It darkens over time. It attracts and incorporates dust at elevated room temperatures. Peeling, shrinking, flaking, spalling. To expose obscured features for research purposes. To improve the aesthetic appearance of a displayed specimen.
Techniques used in removing shellac depend a great deal on the nature of the specimen.
Larger, more robust material is somewhat easier to deal with safely. Cover with a piece of polyethylene sheeting. Let this remain in place for about 5 minutes, then remove and examine the surface coating. Quite often, the shellac surface will bubble up and slide off very nicely, especially on smooth, non-porous bone. Repeat the procedure as needed to soften and remove all the shellac on the surface.