Air abrasion basics

An air abrasive unit (Paasche, Comco, Swam-Blaster and others) fires a powder in a narrow jet of compressed air through a nozzle of 0.4–1.2 mm. The powder does the cutting; the operator controls pressure, distance, angle and dwell. It is the tool of choice for cleaning shale, chalk and soft limestone from ammonites, trilobites, fish and insects, and for the final tidy after mechanical prep.

Powder choice sets the aggressiveness. Sodium bicarbonate (about Mohs 2.5) removes soft matrix and stops at shell and bone; dolomite (Mohs 3.5–4) handles most limestones; aluminium oxide (Mohs 9) cuts anything, including the fossil, and belongs only on hard matrix with a preparator who has practised. Finer grades (under 50 µm) polish; coarser grades (100–250 µm) cut. Start soft and small.

Pressure of 20–40 psi is typical for delicate work; industrial 80 psi settings are for removing bulk. Keep the nozzle moving, work at a low angle to the fossil surface so the jet skims matrix off rather than drilling in, and check under magnification often: the moment the surface texture changes from matrix to fossil, stop.

Dust is the real hazard. Silica-bearing matrix and aluminium oxide both produce respirable dust; use a sealed blast cabinet with a dust extractor, and a fitted P3/N100 respirator for anything outside the cabinet. Bicarbonate is not dangerous to breathe but still needs extraction to see through.

Air abrasion leaves a characteristic matt, slightly frosted surface, and can round fine ornament. Buyers sometimes read that surface as “over-prepped”; the disclosure that matters is not the tool but whether original surface was removed or restored.