by Marc Behrendt, reprinted from Fossil News
Fossil preparation or cleaning can be done in the home or in a fancy lab. Sometimes all it takes is a toothbrush and water. If the fossil is sturdy, like coral and many brachiopods, this method will remove all the mud and loose matrix. However, if your fossil has any cracks in it, if it is fragile, or if it sits on soft shale, do not try to brush it off with water. Fossils like trilobites, bones and fragile brachiopods will dissolve or fragment. To make something fragile look better, you need a different method to clean your specimen.
Let's assume you have a complete trilobite and would like it cleaned. Part of it is buried in the rock and part is exposed but covered with a thin layer of shale.
To expose the buried portion of the trilobite, miniature pneumatic hammers will be used. Ever so carefully the hammer's pounding chips away tiny fragments of the matrix hiding the trilobite. Usually the matrix touching the trilobite shell pops right off after most of the upper matrix is removed. Great care is taken not to touch the trilobite with the hammer.
Before micro-airhammers were used, fossils were exposed using small steel picks like dentists use (this process is still used in many labs today). The method works very well, as you can see in any older museum collection, but it takes a long, long time to accomplish what an airhammer can do in a short time!
OK, the trilobite has been totally exposed, but is still covered by a thin layer of shale. It is time to pull out the microsandblaster. These instruments, using high air pressure, shoot a tiny regulated amount of powder through a hose and nozzle onto the fossil, eroding the soft rock away while leaving the harder trilobite's shell intact.
With experience or careful experimentation, the air pressure and powder flow are adjusted to remove the matrix from the fossil without "burning" the trilobite's shell away with the rock. All the work is done under a microscope under the watchful eye of the preparator, who is alert for new or previously unnoticed cracks in the shell that will need to be stabilized.
The eyes need special attention. Many kinds of trilobites have the lenses still in the eyes, and these are very fragile. With delicate and precise microsandblasting, the entire eye is cleaned so each lens is perfectly exposed without being damaged!
Finally, the rock itself is spruced up. All the chisel marks from the hammers are ground away using either a combination of airhammer and air abrasive, or with a grinder like a dremel tool. The matrix is shaped into the way it best displays the trilobite. Occasionally, new fossils are discovered under the matrix during this step. These are cleaned up and make nice surprise additions for the piece.
When it's all done, sit back and admire your trilobite. It's no longer grey and covered with rock -- it is a beautiful black or brown color, looking like it will crawl off the rock any moment.