I agree with Aydin on the subject of denatured alcohol, but I'll take
it a step further.  The only substance on his list that you are will
find in pharmacy-purchased denatured ethanol is isopropanol.  The
only purpose of denaturation is to make the ethanol non-drinkable
(yes, ethanol, or grain alcohol, is the stuff that gives beer, wine
and liquor its kick).  Any of the substances Aydin listed would
certainly accomplish that purpose, but many of them are toxic not
only if taken internally, but also if applied topically.  (Benzene
for example is a known carcinogen, which is readily absorbed through
the skin).  Therefore, these substances will not be present in
alcohol intended for any medicinal purpose, internal or external.  As
Aydin said, nothing in denatured alcohol will harm shells.
What can be sold in liquor stores is state-regulated.  In some states
you can buy 190 proof (95%) ethanol, or even dehydrated 200 proof
(100%) ethanol, in a liquor store, but it's a mighty expensive way to
dehydrate shells.
 
Paul M.