Ultrasonic Cleaners

Weathered matrix and clay often remain firmly wedged in fine openings in fossils, particularly such pore-bearing fossils as bryozoans and corals and around the compound eyes of trilobites. All the brushing in the world won't remove this material. But just a few minutes in an ultrasonic cleaner will remove this debris, and even some that appears firmly attached. Sand and shale that are not too firmly cemented can be removed neatly by placing the specimen in an ultrasonic cleaner tank for fifteen minutes or so. These cleaners are also used to clean delicate or intricate parts, such as the works of watches. The sonic bombardment loosens dust or dirt.

The ultrasonic cleaner consists of a tank to hold the solvent in which an object is suspended and a source of inaudible, extremely high-pitched sound waves. Bombardment by these sound waves of the liquid in the tank produces cavitation, which is rapid formation of vapor pockets and their collapse. This causes a violent, small-scale, hammering action on an object suspended in the liquid. Anything not firmly attached is torn loose.

There are two basic types on the market: a high-frequency, high-energy unit that is heavy, expensive, and rather dangerous to operate, and a low-frequency unit that is portable, not so expensive, and safe to use. The low-frequency units seem to do a satisfactory cleaning job, and the magnetostrictive transducers that produce the ultrasonics will not overheat and can be used for a long time. Small units with a container large enough to handle a specimen several inches in size are now on the market for around $60. The price increases rapidly for larger units.

While most commercial units distribute the ultrasonics throughout the tank, one has been built with the vibrations coming from the end of a rod, which can be aimed at certain parts of a submerged fossil specimen to give particularly vigorous cleaning action in a localized area. Such a directional ultrasonic cleaner was designed by English paleontologists and displayed in England at a meeting in 1967. This unit did a superlative job of cleaning sandy matrix from fossil bones. A tank-type ultrasonic cleaner used long enough to disintegrate the sandstone would very probably disintegrate the bone as well, but a directional unit could be aimed at the matrix without undue damage to the bones. Such directional probe type ultrasonic cleaners are now being marketed in the United States.

Major manufacturers have announced that they will be producing an ultrasonic cleaner for use in the kitchen sink. The vibrations that will tear loose congealed steak grease may find application in the fossil-cleaning field as well. A cleaner such as this would loosen all but the most stubborn matrix — affording a quick, easy way of preparing fossils for the final detailed work by hand or with other machines.

A typical cleaner, made by the Bendix Company, has a power output of 180 watts and produces sound from 19,000 to 22,000 cycles a second. The cleaning receptacle has a capacity of I4 quarts, more than sufficient for most fossils.

In practice, the specimen is placed in a beaker which is then placed in the tank. Water with detergent or wetting solution is an excellent liquid for cleaning the fossils. With delicate microfossils, immersion time is in the range of one to two minutes. With typical bryozoans, brachiopods, corals, and echinoids, the time may be increased to ten or fifteen minutes. By then most of the material that will come loose has done so. Further treatment may break apart the fossil.

Generally, little damage will be done to the fossil, though long treatment will create some surface abrasion. Delicate appendages, such as brachiopod spines, may be broken off, especially if the power source is strong.

Specimens being treated with acid, or limestone blocks being dissolved to free enclosed silicified fossils, can be placed in the machine in beakers of acid. Short periodic bursts of ultrasonic vibrations will materially speed up the solvent action of the acid.

The ultrasonic cleaner cleans mineral specimens as well, particularly dirty geodes or sturdy crystal groups with clay trapped in crevices. A short blast of the machine will clean embedded grit from pieces of tumbled agate and will remove polishing powder from cavities in slabs or cabochons.

Never add grit to an ultrasonic cleaning solution in expectation of faster cleaning; it will quickly destroy the specimen.

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