Wednesday, February 10, 2016

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

Written by the TreasureGuide for the exclusive use of

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.

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 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.


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.


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.


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

Happy hunting,