Several types of vibrating-point engravers are available; the one most commonly used is the Vibro-Tool. All are priced around $10.
The vibrator in such a tool causes a sharpened point to strike tiny blows many times a second, rather like a miniature jackhammer. An adjusting-screw controls the length of the stroke. The best model for fossil work has a chuck that will hold different points.
The steel points usually provided with vibrating tools are too large and too soft for hard stone, but they are serviceable, particularly if kept sharp and not used too often. For fine work a sharpened dental burr or a phonograph needle is excellent, but since these points will not fit into most chucks, an adaptor must be made of a short piece of brass rod that does fit the chuck. The brass rod should be bored lengthwise for half an inch to accept the phonograph needle or dental burr. Another hole should be drilled at right angles to the first one and tapped to accept a small setscrew, which will hold the small point tightly in the hole. The unit can then be placed in the chuck of the tool and tightened securely.
As the tool hammers away it kicks up a cloud of dust, hiding the work and fouling the air. The dust can be blown or whisked away, which causes constant starting and stopping of the tool, or the work can be done outdoors in a strong wind. A small air hose fastened to the tool and connected to a strong pair of lungs or an air compressor will blow away the dust as it forms. A section of thin plastic tubing can be taped to the side of the tool with its opening near the working end, so that the operator can blow through the other end of the hose.
An inexpensive source of compressed air can be obtained from a small aquarium aerator. A more sturdy arrangement can be made with copper or aluminum tubing carrying the air across the Vibro-Tool from the plastic air hose at the top of the tool. A compressed-air blower system based on the aquarium aerator can also be attached to a stand and aimed at work being done with hand tools.
The action of the vibrating tool can be compared to that of a tiny hammer and chisel. The rapid hammering of the pointed tip of the tool flakes away the matrix. The best action seems to occur when the tool is held almost at right angles to the specimen. A short stroke is used for fine work; the long stroke bangs off larger pieces of matrix and is good for removing large areas of unwanted stone where there is no danger of breaking through into the fossil. As the fossil surface comes near, only fine vibration and short strokes should be used. Usually small pieces will break free at the fossil-matrix interface shortly before the point digs into the fossil surface.
Hours of patient work and a thorough knowledge of the morphology of the underlying fossil are necessary before you will know when to stop. If the fossil is likely to be distorted or bent or to bear unsuspected projections, work should be done by hand, at least until the general topography is revealed. The vibrating tool is too fast and undiscriminating for blind work.
Allowing the tool to work straight down in one place usually will loosen a chip of matrix in a manner similar to the flaking method used with hand tools. If the tool is moved back and forth smoothly the action is rather like that of a hand scraper. Different matrices require different techniques; each specimen is different.
The tool will become uncomfortably warm if used for a long time. If the vibrations carried through the stone reverbrate like a drum from the table below, an effective damper can be made from a small cloth bag filled with sand. The work is laid on the sandbag. The sandbag molds itself to the contours of the matrix and helps hold the specimen while it is being prepared. Sandbags are useful for any type of preparation.
The Vibro-Tool is sold by Burgess Vibrocraf ters, Inc., Grayslake, Illinois 60030. Other vibrating engravers are available at most craft and hobby stores.
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