Acid preparation: acetic and formic

Acid preparation works when the fossil and the matrix are different chemistries. Calcium carbonate matrix (limestone, chalk, calcareous concretions) dissolves in dilute acetic or formic acid; calcium phosphate (bone, teeth, conodonts, some brachiopods) and silica (chert, silicified shells) do not. It is useless — destructive — on ammonites, trilobites and most molluscs, which are carbonate themselves.

Acetic acid at 5–10 % (buffered with calcium acetate or tricalcium phosphate to protect the bone) is the standard; formic acid at 5–10 % works faster and is preferred for large blocks and harder limestones, but is more aggressive and needs more care. Both are run as cycles: immerse for hours to a day, rinse in running water for as long again, dry, consolidate the newly exposed bone with dilute Paraloid B-72, then repeat. A block can take months.

Consolidation between cycles is not optional. Freshly exposed bone is unsupported and will fracture; a thin B-72 coat also masks it from the next bath. Keep an eye on the acid: a spent bath fizzes less and drops in pH more slowly.

Safety: glacial acetic acid and concentrated formic acid burn skin and eyes and their vapour is harmful; dilute outdoors or under extraction, add acid to water, wear nitrile gloves and goggles, and store diluted acid in labelled polyethylene, never glass with a tight stopper (pressure). The rinse water is dilute calcium acetate and can go down the drain.

Acid prep is how most of the fine Cretaceous and Jurassic marine reptile and fish material, the Eocene mammals of the London Clay and the entire micro-vertebrate discipline are produced. Its trace is a fossil with no tool marks and a very natural surface; the disclosure it needs is the consolidant record.