2/11/16 Report - Surface Analysis Of Corroded Reales: Part II.

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.

https://en.wikipedia.org/wiki/Galvanic_corrosion#Anodic_index

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.

Most Cathodic
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
Beryllium	−1.85
Most Anodic

Note that zinc is near the bottom of the list.  It is very anodic.  That probably explains at least partly why zinc pennies hold up so poorly in salt water.  On the other hand, gold is at the top of the list and normally shows no effect of being in the water for centuries.

Excerpt 4.

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.

Excerpt 5.

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.

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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.

Happy hunting,
TreasureGuide@comcast.net

2/10/16 Report - How To Locate More Old Coins - Surface Analysis Of Corroded Reales

Written by the TreasureGuide for the exclusive use of treasurebeachesreport.blogspot.com.

Yesterday I said I was going to present a study of how silver reales are affected by the ocean environment.  The title of the study is Surface analysis of corroded silver coins from the wreck of the San Pedro De Alcantara (1786).  The authors are I.D. MacLeod  and E. Schindelholz.

A detailed morphological study of the corroded surfaces of the seven coins was conducted using low pressure scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis.

You can access the original study by using the following link.

http://www.nma.gov.au/__data/assets/pdf_file/0020/346043/NMA_metals_s2_p02_surface_analysis.pdf

The sample consisted of seven 8-reales that were minted in Lima between 1772 an 1788.  Those are the Carolus III type.  That obviously isn't a large sample size.

As the study says, the wreck site of the San Pedro ...is scattered over 200 metres long in water 3-9 metres deep, lying off a rocky coast near Peniche, Portugal. The sea bottom consists of irregular rocks, with numerous cavities and crevices filled with up to 0.5 metres of stone and sand.

I'll present some excerpts from the study and then add my comments.  Here is the first excerpt.

The degree of preservation of the recovered coins related to their location on the wrecksite. Coins 4324, 4325, 4327 and 4331, which were found buried under 15cm of coarse sediment, have more or less retained their original dimensions. Coins 4315, 4030, and 4342, which were found lying atop the seabed, have lost a majority of their original surface and dimensions. 

The first sentence in the above excerpt supports a main point that I presented yesterday.  Where a coin spends its time determines what happens to it, therefore if you know how various environments affect certain types of coins, you can then look at a coin and get an idea of where it came from.

As I have documented and mentioned in this blog in the past, many beach-found cobs weigh much less than their minted weight.  They have lost significant amounts of material.

This study shows, however, that cobs that were buried under 15cm of sediment were not corroded that much.  They were protected by the sediment.

It should be noted that this wreck site was in 3 to 9 meters of water. The deeper the water, the less the bottom sediments will be churned by the waves.  Also in deeper water, the bottom sediments will generally be more fine.  The passage of a wave only affects the water down to the wave base, which is half the wave length. Below that depth there is negligible water movement, so the sand will not be moved unless conditions are unusually rough.

Unlike the protected coins, coins that were found lying exposed had lost material.  

For coins to wash up onto a beach, they must be uncovered and go through the rough and turbulent zone at the front of the beach where the waves crash and the sand is churned.  Coins that are in excellent shape most likely did not come through that zone.

So how can you find old coins on a beach that look almost like new?  One way is that they can come from the dunes rather than being washed up.  I also supposed that there are rare occasions when coins can remain protected for very long periods of time only to be exposed and washed up by very unusual conditions such as a hurricane without spending much time in the rough surf.

Here is the second excerpt.

There was only one coin (no.4325) with any significant amount of concretion; the deposits consisted of sand-sized to pebble-sized sedimentary particles and small shells bound together by calcareous concretion (see Figure 2). Coins 4325, 4331, 4324, and 4327 all have considerable amounts of iron corrosion products on their surfaces.

Concreted Coin 4325.

Coin 4325 (left) was concreted.  It was one of the four coins that were found covered by 15cm of coarse sediment, and the only one of the four with significant concretion. So why was this one encrusted while the other three were not?

The immediate environment of a coin can affect what happens to it.  Note the appearance of rust on this particular coin.  I've seen that on some beach-found cobs and mentioned that in this blog in the past.

Here is another excerpt from the study.  I'll call it excerpt 3.   It provides a clue as to why coins like 4325 have rust on the surface.

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.

I am not so sure that the Joao Diogo wreck, which occurred later in the area of the San Pedro, is the entire reason for the concretion.  I plan to address that tomorrow or sometime soon when I continue this discussion.  That is as far as I can go with it today.

You might wonder why all of this is important.  The answer is that if you understand how the environment affects coins and other objects, you can look at the object and get a good idea where it has been.  That can help lead you to the source and the possible location of additional items.

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I didn't get many emails concerning the proposed citizen archaeology permit that is being considered in the Florida legislation.  I hope that does not mean that the metal detecting community is becoming apathetic.  If you don't stay informed and active, you can expect to encounter more and more legislation that will prohibit or limit metal detecting.

If you did not read that post, I'd recommend that you go back and check out the sources and become informed on the issues.

One link provided a list of emails of Florida representatives.  Please make your feelings on the subject known to them.  Apathy is a good way to lose freedom.

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There was a whale and baby in the Sebastian Inlet.  Crowds were able to see these magnificent mammals.  That was a rare opportunity.

Here is a link to the story.

http://www.fox35orlando.com/home/87340117-story

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We got another cool front and will have the west wind for another day.  The surf remains small.

Happy hunting,
TreasureGuide@comcast.net