Salt, sand and drying
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Why salt matters
It is hygroscopic: it pulls moisture out of the air. A salty shell in a sealed jar raises the humidity inside it and keeps raising it, which is the opposite of what a sealed container is for.
Rinse, then dry, then wait
Fresh water to remove the salt, then air drying for longer than seems necessary. Shells hold water in their interior whorls for days after the outside is dry.
Sand is worse
It holds moisture throughout and it clumps permanently if sealed damp. A sand sample dried in a warm place for a week before it goes in a jar stays loose for decades.
Driftwood is worse again
Wood holds a lot of water, releases it slowly, and supports mould while it does. It is the one find in this property that arguably should not be sealed at all until it is thoroughly dry.
A desiccant helps and does not substitute
A pouch in the jar manages the remaining moisture. It cannot dry a wet find, and using it as though it can simply exhausts it.
Smell is the early warning
An unrinsed shell or a damp piece of wood announces itself within weeks. By the time it smells, something is growing, and the fix is to open, clean and dry rather than to seal harder.
Which argues against sealing immediately
Bring the find home, clean it, dry it in an open container, and seal it later. The jar is for the finished state rather than for the journey.
Warm and moving air, not hot
A warm room with air movement dries thoroughly. An oven or direct sun can crack shells and craze ceramic, which is a different kind of damage arrived at from impatience.
Weigh it if you want to know
A find that has stopped losing weight has stopped losing water. For sand and wood that is a more reliable signal than how it feels.