The Geological Survey of Alabama has several fossil shells that were
probably coated with shellac in the 1930's or 1940's. By 1990, the shellac
had yellowed, darkened, cracked, and peeled, ruining most of the fragile
specimens. Because some of the fossils were collected at sites that are now
permanently underwater, the loss cannot be repaired by collecting
additional specimens.

I do not recommend the use of shellac for preserving shells.

Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA