Marlo,
 
I am not convinced that denatured ethanol sold in drug stores contains
anything that "can also eat up the calcium carbonate in the shell". According
to the Merck Index, the most common denaturants, either alone or in
combination, added to ethanol are: methanol, camphor, amyl alcohol, gasoline,
isopropanol, terpineol, benzene, castor oil, acetone, nicotine, aniline dyes,
ether, cadmium iodide, pyridine bases, sulfuric acid, kerosene, diethyl
phthalate. The only thing on this list that would dissolve CaCO3 is sulfuric
acid, but you wouldn't find that in ethanol sold in drug stores.
 
In any case, I am not sure where someone who does not work in a research lab
could get undenatured ethanol (95%) from. Do they sell it in liquor stores?
 
A.