9/9/16 Report - Two Systems Hurricane Trackers Are Watching. Deep Sea Fishing Effects On Shipwrecks. Cobs With One Good and One Bad Side.

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

Two Atlantic Disturbances To Watch.

As you can see there are now two systems that we need to watch, both of which seem to be headed to the east of the Treasure Coast.

The second (red) has a higher chance of becoming a cyclone in the next couple of days.

Both of these could bring us some north/northeast winds.  There is no big increase in surf predicted yet.  In fact the predictions are showing mostly a two-foot surf for the next few days.

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You might think that deep sea wrecks rest undisturbed.  That is not usually the case.  Various types of fishing trawlers and things rake through wreck sites, scattering the remains and breaking artifacts.

Many pictures and examples are shown in an Odyssey Marine study.

One of Many Pictures Showing Effects of Deep Sea Fishing.

Here is the link.

http://www.shipwreck.net/pdf/OMEPapers4Final_000.pdf

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In recent weeks I've talked about cobs found as a cache that were in remarkable condition.  Many cobs found on a beach will be very much corroded and some will be so corroded that it is difficult or impossible to tell for sure what they are.

In the past I've found some cobs that were in very good or excellent condition on one side and very poor condition on the other side.

Below is a grading system used in one Odyssey Marine paper.

Grade One coins display little or no visible ocean wear, show little or no roughness or pitting, and, therefore, look much as they did when new. Both sides are in Very Good to Excellent condition and the obverse and reverse features resulting from the original strike are defined and easily identifiable.

Grade Two coins were partly exposed to the elements. The coin may not be completely intact or it may look more ‘sand-blasted’ than a Grade One, but the quality is still Good and most of the features resulting from the original strike are easily identifiable.

Grade Three coins are in Fair overall condition, but ocean wear is very apparent. Many Grade Three coins were typically located on the outer layer of a shipping chest and, therefore, the side of the coin that faced into the chest will be of Grade One or Grade Two quality. The opposite side, exposed to the elements following the wooden chest’s deterioration, is completely worn away. Other Grade Three coins might incorporate ocean wear distributed across both sides of the coin. On Grade Three coins, obverse and/or reverse sides will still offer easily identifiable characteristics that have definition.

Grade Four coins are still identifiable as Spanish Colonial coins, but they have been subject to much wear and tear and the markings are faint and have little or no definition.

Grade Five coins are just above a fragment in quality, but can still be identified as a coin taking into consideration factors such as shape and provenience. • A Fragment is a piece of silver recovered among concentrations of coins, but with no markings to identify it as a coin.

Here is the link for that source.

http://www.shipwreck.net/pdf/OMEPapers30-2013_001.pdf

They talk about one way that cobs end up being good on one side and poor on another side. They are referring to cobs that were salvaged from the sea and probably never spent any time on a beach.

One cob I'm reminded of was found on a beach and appeared to have been just washed out of the dunes.  It had a sand and shell crust covering one side.  That side was not visible at all because of the crust when it was dug.  The other side was not covered and displayed fairly good detail.

When the sand/shell crust was removed, that side then showed remarkable detail - much better than the exposed side.  The crust protected that surface of the cob, while the other side was worn down by washing around on the beach.

I'm not certain why one side was covered with a crust while the other side wasn't.  I do believe that that cob spent many years in the dunes.

How a cob corrodes can depend upon what it is next to.or touches.  I once did a couple of posts on surface analysis of cobs and talked about galvanic corrosion.

You can go to the following post to read about that.

http://treasurebeachesreport.blogspot.com/search?q=surface+analysis

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Happy hunting,
TreasureGuide@comcast.net

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