Bill Clendenin asks how best to clean and preserve small, very fragile
shells in clay. The answer depends on how fragile they are, and whether the
clay will swell when moistened (which will tend to break up the shells).
Clay can be crumbly or stiff. General guidelines:
 
1. Proceed cautiously. Whatever you decide to do, test it on the least
desirable shells first. Test it on a small part before trying the whole
piece.
 
2. If shells are relatively robust, they can be entirely freed from the
clay. Some clay can be removed by immersing the whole piece in water; some
clay is better removed my moistening a small part at a time; some clay is
best removed dry, by gently scraping it with a dental pick.
 
3. If shells are so fragile that removing the clay breaks them, then only
one side of the shell should be cleaned, with enough clay left as a backing
for the shell. Excess stiff clay can often be cut or scraped away with a
small knife, so the specimen can be squared or rounded off neatly. Leave
enough sediment so you can handle the specimen without touching the shell.
 
If you can clean only one side of the shells, then try to clean different
sides of specimens so you end up being able to see both sides of the same
species. Generally it is easier to clean the exterior than the interior,
because the clay tends to break that way; but identification is much easier
if you can see the interior as well.
 
Charles Sturm's advice on stiffening or gluing the shells after cleaning is
excellent.
 
Don't try to bleach fossil shells. Most people prefer as little as possible
to be done to fossil material. Do record any chemicals that you have used
on the label, in case they need to be removed later.
 
Andrew K. Rindsberg
Geological Survey of Alabama