The sandblasting process, used to clean buildings and to etch tombstones, can also be used to clean fossils. A few years ago a small sandblaster that allowed localized and precise cutting was put on the market by Pennwalt-S.S. White Dental Products Division, 3 Parkway, Philadelphia, Pennsylvania 19102. It was designed to replace the dentists' drill. The idea was excellent, but the machine proved impractical because the abrasive clogged the fine orifice. Similar units are now available from the same company for etching metals, glass, and plastic and are easily adapted for use in cleaning fossils.
Almost every major university and museum in the United States now has one of these machines, and some private collectors have them, too. They are the ultimate in fossil cleaners. The abrasive stream can be directed so that layers of fossils are revealed without removing them from the matrix block. On crinoid slabs from Le Grand, Iowa, several feathery crinoids may lie atop one another. Old preparation methods could prepare the top layer well and perhaps part of the underlying one, but too much work on the bottom layers would break away parts of the top. With the gentle action of the sandblaster, all parts can be undercut and cleaned perfectly in areas where the tiniest brush couldn't reach.
Unfortunately, there are some drawbacks: a sandblaster costs several hundred dollars, the abrasives are somewhat expensive, and the machine is rather delicate. Dried compressed air or carbon dioxide must be used
because the machine will clog if there is any humidity in the air. The type of fossil and matrix must be well understood —use of the wrong abrasive will destroy the fossil.
Basically, the machine consists of a vibrating hopper filled with fine abrasive which is fed into a stream of rapidly moving gas. The abrasive particles shoot out of a tungsten-carbide nozzle, bounce off the specimen, and wear away the material. The actual speed of the particles approaches 1,100 feet a second —about the speed of sound —and at this speed the abrasive will cut rapidly even though it is of the same hardness and composition as the matrix being removed.
The speed of cutting is controlled by the amount of abrasive fed into the stream of gas. The nozzle comes with a round hole about 1/50 inch in diameter for general work or with a narrow rectangular hole for special work such as precision cutting. The hand piece is jointed so that the nozzle can be aimed in any direction for working in difficult areas.
Coarse removal of matrix is done with aluminum oxide of about 50-micron diameter, and finer work with a finer size of the same material.
Powdered dolomite (calcium magnesium carbonate) is most commonly used for general work when not too much matrix must be removed from calcified fossils. For exceedingly delicate specimens, such as paper-thin shells, trilobites, and teeth, powdered sodium bicarbonate is used.
Work is done in an airtight box fitted with arm holes that have tight rubber sleeves. The top has a window of glass made to be replaced easily as it will become frosted through use of the machine.
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