How to Preserve Dug Coins
Coins fresh out of the ground need attention to prevent further deterioration. The goal is to stabilize them without destroying the details or patina that remain. Different metals require different approaches.
Immediate Steps
In the field, don't rub coins clean. Place them in a bag or container without letting them rub against each other or against hard surfaces. At home, rinse gently with distilled water and let air dry. Examine under magnification before deciding on any further cleaning. See reading corroded dates for techniques to reveal hidden details non-destructively.
Copper and Bronze Coins
A stable brown or green patina on copper is desirable and should be left alone. If encrusted soil obscures the coin, a distilled water soak (days to weeks) followed by gentle brushing with a soft brush is the safest approach. Olive oil soaks are slower but even gentler. See should you clean old coins for the value implications.
Silver Coins
Dug silver often has a dark gray to black tarnish. Many collectors prefer this "original" look. If the coin is heavily encrusted, an aluminum foil bath (aluminum foil in a dish with baking soda and hot water, place the coin touching the foil) will remove sulfide tarnish without removing any silver. This can be done gradually with multiple short treatments.
Storage
Store coins in inert holders — Saflip-type non-PVC flips, hard plastic capsules, or acid-free cardboard holders. Avoid PVC flips (they release acids over time that damage coins) and cotton or fabric that can trap moisture. Store in a dry location with stable temperature.