I was watching a recent video discussing the recent underwater archaeology in the harbor of Caesarea. My question is how well could coins be preserved underwater? I guess it would depend on the microenvironment: salt content, temperature, storms, etc. The video suggested better preservation in water. I’m not sure of that, but I will defer to those more knowledgeable (which is about everyone else on this forum). I also found this discovery in the area interesting: “Among these were a beautiful red gemstone for setting in a ‘gemma’ ring; the carving of the gemstone shows a lyre. In Jewish tradition, the lyre it is called Kinor David (‘David’s harp’).” https://www.israelnationalnews.com/news/319062
From the released photographs, the coins are Roman sestertii of the 2nd and 3rd c., from Antoninus Pius to Gallienus. The shape of the gold ring is also attested in the mid-3rd c.
I'm no chemist, but so far as how well coins are preserved in saltwater, I'd say that it depends on the metal. More precisely, how susceptible the metal is to oxidation. Gold is probably preserved just fine, but then again, gold is well preserved under just about any conditions. Silver would be affected more severely than gold, but copper/bronze would be the worst. Another consideration would be whether the coins were accessible to encrustations. I think those guys can build up difficult to remove solids in even the smallest void spaces if given enough time. Again, all speculation on my part. I wouldn't advocate for this to happen, as it is something that more properly belongs in a museum, but it sure would be interesting to see what that ring would bring at auction!
Consider also that not all water contains the same amount of oxygen. Coins buried in anaerobic muck might do better than others where the water had a higher oxygen level.
I read about this discovery today, and I though I saw a couple of tetradrachms, possibly from Antioch. As for preservation, the state of any given coin salvaged from the ocean depends on where it is buried, the amount of oxygen it is exposed to and the salinity of the water. My experience with salvaged coins, all silver, is that coins exposed to these elements, say, on the outside of a pile or found individually on the ocean floor, are often very corroded, while those that are fortunate to be more protected can retain much, if not all their original surfaces and detail. Another critical element in a salvaged coin's condition is the restoration or preservation method employed after the coins are removed from their oceanic environment. Some salvors opt for a relatively quick method of cleaning these coins, such as electrolysis, a method that basically strips away the heavy oxides that often accompany these coins. While fast and economic, the coins usually emerge overly bright and basically quite unappealing in appearance. A more labor intensive method, such as careful manual cleaning, can yield much better results.
