Written by the TreasureGuide for the exclusive use of treasurebeachesreport.blogspot.com
Yesterday I left off discussing the following excerpt from a study of seven 8-reales from the wrecksite of the San Pedro. I'll discuss a little more of that study today.
Here is that excerpt (no. 3) again.
The large amounts of iron found in the concretions and as distinct films on the coins recovered from a buried microenvironment is consistent with the history of the site. The majority of the surfaces of coins 4325, 4327, and 4331 were covered with a layer of pure hydrated iron oxides, such as FeOOH.xH2O. The films are compact and of relatively uniform thickness. The deposition of this material is likely to have occurred during the salvaging of the João Diogo as a continuous film that was subsequently eroded/corroded away in areas to reveal underlying layers of silver halides. The absence of iron on coins 4315, 4030 and 4342 is a reflection that these coins are highly eroded and corroded and have very little concretion.
The authors of the study believed that iron oxide accumulated on the coins came from the Joao Diogo wreck, a later wreck that scattered over the San Pedro wrecksite. I feel certain that if they later wreck was not in the area, there would still be some coins showing some iron oxide. Cobs from 1715 Fleet beaches sometimes show the iron rust appearance. All it takes is for a silver coin to rest on or near iron objects such as cannons or spikes. I've commented on the rust-like residue seen on some reales found on Treasure Coast beaches.
As the last sentence in the excerpt suggests, layers of corrosion can also be eroded off of coins, especially when they are end up getting tossed about in the surf and sand.
In the same way that iron products get transferred to coins at times, a similar thing can happen with other metals.
In nautical archaeology galvanic corrosion is an important topic. It was observed that copper sheathing would cause iron spikes to rapidly corrode, for example. Also, cleaning by electrolysis makes use of a similar process.
Here is link to a web site about galvanic corrosion.
And here is a table showing how susceptible different metals are to galvanic corrosion. The table shows copper as being more cathodic than iron, which means iron will be drawn to copper, as in the example I mentioned concerning copper sheathing and iron spikes. Forgive my simple explanation. Chemistry was one of my least favorite subjects, and really don't know much about it.
|Gold, solid and plated, Gold-platinum alloy||−0.00|
|Rhodium plated on silver-plated copper||−0.05|
|Silver, solid or plated; monel metal. High nickel-copper alloys||−0.15|
|Nickel, solid or plated, titanium an s alloys, Monel||−0.30|
|Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys||−0.35|
|Brass and bronzes||−0.40|
|High brasses and bronzes||−0.45|
|18% chromium type corrosion-resistant steels||−0.50|
|Chromium plated; tin plated; 12% chromium type corrosion-resistant steels||−0.60|
|Tin-plate; tin-lead solder||−0.65|
|Lead, solid or plated; high lead alloys||−0.70|
|2000 series wrought aluminum||−0.75|
|Iron, wrought, gray or malleable, plain carbon and low alloy steels||−0.85|
|Aluminum, wrought alloys other than 2000 series aluminum, cast alloys of the silicon type||−0.90|
|Aluminum, cast alloys other than silicon type, cadmium, plated and chromate||−0.95|
|Hot-dip-zinc plate; galvanized steel||−1.20|
|Zinc, wrought; zinc-base die-casting alloys; zinc plated||−1.25|
|Magnesium & magnesium-base alloys, cast or wrought||−1.75|
Since the San Pedro de Alcantara site is an area of constant and aggressive surge action, erosion corrosion plays a major role in the deterioration of metals on this site. The coins 4342, 4030 and 4315 were found lying on top of the sediment and their much higher mass loss is a measure of the erosion effect. Prior to recovery, coin 4315 was lying flat on top of a thin layer of sediment in a shallow bedrock hole, which meant it was fully exposed to water and sediment movement, and this resulted in more than a 60% mass loss.
I've commented about the loss of mass seen on many Treasure Coast beach-found cobs in previous posts. Some are less than half of their minted weight. Such coins likely come from exposed areas with rough surf like those mentioned in this excerpt. Tumbling in the surf zone is surely a factor.
Calcareous concretions, especially those that grow in tropical waters tend to be somewhat porous and good conductors for the corroding metal they cover (MacLeod, 1982). In contrast, iron oxide films tend to be poor ionic and electrical conductors and can passivate the underlying corroding metal. There is a clear discrepancy between the preservation of those coin surfaces that are covered by iron oxide films and those that are not. Where the surface of coins 4325 is covered by an iron oxide film the raised design is well preserved, while the surfaces without the protective iron have the design obliterated. Clearly the iron oxide films are providing erosion and corrosion resistance to the coin surfaces they cover.
That does not require additional comment.
I've probably carried on with this too long, but many of my casual observations and conclusions were verified and elaborated by this study. To me it was validation. It added to my understanding of the processes involved. I hope you found it as interesting as I did.
Whether you understand all of it or not (I don't) you might benefit from wading through it to take what you can from it.
I didn't comment on the parts the explain the corrosion processes, such as how silver coins develop different layers, even though that information also expanded upon some of my personal observations.
The main point in looking at this study is to reinforce the idea that if you study your finds and understand how they got to look like they do, you'll then be able to draw conclusions about where they have been and where they are coming from. When you know something about the source of those coins, you'll have some good information about where to look for more of them.
I received some good information relative to the proposed citizen archaeology permit. I need to discuss that some day.
I also have some other good things too discuss in the future.
On the Treasure Coast beaches we've been having a small surf and will for a couple more days.
I have to go.