Salt damage

Distinct from efflorescence, which is a reaction with the air. This is salt that was already in the specimen moving to the surface.

Soluble salts inside the stone

Porous matrix can hold soluble salts. Humidity cycling dissolves them, draws them to the surface as water migrates, and deposits them there as it evaporates.

The crystallization happens inside pore spaces as well as on the surface, and the pressure that generates is what spalls flakes off a specimen.

Where they come from

The occurrence itself: material from a saline environment, a playa, an evaporite or a coastal site.

Mine water, which is often heavily mineralized.

Seawater on anything beach-collected.

And cleaning: tap water in a hard-water area leaves its own dissolved solids behind, which is why distilled or deionized water is used for the final rinse.

What it looks like

A white crust that returns after cleaning, like efflorescence but usually harder and more granular.

Flaking or spalling of the matrix surface.

A specimen that feels faintly damp or sticky in humid weather, which is deliquescence.

The distinguishing test is history rather than appearance: efflorescence needs a carbonate specimen and an acid source, salt damage needs a porous specimen with a saline origin.

What to do

Stabilize the humidity first. Salt damage is driven by cycling rather than by any particular level.

Desalination by repeated soaking in changes of deionized water is the museum treatment, and it is not available for anything water-sensitive, which is much of the material that has this problem.

Rinse in deionized water at the end of cleaning, always, so you are not adding to the problem.

And keep beach and playa material apart from the rest until you know how it behaves.

References