Cleaning shells: what bleach does and does not do

Modern shells collected live carry animal tissue and periostracum (the brown organic skin). Collectors soak them in a 1:1 dilution of household bleach (sodium hypochlorite) for hours to a day, which dissolves the tissue and lifts the periostracum. That is what bleach is for, and it should be the only thing it is used for.

Bleach does not clean mineral deposits, algal stain or dirt in sculpture; it does damage nacre and colour with long exposure, and it soaks into the shell's porous structure. Any residue left behind is a salt that will crystallise out later as a white bloom that looks like Byne's disease. Rinse for as long as you soaked, in several changes of fresh water, then dry slowly out of the sun.

Dirt and algae come off with a soft brush and water, a dental pick for sculpture, and patience. Calcareous crust (coralline algae, barnacle bases) can be thinned with a fine dental tool; dealers use dilute muriatic (hydrochloric) acid dips of a few seconds, which also eats the shell — disclose it if you do it. Ultrasonic cleaners work on robust shells and shatter thin ones.

Fossil shells and shells with original material (Pleistocene and many Cenozoic molluscs) get water, brushes and picks only; no bleach, no acid. Their surfaces are already fragile and the original colour patterns some carry are irreplaceable.

Oil, glycerine and “shell shine” products give a wet look that fades unevenly and traps dust; museums do not use them. If a shell is to be polished or coated for décor, say so on the record — polished nautilus and “pearlised” abalone are treated specimens under every disclosure code.