There is a temptation to trim off as much matrix as possible in the field to save weight. Especially with limestone slabs, this can result in breaking the fossil in half where the fossil creates a weak spot in the slab. It is safer to take the heavy slab home and trim and saw it to size there.
If the block enclosing the fossil is too large to transport, a channel should be cut around the fossils with a small cold chisel and hammer. The chisel should be held so that it points away from the fossil, and the channel should be a reasonable distance away from the fossil. After much labor, the fossil will be atop a pedestal, which can then be broken from the block with a single blow from the hammer against the chisel placed at the base of the pedestal. The deeper the channel and the higher the pedestal, the better the chance of removing the fossil in one piece.
If the block, however unwieldy, can be hauled home, the chances of removing the fossil safely are much better. Limestones, hard shales, slates, and sandstones can be sawed to convenient sizes and shapes with a diamond saw blade such as is used in lapidary work. Small slabs can be cut
quickly into square pieces, and individual fossils can be separated with little or no damage with a small trim saw using a six-inch to ten-inch blade. Such trim saws can be purchased new for $25 to $50, not including blade and motor. A six-inch blade costs less than $15 and a ten-inch blade about $25. If used only for sawing sedimentary rock, the blade will last the fossil collector many years. The saw coolant should be either a water-soluble oil or one of the materials marketed to be added to tap water. Water alone will rust the blade.
For lapidary work, in which rocks much harder than those usually encountered in fossil work must be sawed, a light cutting oil or kerosene is used as a coolant. If sawing must be done on a saw that uses oil as a coolant, the specimen should be soaked in water for several hours prior
Removing a fossil from a block of limestone. Step 1: First chisel a groove about an inch from the specimen. Tilt the chisel away from the specimen to avoid chipping it.
Step 2: After the groove is 3/4 inch deep (or more if the fossil is large) place the chisel at the bottom of the groove and angle it sharply toward the fossil. A sharp blow with a hammer on the chisel should then break loose the fossil atop its pedestal.
Step 3: The result. The fossil can be trimmed further at home if desired.
to sawing. After sawing it should be plunged into a strong detergent solution. Soaking will prevent too much oil from penetrating porous limestones or shales. If matrix will not disintegrate, the specimen should soak for several hours or days in the detergent.
Specimens too large for a trim saw may require commercial sawing, a service usually available at a rock shop at a cost of about ten cents a square inch.
Thin slabs of shale or limestone can be sawed with a silicon carbide cutoff disc on an electric drill or flexible shaft machine. These cutoff discs are not expensive; but they create annoying dust, and they are fragile. A steady hand is needed to keep them from breaking.
Cement and tile contractors use a special saw for trimming tile or marble slabs or for sawing concrete. This outfit uses a much thicker blade than a lapidary saw, usually with a water coolant. The saw blade is overhead and is lowered onto the work by a foot-operated lever. The blade is usually a diamond blade, but it may be silicon carbide, which will easily cut limestones or shales. Such a saw is excellent for trimming large pieces.
Carbide hacksaw blades and diamond-impregnated wire blades that fit in a coping-saw frame are also useful for sawing matrix.
If no saw is available, the waste material can be removed carefully with
hammer and chisel. A small channel should be chiseled across the matrix where it should break. The slab can then be placed on a flat block, a curb, a concrete step, or some other hard surface with a square edge. The part to be broken away should hang over with the chiseled line facing up and at the edge of the support block. While the piece is held firmly, the protruding end should be given a sharp blow with a heavy hammer. If all goes well, the piece will snap cleanly along the line.
Small edges can be chipped off, using the chisel tilted slightly away from the fossil. With solid, fine-grained limestone that breaks with a conchoidal (shell-like) fracture, the edge can be chipped away quite rapidly with sharp blows from a small hammer. If a piece is thin, the edge can be nibbled away with pliers. Small bites, not more than a fraction of an inch, should be taken, then the pliers should be tightened and snapped down or up sharply.
If only a small amount of material is to be removed, it can be ground
away on lapidary-type silicon carbide grinding wheels. Small high-speed wheels used for sharpening tools are not well suited for grinding fossil specimens. Pieces weighing more than a few ounces should never be ground on a wheel, as they will quickly create a bumpy wheel surface or even cause the wheel to shatter, shooting out pieces of sharp grinding wheel at the speed of a bullet. Grinding should be done on an individual fossil only to remove unsightly matrix or to flatten a side or back so that the specimen will sit properly when displayed.
Large pieces can be broken cleanly with a hydraulic press using a hardened steel or carbide point. Grooving the piece with a chisel will help to make a long, clean break, or the specimen can be trimmed more safely by biting off small corners, one at a time. Such hydraulic units are expensive.
Some soft shales cannot be sawed because the water or oil coolant will disintegrate the shale. Such specimens can be trimmed by nipping with pliers, breaking with a hydraulic press, or sawing dry with an old hacksaw blade. The hacksaw blade will wear quickly, but as long as some teeth remain it will continue to cut.
When the fossil is a convenient size to handle, the delicate task of removing the remaining matrix can be begun.
Trimming slabs of stone by nibbling at the edges with pliers. FINISHING TOUCHES WITH HAND TOOLS
Fossils that are harder than their matrices are the easiest to clean and luckily the most common. Pyritized, calcified, or silicified fossils in weathered limestone or shales soft enough to be scratched with a fingernail fall into this class. So do loose fossils weathered from shales or chalky, impure limestones.
A vigorous brushing of adhering matrix with an old toothbrush and water will show whether the matrix can be removed this way. Such brushing will not damage the surface of any sturdy fossil (brachiopods, most trilobites, corals, blastoids, crinoid cups, etc.), but should be done with care on lacy bryozoans, graptolites, crinoid crowns, and the like. If the matrix is not too thick, a surprising amount of cleaning can be done in a short time. This is the only way to clean out the fine furrows on the surface of a brachiopod. Soaking a specimen for several days in water may help loosen matrix.
If the fossil is considerably harder than the matrix, the brushing can be done with a fine-bristled brass brush, such as a suede brush. On pyritized, silicified, and even some calcified specimens, brass brushing will remove matrix more rapidly than a toothbrush and will remove some hard shales untouched by a nylon brush. Pyritized Devonian starfish and crinoids from Bundenbach, Germany, can be cleaned this way, as the pyritized fossils are encased in a tough shale. Devonian trilobites from western New York and Ohio (generally black or dark-colored and preserved in a gray shale) can be quickly cleaned by gentle use of a brass brush. Pyritized brachiopods from Ohio respond equally well.
It is easy to tell whether the fossil is harder than the brass brush and therefore is not likely to be damaged. Brush a specimen, and if the fossil quickly picks up a brassy shine it is harder than the brush. If the matrix wears away and does not turn a brass color, it is softer. The brassy color can be removed from the fossil by scrubbing with a nylon toothbrush and a detergent.
Fine steel-wire brushes can be used, but only on silicified or pyritized specimens. Try a sample first to see whether the brush wears away the fossil. Both steel and brass brushes are made for use on a flexible-shaft machine.
Most specimens will have some areas of thick matrix that can be removed more rapidly with metal scrapers. Small-bladed pocketknives make fine scrapers for large areas. Screwdrivers can be sharpened to various shapes for scraping or prying loose larger pieces of matrix. For delicate work, fine, needlelike scrapers should be made. Discarded dental tools fit the hand and are usually available from your dentist. Crochet needles work well if the hooked point is ground away. A handle can be made for them from a piece of wooden dowel drilled to accept the shaft. If one can still be found, a steel phonograph needle is a particularly tough, fine point for delicate work. Ordinary needles, mounted in a wooden dowel, work well. Large needles used for sewing carpets can be used for large-scale work. Points must be kept sharp, as the matrix quickly dulls them. A dull tool may slip and damage the surface of the fossil.
The scraping action should be much like cleaning fingernails, with just enough pressure to move the matrix but not enough to bite into the fossil.
Brush and wash frequently. Don't try to scrape off that last thousandth of an inch. With practice, little damage will be done to the fossil.
A trickier technique is flaking away adhering matrix. This works particularly well on fossils that are smooth and shiny and do not have many indentations. It also work well on some shale-encased fossils, such as the Wyoming fish or the plant fossils of the Mazon Creek area in Illinois. The trick here is to start a few millimeters away from some exposed part of the fossil, dig the pointed scraper into the matrix, and with a sharp push and twist flake away a piece of the matrix. It should part cleanly from the fossil. This can also be done using the scraper and a tiny hammer, angling the tool toward the exposed fossil at about 45 degrees. A single blow should dislodge the flake. Never try to slide the scraper between the matrix and the exposed fossil. The matrix will flake away, but a splendid scratch will remain on the fossil. This flaking technique can be done rapidly in a routine of dig in, flick upwards, and blow away the chip.
Flaking works best when the matrix is thin. This may require a bit of preliminary grinding, a rather dangerous adventure that can also destroy parts of the fossil that protrude unexpectedly.
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