting a carrot into slices) and another at right angles to it (like cutting the carrot lengthwise). Before sawing the specimen, study illustrations of sections in a textbook to see how to cut the specimen and what a section should look like when sawed.

Only a small piece of the fossil is needed, rarely more than half an inch. Don't try to saw it too thin; it may break.

Most fossils are too small or too irregular in shape to be held in the saw vise; they must be cut by hand. The piece should be fed slowly and carefully to avoid breaking off the tip of the slice at the end of the cut. Since a diamond sawblade will not cut flesh, there is little danger from the saw unless a thin slice shatters while being held.

The sections should be washed at once to remove the coolant oil. Undiluted liquid detergent can be rubbed over the specimen. It can then be rinsed in water that is neither very hot nor very cold, as extremes of temperature can cause a specimen to crack.

If the saw cut is smooth, the piece can be cemented directly onto a glass microscope slide. If both surfaces are rough, one should be carefully ground or sanded flat. This can be done more easily if the surface exposed by the first cut is sanded to a flat surface before the thin section is sawed off. This leaves at least one flat surface on the slice.

Cement the thin section to the slide with epoxy or one of the fast-drying jewelry cements. Whatever cement is used, it must dry clear. Spread the cement evenly and thinly on both specimen and slide and put them together carefully to avoid trapping bubbles. Gentle heating at 200 degrees for fifteen minutes will cure epoxy sufficiently so that the specimen can be ground; without heating, it must cure at room temperature for a day.

The mounted piece should be ground as thin as possible. Ideally this should be done on a small flat lap, a cast-iron disc with a true flat surface. The disc rotates horizontally like a potter's wheel. The lap wheel is mounted atop an arbor, and the whole is placed in a metal tub to catch the grit cast off by the rotating lap. A motor, usually mounted underneath, turns the wheel at a speed determined by the diameter of the wheel. The speed should be just short of that which will sling off the grit as fast as it is placed on the lap wheel. These flat laps can be purchased for reasonable sums from a rock shop or built from an arbor and a lap wheel.

If a great deal of material must be ground away, the lap wheel should be sprinkled with 220 silicon carbide grit. If a large surface on a piece of petrified wood is to be prepared, 100 grit may be necessary for the first grind. The thin section should be ground until it is almost thin enough, which will be perhaps 1/25 inch. At this stage, the specimen should be translucent when held to a bright light. The lap wheel should then be cleaned, and the grinding continued with 400 or 600 grit. If the piece is

Cast-iron flat lap grinds flat sections for microscope examination or for preparing coal balls or petrified wood.

thin, and the fossil is composed of a soft material like calcite (most animal fossils), the grinding can be done entirely with 400 or 600. Grinding should proceed slowly, and the flat must be inspected often to avoid grinding through the fossil. The specimen will be complete when it becomes paper-thin and quite translucent.

An alternative method is to use a silicon-carbide grinding wheel, such as is used for lapidary work, and to grind gently on the flat side of the wheel. Specimens of more than an ounce or two should not be worked in this way. Some pieces of lapidary equipment, particularly the compact machines, are designed to run horizontally, making flat-section grinding easier.

Do not use much pressure while grinding on the side of the wheel, as it is not designed for side pressure. Use plenty of water for cooling and use a fine-grit wheel, such as 220. Move the specimen back and forth to avoid grinding a hollow in the wheel.

When using the grinding wheel, leave a slightly thicker section than when using the flat lap. The grinding wheels can also be used to remove enough material to proceed directly to the 400 or 600 grit on the flat lap, saving one lapping step there.

If no flat lap is available, the specimen can be given the final grind by hand on plate glass with loose silicon carbide grit. The grit (400) is sprinkled on the glass and enough water added to make a soupy mixture. The slide and its attached specimen are then ground vigorously with a circular motion. As soon as the specimen is ground to the proper thinness, it can be finished on another piece of glass with 600 grit. A piece of wet 400- or 600-grit silicon-carbide paper attached to a hard, flat surface such as a linoleum tile or sheet of glass can also be used for finishing.

It is not necessary to polish the thin section. A thin cover glass is usually cemented over it to provide a durable surface. The cementing can be done with a thin layer of epoxy. The cover glass should be pushed around until all bubbles and excess cement are squeezed out. Cover glasses can be bought at any store that handles laboratory equipment. They are less than a millimeter thick and must be handled carefully.

After the cement has hardened, the slide is ready to be examined under the microscope. A label should be attached, describing the section and the specimen it came from.

If an entire specimen is being ground in section, the operations are carried out as with the thin section. It is far easier to work with the whole fossil, as there is no danger of grinding through it. A cover glass can be glued with epoxy to an appropriate spot to make a window, or the specimen can be kept wet while being examined. This gives the appearance of a polished surface and makes the structures much easier to see. The surface should not be polished, because polishing pulls out soft areas and makes a rough surface rather than a smooth one. Pieces of solidly silicified wood can be polished, using standard lapidary procedures, after a good fine grind has been achieved.

After the thin section is made, or a suitably oriented flat spot is ground on a specimen, the internal arrangement of compartments can be compared to the illustrations in an identification book. It may be easier to make a pencil drawing of one area of the specimen rather than refer to the microscope repeatedly.

The Peel Technique

Peels are used mainly to examine coal balls, those mysterious mason jars of perfectly preserved vegetable matter 250 million years old. Coal balls have been described in Chapter IV —they are rounded masses of seeds, leaves, stems, roots, and bark of Coal Age trees "petrified" in complete cellular detail by a mass of calcium carbonate. These coal balls lie in the coal seam which was formed from similar plant material. The coal balls, impregnated by calcium carbonate, hardened into stone before the rest of the plant remains were compressed into coal.

Early peels were made by pouring liquid plastic over the flat acid-etched section of the coal ball. When the plastic was dry, usually hours or days later, it was pulled and scraped off the specimen. This process is rarely used now except when extremely small structures must be preserved.

The modern method is to use a sheet of cellulose acetate dissolved by acetone directly onto the specimen. It flows into all cellular spaces left after the matrix has been dissolved by acid. As the acetone evaporates, the plastic hardens again and can be "peeled" in less than an hour.

The peel technique can be used on some corals, brachiopods, and other fossils. The fossil must contain some acid-resistant organic substance. Most plant petrifications do contain such organic material, even the hardest silicified woods. The acid dissolves the matrix, which is calcite (calcium carbonate). Even agatized woods can be peeled if the silica is dissolved with hydrofluoric acid. A short bath in acid removes a layer of matrix perhaps 1/1000 inch thick, leaving the cell walls and other organic material standing free. The plastic fills the voids and tenaciously holds the organic material in exactly the same position when the plastic is ripped loose from the specimen. The peel needs no further preparation to be examined, identified, or displayed.

The specimen must first have a flat surface. This is usually made with a diamond saw. A coral may need to be oriented carefully in order to obtain a useful peel, but a coal ball may be cut into thick slabs at random. (It is impossible to know what structures will be where in a coal ball.) After sawing, the sections are ground perfectly flat, like a thin section, on a flat lap, or with loose grit on a piece of plate glass. The specimen is finished by grinding it on plate glass with 400 grit for a few minutes to make a smooth surface. The grit should be washed from the specimen.

A solution of 10 percent hydrochloric acid is prepared (Always Add Acid to water) in a shallow plastic, glass, or enameled flat-bottomed container. At least an inch of acid should cover the bottom. Grasp the specimen with rubber gloves and hold it, prepared surface down, in the acid, but do not let it touch the bottom. Any fine exposed structures will be broken off if the specimen touches the bottom. It should fizz violently. The time of the bath will vary from five to fifteen seconds, depending on the type of matrix, the strength of the acid, and how many times the acid has been used. It does wear out. Rinse the specimen by letting warm water flow across it gently for ten seconds or so. Be careful not to bump or touch the etched surface.

Place the wet specimen in a box of sand or gravel with the etched surface up and parallel to the floor. It can be allowed to dry for an hour or so or the drying can be reduced to a few minutes if acetone is poured gently across the surface a few times. This should be done in a ventilated place to avoid breathing the acetone fumes. While the specimen is drying, cut a sheet of cellulose acetate to a size somewhat larger than the specimen. The acetate is a clear plastic and should be about three mils (3/1000 inch) thick, or about the thickness of heavy paper. Do not confuse cellulose acetate with polyethylene or Mylar; it should be ordered specifically from a company found under Plastics in the Yellow Pages of the telephone book. If none is available locally, it can be ordered from the Colonial Kolonite Company, 2232 West Armitage Avenue, Chicago, Illinois 60647. Acetate comes in large sheets that can be cut to size.

Acetone should now be poured over the specimen. It evaporates rapidly, so it is necessary to work quickly or the peel will be incomplete. Start the acetate sheet at one corner of the specimen, the lowest corner if the acetone is running off that end. The sheet should be held slightly curved (see illustration) so that it is rolled across the specimen pushing a little wave of acetone ahead of it. Do not touch the specimen or wiggle the sheet to remove air bubbles. Allow it to dry for at least half an hour or until there is no more acetone odor.

When it is thoroughly dry, carefully pull on one corner of the sheet. It

Peel Technique Fossil
Making a coal-ball peel. Step 1: After the coal ball has been sawed, it must be ground perfectly flat. A small amount of number 400 silicon carbide grit is used on a sheet of plate glass.

Step 2: The grit is moistened to a soupy consistency, and the coal ball is ground for several minutes with a rotating motion. It is then washed thoroughly.

Step 3: The prepared surface is lowered into a shallow pan containing 10 percent hydrochloric acid. Immersion time varies from five to fifteen seconds. Care must be taken not to damage the prepared surface by striking it on the bottom of the pan.

Step 4: After the acid bath, water is run gently across the surface to remove the acid. Care must be taken from here on not to touch the delicate, etched surface. The coal ball is propped with the top horizontal and allowed to dry. When dry, the surface is flooded with acetone, and a sheet of cellulose acetate is rolled onto it.

Peel Technique Procedure Fossil

Step 5: The peel normally dries in half an hour. It can then be pulled carefully from the coal ball and is ready for study under a microscope.

should separate easily from the specimen. Pull it off slowly and gently, and the peel is complete. If another peel is desired, the specimen should again be ground for a minute or so on the glass with 400 grit to prepare a new flat surface, and the peel operation repeated. With care, more than a hundred peels can be made from I inch of coal ball.

Peels made from corals, brachiopods, or bryozoans may have to be made several times to arrive at the optimum time of acid etch. Their small size may make the placing of the acetate film more difficult. It is advisable to etch silicified woods or coral with hydrofluoric acid, but only in a properly equipped laboratory.

The peels can be stored in envelopes, or particularly interesting structures can be cut out with scissors and mounted on microscope slides. They can also be placed in a projection slide, mounted between thin glass, and projected onto a screen. If some grains of matrix remain, the peel can be plunged into acid for a few seconds, washed carefully, and dried. The plastic is unaffected by acid. Names or catalog numbers can be written with a grease pen or marking pen on a corner of the peel.

Many coal-ball structures can be identified from paleobotany texts. Much specialized work has been done recently at the University of Illinois on such structures, especially of seeds. Workers there have published a number of papers illustrating coal-ball fossils. These are still available.

Peel sections of corals and other marine fossils are identified like standard thin sections. Most books dealing with identification of corals and bryozoans publish illustrations of thin sections.


Liquid plastics are being used by hobbyists to create colorful wall decorations, jewelry, and tabletops with slabs of agate and other gemstones embedded in the glass-clear resin. This casting plastic is a polyester resin —a thick, sticky, slightly bluish liquid that hardens in a few hours at room temperature when several drops of a catalyst are added. The material can be poured into a plastic or ceramic mold; it can also be cut and polished after it hardens. It sells for $4 to $10 a gallon at hobby stores and rock shops.

Museums have been experimenting with similar substances, and the casting techniques they have developed can easily be adapted by the amateur.

Some fossils that are not dissolved by acid and that lie on the surface of limestone or limy shale slabs soluble in acid are excellent candidates for

Fossil fish preserved in plastic after matrix was removed by acid. Specimen is in the British Museum.

making a plastic embedment. The method permits a view of both sides of very thin, filmlike fossils such as carbonized plants, carbonized worms, or graptolites. It is excellent for preparing fragile bony fossils, such as fossil fish. It can also be used for pyritized or silicified fossils that are so paper thin or fragile or so badly fractured that they would fall apart if freed from the matrix. Shattered bones exposed at the surface of limestone can be kept intact with the plastic.

The specimen should first be prepared as well as possible on the exposed side while the other side is still locked in the matrix. All matrix clinging to the surface should be removed. The slab of rock should be sawed as close to the fossil as possible without risking damage to hidden parts or weakening the slab. If the matrix will not stand such treatment, or if saw oil and water would damage the fossil, the specimen should be trimmed by nibbling away at the edges with strong pliers or chipping it with a small hammer and chisel.

The surface of the trimmed block should be dry and clean. A piece of plate glass should be scrupulously cleaned and treated with a mold release (or wax such as Pledge) over an area larger than the fossil. The glass should be placed quite level, and a retaining wall should be built about an inch from the edge of the specimen on the surface of the glass. The easiest material for such a wall is an inch-high strip of Mylar plastic sheet. Only Mylar will work; polyethylene or cellulose acetate will not. It should be thick enough to stand by itself, about the thickness of heavy paper, which is about 5/1000 inch. Mylar is available at most stores that sell liquid plastics, or it can be obtained from large mail-order houses. The Mylar strip can be affixed to the glass with a leakproof band of modeling clay, self-

adhesive rubber molding strips, or masking tape. The band is placed outside the strip.

Enough plastic should be mixed with the catalyst (following directions on the can) to cover the bottom of this modernistic pool to a depth of \ inch or less for small specimens, a bit deeper for specimens more than 6 inches in length. The layer should never be more than \ inch thick in one pouring — heat is generated as the plastic sets, and this may cause a thick pouring to crack. The plastic can be mixed conveniently in a disposable paper cup. The plastic should be stirred carefully to prevent bubbles from forming. If bubbles are trapped in the poured layer, they can be forced to the top with a toothpick.

Place a piece of paper loosely over the pool to keep dust from the newly poured surface. The material will become tacky and hard enough to support the specimen in half an hour to an hour, depending on the amount of catalyst and the ambient temperature. It is necessary to wait only until it has set firmly before adding another layer. Mix another batch of the plastic, and pour a thin layer —again less than J inch —over the first pouring. Paint the fossil and matrix surface with a heavy coat of the liquid plastic, and carefully place the fossil facedown in the liquid in the mold. If one corner is submerged first and then the specimen is slowly rolled into the liquid, there is less chance of capturing bubbles. Lift the glass slab and peer underneath to see whether the piece is relatively free of bubbles; if there are many large ones, pull out the slab and try again. Add more plastic so that it rises around the sides of the specimen at least a half inch for small pieces, up to an inch for larger ones. Cover and allow to set for a day.

The top plastic may still feel sticky even after it has set for a long time. This is normal. The side against the glass will be hard and dry. Remove the side walls and try to separate the plastic block from the glass. If a release agent was used on the glass, it should come right off; if not, try a few gentle taps. It may be necessary to tap a table-knife blade under one corner to free the block. If it still sticks, place fossil and glass slab in the refrigerator freezer for half an hour. The plastic should then come off easily.

The fossil should be clearly visible through the thin window of plastic. Submerge the block in acid —10 percent hydrochloric for most fossils, and acetic for bones—in a plastic or glass container. The block should be placed with the fossil window facing up, and one side of the bottom blocked up on a piece of glass or other acidproof material. Refresh the acid when necessary, and allow the entire matrix block to dissolve. In the last stages it may be necessary to remove the block and gently wash, the surface to remove clinging matrix.

Bony fossils may shed a few bones that were not attached to the skeleton and not exposed enough at the surface to be held by the plastic. When all matrix has been dissolved away, a skeleton should remain resting on the plastic, or a delicate carbon film of a plant or graptolite firmly fixed on the plastic. Rinse the block gently for half an hour in water to which a spoonful of sodium bicarbonate has been added. This will neutralize acid remaining on the fossil. Rinse the specimen again with clear water and allow it to dry.

When the fossil is thoroughly dried (bones can hold moisture for a day or more), more plastic can be mixed with catalyst and gently poured into the well and over the newly exposed back of the fossil. No more than 7 inch should be poured at one time, and the plastic must be allowed to set between pourings. Few fossils will need more than one or two pourings. The plastic should be kept as thin as possible, just enough to cover the back of the fossil.

The final pouring can be covered with a liquid available from the store that supplied the plastic. The liquid will keep air from the surface and allow it to dry hard. If the plastic is still tacky, the piece can be heated gently in an oven or an electric fry pan until the plastic sets. You will notice that the plastic will set more rapidly on a warm day than a cold one. However, plastic should not be poured on an excessively warm and humid day, because the moisture will cloud the plastic.

The fossil is now preserved with all delicate details of both sides clearly exposed. Protruding edges of the plastic can be ground away on a wheel and sanded with sandpaper but the article should be allowed to dry for several days before it is handled or polished. Final polishing can be done with a cotton wheel and buffing compound, or on leather or felt sheets with standard polishing agents such as cerium oxide, tin oxide, or tripoli. If the surface becomes scratched, it can be repolished.

Casting resins do not have a long shelf-life. Make sure to buy fresh resin and keep it in a cool place, such as in a refrigerator. Once it is opened, use the contents of the container in a short time. Never mix more than can be used in fifteen minutes, because in that time it will set. Spic and Span or other detergents can be used to remove the inevitable sticky messes from the hands. Acetone is a solvent for the liquid resin but is not recommended for use on the hands. Work should not be done in the kitchen because the odor of the casting resin can affect the taste of food.

Liquid casting plastics have other uses in the fossil field. A specimen, such as a clam shell or a delicate snail embedded halfway in matrix but too badly fractured to be loosened further, can be coated with a thick layer of plastic and then prepared from the other side. Hand preparation can be used on shales and sandstones that would not be affected by acids. The

This fossil tooth once belonged to a shark; it now adorns a collector. A standard ceramic mold was used. Two pourings of casting resin were made: the first was of clear resin (done upside-down) with the tooth embedded, followed by a backing of white resin. With bola-tie slide attached, the fossil becomes a piece of jewelry. (Jewelry by Cecelia Duluk)

This fossil tooth once belonged to a shark; it now adorns a collector. A standard ceramic mold was used. Two pourings of casting resin were made: the first was of clear resin (done upside-down) with the tooth embedded, followed by a backing of white resin. With bola-tie slide attached, the fossil becomes a piece of jewelry. (Jewelry by Cecelia Duluk)

plastic acts as a glue and a solid base for the specimen. Extremly thin fossils, or fossils on thin sheets of shaly matrix, can be strengthened on the back with layers of the plastic. The plastic can also be used to cement a fragile but irregular piece of matrix to a wooden mount. Of course, fully prepared fossils can be embedded in the plastic, either in a polished block for a decoration or in a mold to make a shark-tooth bola tie slide, or a pyritized brachiopod pin. Commercial molds are available in ring, bola tie slide, and pin shapes.


Most fossil collectors have been struck, perhaps at Christmas time, with the beauty of a fossil used as jewelry. Since most fossils will not wear well if cemented to a ring or pin backing, the obvious answer is to cast the specimen in harder material.

Casts of fossils are used in schools, where there is danger of damage to irreplaceable specimens that will be handled a great deal. Casts can be made by the hobbyist for donation to schools and interested junior collectors.

Some models made commercially are difficult to tell from the real fossil. One collector found a superb trilobite in a dusty drawer in a London mineral shop a few years ago and bought it for a reasonable price. Later, when he was washing the specimen, one stark white corner appeared on the otherwise brown trilobite. The specimen turned out to be a painted plaster cast, made perhaps fifty years ago.

The black, shiny, fat trilobites found in the Devonian shales at Sylvania, Ohio, are particularly well adapted to casting. A black plastic cast is startlingly lifelike. One collector cast a number of these trilobites and then on a club trip to the quarry scattered them in likely places before anyone else got there. All day there were shrieks of excitement as one person after another unearthed these perfect trilobites. Many found it hard to believe that the joke was on them. At the quarry some weeks later a woman excitedly showed friends her prize find of the day. It was one of the plastic trilobites, speckled with mud from intervening rains. Only its light weight betrayed it. She was heartbroken when told the truth. Undoubtedly some of these fake arthropods, labeled in good faith as the real thing, reside in collections.

Casts of type-specimens of fossils are professionally made to be sent to other museums and universities for their study collections. A good cast is

Cast in plastic of the crinoid Cactocrinus arnoldi, made in a rubber mold.

in every way as useful as the real thing. It shows all details, even the smallest pit and pore. It does not show color variations and suggestions of organic films as well as the original does and, of course, cannot be further prepared or examined under a microscope to see crystalline detail.

Casts are also made in natural molds, which are the only fossils found in some rock layers. This is particularly true of dolomites. Sometimes a calcified fossil is found in an extremely hard sandstone, slate, or shale that cannot be chipped away without damaging the softer fossil. In such cases, the fossil is dissolved with acid, and the mold cavity filled with rubber. This yields a perfect cast of the fossil when the hard matrix is broken away.

A century ago, casts were complicated things to make. One method was to coat the specimen with shellac to seal the pores and then press it into wax or paraffin. By another method the fossil was lightly coated with oil and covered with plaster. The mold was removed from the fossil, and plaster casts were made from it. Often only one could be made. Fossils that had projections or overhangs had to be cast from elaborate split molds, sometimes half-a-dozen pieces for each specimen. Such molds were not only difficult to make but difficult to cast.

Cast And Mold Fossils

Relatively smooth fossils with no undercuts can be cast using plaster molds. The fossil is coated with oil and half buried in plaster (left mold). When thoroughly dry, the plaster mold is treated with a release agent such as petroleum jelly, and the rest of the fossil is covered with plaster. When dry, the two halves of the mold will separate easily from the fossil and from each other. A filling hole must be cut into the mold at the highest point of the cavity. The mold must be treated with a release agent before each casting. This blastoid was cast in plaster in this mold.

Relatively smooth fossils with no undercuts can be cast using plaster molds. The fossil is coated with oil and half buried in plaster (left mold). When thoroughly dry, the plaster mold is treated with a release agent such as petroleum jelly, and the rest of the fossil is covered with plaster. When dry, the two halves of the mold will separate easily from the fossil and from each other. A filling hole must be cut into the mold at the highest point of the cavity. The mold must be treated with a release agent before each casting. This blastoid was cast in plaster in this mold.

Casts made well over a century ago by James Hall turned up when his collection was unwrapped from ancient newspapers at the Field Museum of Natural History in Chicago, which acquired the collection. The casts were made of sulfur and of lead.

The amateur can make simple casts of fossils such as flat trilobites, small ammonites, flat snails, brachiopods, and other fossils that do not have exceptionally intricate detail or overhangs. This is easily done by coating them lightly with oil (which is not well adapted to absorbent or porous matrices) and pouring melted wax over them. Papier-mache works well to cast fossils that are without fine detail, such as bones or shark teeth. Modeling clay will hold an impression of any fossil sturdy enough to be pushed into it. These molds can be cast in plaster but are usually good for only one cast.

Multiple castings from all except very fragile fossils can be made with rubber molds, the contribution of this century to casting. Hobby- and art-supply shop sell liquid latex in bottles. A pint bottle makes many molds. The fossil is painted with several layers of this material, and then the

John Harris of the Field Museum shows use of rubber molds in casting large fossils. He is touching the lower jaw; the plaster cast, a faithful reproduction, is in front of it. This jaw was cast with only two molds (at right side of picture). Such large rubber molds are good for making only about a dozen casts.

John Harris of the Field Museum shows use of rubber molds in casting large fossils. He is touching the lower jaw; the plaster cast, a faithful reproduction, is in front of it. This jaw was cast with only two molds (at right side of picture). Such large rubber molds are good for making only about a dozen casts.

rubber mold is pulled from the fossil. Since rubber stretches, it can be used on fossils with some overhangs, or even on rolled trilobites or other three-dimensional fossils. If it can be peeled off the fossil without stretching out of shape or splitting the rubber, it can be used as a mold.

The rubber mold is filled with casting plaster, which is allowed to dry and is then removed. Rubber molds can be used dozens of times before they split. Even split molds can be repaired with liquid latex. Casting plastic (polyester resin) can be cast in these rubber molds, but it must be removed carefully, as the surface will be sticky no matter how long it cures. It can be hardened by gentle heating in an oven or electric fry pan.

The liquid plastic can be tinted with special dyes mixed in along with the catalyst. The surface of plastic casts may be shiny and may need to be dulled by a rubbing with fine grit. Plaster casts can be painted with water-colors to resemble the natural color of the fossil.

Rubber is the best material for making casts from natural fossil molds in the rocks. The rubber cast can be painted, or it can be used to make

Mold of a snail in dolomite. A release powder is dusted into the mold and casting rubber is poured into it. The rubber cast is much easier to study than the fossil mold itself.

another mold that can be cast in plaster or plastic. Unless the collector leaves some sort of handle protruding from the rubber when he pours it into a natural fossil mold, he will wind up with a hole full of rubber and no way to pull out the cast.

Molds have been made of silicone rubber and of polyvinyl chloride, a type of plastic. But the amateur will not have use for them. Molds designed for making lead or other hot-metal casts require special techniques.

If a simple natural mold is seen in a rock, a field cast can be made by coating the mold lightly with thin oil, pressing modeling clay into the cavity, and gently pulling it out. Even though distorted, the mold will pick up fine details and disclose what once occupied the hole.


The use of X rays is a recent development that has become the standard method of identifying mineral specimens. The use of X rays in identifying fossils is less well known. It is beyond the capacities of most amateurs except persons such as physicians who may have access to such machines; but the technique should be understood by all collectors so that they can avoid destroying a rare fossil by attempting to prepare it by other methods, when it would have been a perfect candidate for an X-ray picture.

X rays reveal fossils locked deep within hard matrix just as they show bones in a human body. Under the right conditions, a fine X-ray picture can be made of a fossil that is not visible at the surface. Preparation might have been impossible because of the nature of the matrix or so difficult that there would have been much damage.

X rays produce a picture because there is a difference in the absorption of the rays by bone and flesh or fossil and matrix as the rays pass through the object to the film. Absorption of rays increases to the fourth power of the atomic number of the atoms that compose the fossil and matrix. Thus, if a pyritized fossil is embedded in a carbonaceous shale, a fine X-ray picture can be made. Pyrite is an iron sulfide, and iron has an atomic number of 26. Carbon has an atomic number of 6. This difference allows strong contrast because of the much greater absorption of the X rays by the pyrite.

On the other hand, a pyritized fossil inside an ironstone concretion will make a very poor X ray. Both fossil and matrix contain iron. Limestones are hard to X-ray unless very thin because they absorb all X rays. A calcite fossil would not show up; it is chemically the same as limestone. Carbonaceous shales with pyritized fossils are excellent subjects. This is fortunate, because they are the most difficult fossils to prepare mechanically.

Dr. Eugene Richardson and Dr. Rainer Zangerl of the Field Museum in Chicago did a massive research project on a fossil occurrence in central Indiana — a mass burial ground for Pennsylvanian fish. The fossils required slow, painstaking preparation. There were thousands of specimens. After much experimentation and the addition of an electronic dodging machine to compensate for the wild contrasts in the negatives, superb X-ray pictures of the fish were produced. The pictures were more useful than even the best-prepared specimens. This technique is described in Handbook of Paleontological Techniques (see Appendix: Recommended Books.)

If an amateur finds black, sheety shales having suspicious swellings and some exposed scales or bones in them, which he is unable to prepare, he may find they are of great interest to a well-equipped museum.

There will probably be other developments in this area. Research is being done with neutron radiography, which can be used to produce pictures similar to X rays. A neutron radiograph of an object containing a variety of substances, such as wood, plastic, and several different metals,

Dr. Rainer Zangerl of the Field Museum of Natural History preparing fossil shark in hard shale. Broken fossil was reassembled in a frame and prepared with small scrapers and flaking tools. Pennsylvanian; Mecca, Indiana. (Photo Field Museum of Natural History)
Fossil fish apparent only as a swelling on surface of the slab but disclosed in X-ray photograph in perfect detail. Pennsylvanian; Mecca, Indiana. (Photo by Eugene Richardson)

would show each hidden object distinctly in a different shade of gray. A standard X ray would not differentiate as well and would ignore some of the objects.

While this has not yet been applied experimentally to fossils, it undoubtedly would give much better results than X rays with fossils and matrices. Its drawbacks are the high cost of the equipment, which is now limited to nuclear laboratories, and the danger to users and passersby from radiation.


While examining a collection of the Jurassic fossils from Solnhofen, Germany, a researcher was startled to see a beautifully detailed insect fluo-rescing brilliantly on one slab. When he turned on the room lights, the insect could not be seen on the piece of limestone. Other disappearing insects were found on other slabs. The conclusion was that the insect-shapes were indeed fossils, but fossils whose substance had decomposed before the rock hardened. No swellings or visible fossils remained, but organic fluorescent substances once contained in the insects were left behind, creating ghostly images of their bodies. Visible fossils were found to show details not visible before —for instance, shrimp showed antennae and legs under the ultraviolet light that could not be seen under the most careful scrutiny in daylight.

This was an exceptional case of fossil fluorescence — the insects that weren't there. Some fossils fluoresce, but none as brilliantly as minerals do; and the minerals that replace the organic material of fossilized organisms are not fluorescent at all or only feebly so. Consequently, the cause of the fluorescence of some fossils is a challenge to science.

It has been suggested that, as with the Solnhofen fossils, the fluorescing material is a substance remaining from the organism. A few snail shells display fluorescent bands and stripes, perhaps the remains of long-vanished color patterns.

Just as crabmeat canners separate the fluorescent crab shell from the nonfluorescing meat by ultraviolet light, so the collector occasionally may use the same technique to distinguish small fluorescent fossils from the matrix. In the same way, he may find that fossils from one locality closely resemble those from another, but those from one locality are fluorescent and the others are not. Here he has a ready means of separating them.

One further use is akin to the use of ultraviolet light to detect alterations and forgeries in art works, rare stamps, etc. Most glues and cements used in patching fossils are fluorescent, so that the lamp may be a means of detecting even the cleverest repair work.

The ultraviolet lamps, both shortwave and long, that are used for minerals are satisfactory for fossils. They come in a wide range of prices and sizes, varying degrees of portability and powers.

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  • Cornelia
    What is a mold and cast fossil?
    8 years ago

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