Acid maceration is a technique to extract organic microfossils from a surrounding rock matrix using acid.
Hydrochloric acid or acetic acid may be used to extract phosphatic fossils, such as the small shelly fossils, from a carbonate matrix.
Hydrofluoric acid is also used in acid macerations to extract organic fossils from silicate rocks. Fossiliferous rock may be immersed directly into the acid, or a cellulose nitrate film may be applied (dissolved in amyl acetate), which adheres to the organic component and allows the rock to be dissolved around it.
References
Acid maceration Wikipedia(Text) CC BY-SA