ARTICLE EXAMINES SS CENTRAL AMERICA COIN RUST

I am delighted to announce the publication of our peer-reviewed article about biogeochemical growths ("rust" to non-scientific folks ) on gold coins from the S.S. Central America treasure. For those of you who regard rust as simple and dirty, this article will do its best to change your minds.

With my background as a geologist, early on in the SSCA project I became fascinated with the environmental processes of shipwreck degradation, corrosion and re-deposition of mineral deposits on the coins and ingots. The rust fascinated me from the very first moments we saw the shipwreck on our monitor screens in 1988. At that time there was nothing in the literature about rust on gold. Why would there be? Gold doesn't rust. But in the very unusual chemical and biological environment of a wooden-hulled steamship festooned with gold deposits, such quirks of nature are actually commonplace. Bacteria play a major role in the formation of such deposits. In this study my co-authors and I explore and analyze the different microscopic layers of a typical rust-on-gold deposit that flaked off a double eagle. This seemingly innocuous flake of rust became the subject of an excellent Senior Thesis (by Bryan Seymour, one of my co-authors for the article) and related geological study performed using a scanning electron microscope and associated scientific tools.

A fortuitous meeting: Professor Erik Melchiorre of California State University San Bernardino came to see the Ship of Gold exhibit at the Long Beach Coin & Collectible Expo in February of last year. I saw him looking at the gold with his kids, spending more than the usual amount of time in front of the porthole-like display of gold nuggets and gold dust. Most people focus on the gleaming displays of ingots and mint-state gold coins. There was a lot of that on display. But we also found whole bags (pokes) of gold dust and nuggets during the 2014 recoveries. As I engaged the pony-tailed professor in conversation I immediately realized that he understood the deeper significance of what was on display, geological samples from a specific year of the California Gold Rush. This allowed us to ask questions such as, "Exactly where were they mining in 1857?" -- the kinds of possibilities that appeal to those who read the stories in the earth. There are chemical, mineralogical and crystallographic clues spread throughout these very special samples of rocks, sand and dust. Under very high magnification and subject to modern scientific techniques, the secrets have begun to reveal themselves.

We have studies addressing the mineralogy of the gold itself, but have also expanded into the clues presented by microscopic grains of other minerals found associated with the gold dust. These topics address the geology of the gold and its origins in California.

Additional opportunities for research come from the shipwreck itself, a geological environment with its own suite of wonderful scientific phenomena to discover and investigate. This article is one of the fruits of that type of investigation. As the chief scientist of the shipwreck project and the curator of the treasure I have dealt with "rust" in a wide variety of forms on gold and other objects for 30 years. It pleases me very much that we are gaining a greater understanding of the incredible complexities involved in such a "simple" substance.

Abstract: Black crusts that formed on gold coins recovered from the 1857 shipwreck of the SS Central America played a key role in their preservation in a near original state. Within a few years of the sinking, the significant quantities of iron and steel on the shipwreck produced laminar geochemical precipitates of fine-grained iron minerals on the coins. This coating served to armor the coins from future chemical or biological attacks. Once coated, the coins were colonized by at least two distinct populations of gold-tolerant bacteria that precipitated abundant nanoparticulate gold in the black crust material and produced biomineralized bacteria in a web-like mat.

Above this middle layer of black crust, the outer layer consisted of a geochemical reaction front of euhedral crystals of iron sulfate and iron oxy-hydroxide species, formed by the interaction of seawater with the chemical wastes of the bacterial mat. Understanding this process has application for assessing the diverse and extreme conditions under which nano-particulate gold may form through biological processes, as well as understanding the conditions that contribute to the preservation or degradation of marine archaeological materials.

Figure 4. Image of an 1857-San Francisco mint $20 gold coin from the shipwreck, showing the preserved cast of the coin on a fragment of black crust (red box). The image of the crust has been inverted to align with the coin, as it is a negative-relief cast. The yellow areas on the crust are not gold, but hydrated iron oxides. Coin image courtesy of Professional Coin Grading Service.

To read the complete article, see:
The Interpretation of Biogeochemical Growths on Gold Coins from the SS Central America Shipwreck: Applications for Biogeochemistry and Geoarchaeology (https://www.mdpi.com/2077-1312/7/7/209/pdf)

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