Question about seawater surfaces

Longacre

I assume that a "seawater" surface on a coin from a shipwreck results from the salt that is in the water (or is it from the movement of the sand at the bottom of the ocean)? The coins are described as having a matte surface. It doesn't seem like gold is a very reactive metal, so it must be a physical phenomenom. Can someone explain? Also, does anyone know what seawater does to a copper coin or a silver coin?

Hedger

I believe this seawater surface occurs after the coin is removed from the salt water environment and then immediately exposed to air. This is why coins extracted from salt water are immediately neutralized in distilled water to prevent the corrosive effects of salt afterward.

Pistareen

You correctly surmise that gold is not terribly reactive -- quite the opposite.

Seawater surfaces in general show sanblast-like abrasions from the constant action of sand and microscopic salt particles over it. This is why coins like the SS Central American coins look so nice -- they were brought from a depth where the currents are relatively still and, from my amateur understanding, the water is cold enough that there is essentially no chemical reaction. Compare this to gold from the Fleet of 1715 wrecks along the coast of Florida, some of which were found in 10-15 feet of water. The gold from those wrecks always looks very abraded, classic "seawater" surfaces. The pieces from the Ft. Capron find are the same way, as are those from the SS Yankee Blade.

Silver and copper have a much worse time in the ocean, because they are so reactive. They get both corroded and abraded, a real two-edged sword that generally makes them look awful except where protected by being inside a clump of coins or at very still, very cold depths. Some of the silver from the SS Republic looks very nice and was protected from reaction and abrasion.

TUMUSS

Seawater Composition

griffin6

Bear in mind that all US coins are made of alloys, and while the silver and gold in the alloy composition may not be particularly reactive, the other elements... particularly copper... are!