Shipwreck Coin With Low Specific Gravity (1736 8-Real "Rooswijk")

Thanks for your responses thq, tdziemia, llewellen.
A particular thanks to swamperbob for his very insightful response re. the idea of "shipwreck sponge coins".
This could explain all the observations on my Spanish shipwreck coins.
It would also explain why the coin does not "ring" when I tap it with another. The acoustic properties of spongy silver would be quite different from solid silver.

To answer swamperbob's question, yes the reported figures 24.62g represented the weight of the coin in air and 2.46 g represented the apparent loss of weight when the coin was immersed in pure (distilled) water.
From this I was able to obtain the SG = 24.62/2.46 = 10.03.

Being a careful fellow I measured the coin multiple times on the 0.01gr scales. I also measured the weight displaced by the coin within the pure distilled water multiple times. An average of both values was taken. The variations in weight across these measurements were very small.

Also, as swamperbob noted, the total weight of the coin 24.62 grams shows considerable loss of metal. If you look at the image of the coin in this post you can see this metal loss on the bottom right hand quadrant of the Pillar side - the shiny silver swath with the black "shadow". Perhaps another 8 Real piece rested upon and protected the relatively undamaged part of the coin.
I have examined similar shipwreck coins on the Internet and the appearance of the loss of metal from these coins look similar to mine.

I am assured by the reputable Coin Dealer that I can return this coin for a refund if I am not entirely happy with it. For the moment though the coin really intrigues me and I am prepared to declare "innocent until proven guilty".

Squire

I like Bob's sponge analogy, but I can't believe that water remains trapped inside for very long once the coin is on dry land. Vapor pressure and fissures would allow it to escape. You might oven the coin above 100 C for a few minutes and check the weight.

I think that it's unlikely that water repermeates the coin's interior in the time it takes to run an sg test (though that's worth checking too - does the coin get heavier right after immersion?). My earlier point about state of preservation is pertinent to sg, which is based on water displacement. If the coin is externally the same size as uncorroded, a 10% loss in internal weight would cause a 10% drop in sg.

I apologize for the confusion in terminology. I was referring to the old practice of putting silver coins in a bag and shaking them to "sweat" off some of the silver powder that would then be refined, an old dishonest way of making some money.

This is separate from the thread but to recap in the case of your coin you show, you put it in a pouch with several other silver coins, walked around with the pouch for a couple days, then observed deposits of black stuff on the surface that could be removed with a fingernail. What I was claiming is that the only logical explanation for these deposits is the "sweated" silver, ie the fine powder resulting from rubbing of coins together, rather than any sort of chemical reaction like those discussed, which require different environments and much much longer time scales to happen.

Interesting thread folks. I don't know much about the chemical component of your discussion but I can certainly attest that porous coins without a doubt ring differently. I have several older thalers and European crowns that are not sea salvaged. Their planchet is porous (whether it was so when struck is unknown) and they ring completely differently when flipped than other silver coinage.

Also, I think Bob is spot on about saltwater coins containing micro-fissures and being "sponge like". I don't know necessarily if the salt water remains (also seems unlikely to me), but I don't think it is unreasonable for salt particles to be present. Secondly, if the coin is porous and contains micro-fissures, doesn't it by definition imply that tiny amounts of air get trapped inside the coin? Perhaps it is this air that raises the buoyancy of the coin and is responsible for the lower than expected SG measure. If you were to do an SG test and keep the coin submerged for a much longer period of time so as to allow the air particles to exit the coin (perhaps rotating the coin may be required), will the SG measure rise?

I told you I wasn't a chemist. Forgive my ignorance here but I'm not sure I understand how air inside a coin doesn't make it float? Hypothetically speaking, if the coin was hollow or had a hollow center filled with air, wouldn't it float, or at least be much more buoyant than a regular coin? If so, wouldn't that affect the SG of the coin as like you said, it would displace much less water?

Now that I'm thinking more about it, I guess I see the fallacy in my thought process. The hollow coin would also weigh much less outside the water. So theoretically speaking, would the SG of two identical coins, one hollow and one regular be identical?

This thread covers a lot of material that is found in undergraduate science classes and it can be very confusing for people who are not versed in those sciences.

Keep in mind that when we discuss coins we need to consider changes to the outside of the coin as well as changes to the internal structure of the metal alloy iteself. These are normally somewhat similar but simultaneously not at all the same.

When I was first taking courses in geology in High School, I was surprised to find that many rocks were not as solid as I initially thought they were. Rocks, it turns out, can be very low density like pumice which floats on water (S.G. under 1.00) or basalt which is quite dense (S.G. of 3 ). All rocks also have internal structures which are studied to establish two important characteristics associated with rocks and soils - porosity and permeability. Porosity is a measure the relative amount of open space between the individual granules in a rock or soil while permeability refers to the ability of a liquid or gas to pass through a rock or soil. A rock or soil can be porous without being permeable.

In practical terms this distinction is useful for rock climbers who want to avoid soft rocks that are permeable - in particular right AFTER a rainstorm. For that reason granite is preferred by climbers over sandstone because a granite surface dries quicker. Internal moisture in impermeable granite is often trapped for all intents and purposes while water moves easily in sandstone which is normally highly permeable.

Something similar can be thought of as applying to silver copper alloys that have been operated on by electrolysis, corrosion and abrasion as we see in sea water wrecks. Near the surface very large pores can form in the alloy from a combination of electrolysis, corrosion and mechanical actions. These large pores are sufficiently large to allow water to easily drain out or evaporate from the metal. This outer portion of the coin can be viewed as porous and very permeable. However, micro-pores which are the result of electrolysis along individual grain lines without any corrosion or mechanical action are rarely large enough to allow for any permeability of liquid water. This water would migrate out of the coin very slowly over time even more slowly than a low permeability rock.

When we measure SG using water - the water fills the outer pores but not the inner micro-pores. That is why using water actually works.

The volume of the salvaged coin decreases due to all of the surface effects of electrolysis, corrosion and abrasion. The SG is altered only when the composition of the alloy is changed by the removal of one element selectively. In the case discussed here Copper is the reactive element and it will be removed. The loss of copper faster than silver reduces the density of the remaining coin.

To make a coin float you would need to make the density drop below 1.

Someone mentioned a hollow coin. If you had two coins of the same size (volume) they would need to be the same total weight to get the same SG. So theoretically if you had a dense enough material you could make a hollow coin with the same density of a solid silver coin, but such a material is not known to exist. The densest element known is osmium at under 23 only about 2 times that of silver.

Now lets look at the silver copper alloy used in most coins (about 90% Ag 10% Cu). In the case of this alloy the speed of cooling results in a differentiation of the components of the alloy - the slower the cooling progresses the larger the grains become and the more the components tend to differentiate from each other. The larger the elemental grains the more susceptible to electrolysis the coin becomes.

Cast metal (silver) has a lower Specific Gravity (or density) than worked or struck silver. So you need to consider how a coin was fabricated to see what density it should have. This is also a reason a cast coin will not ring but a struck coin will. The small spaces between the grains are compressed by the strike.

The fillet ingots used in Spanish and Mexican mints were small in size so they could be worked on manually powered machines. This small size meant there would be fewer inclusions in the alloy as well as resulting in a rapid cooling of the metal which produced a nearly uniform alloy.

Inclusions seen in silver fillet ingots were primarily minute traces of fire scale (debris from working and re-melting) and tiny air or vapor pockets. These pockets might contain air or not. The normal vapor found in casting bubbles is usually metallic and condensation of the vapor into metal leaves a vacuum in the silver. The the metal condenses on the inside of the bubble. The fire scale is a solid material consisting of impurities or corrosion products (neither being metallic silver or copper). Too much of either kind of inclusions in the ingot can effect the density of the entire ingot and result in re-melting or even re-purification of the metal.

This was why the development of fillet ingots produced increased output, because it reduced the number of poor ingots.

Density as noted above is a relationship between weight and volume. Water is the standard against which other things are measured. So coined silver 900 fine is 10.31 times as heavy as an equal volume of water. Only things with a density under 1.00 will float.

The micro-structure of silver even when subjected to electrolysis is NOT permiable to water. If it was then water would pass through the coins readily. As an expiriment take a sea water recovered coin and place it on a flat level surface covered with tissue paper. Then take an eye dropper and apply water to the upper surface of the coin. How long is it before the tissue paper gets wet?

I agree that some water can be evolved from a sea salvaged coin by heating it to change the residual water into vapor. But how long it takes to completely dry a silver alloy must be far higher than stones. BTW a one inch thick slab of granite can take one week of heating to dry fully.

English? They sound Scottish to me Squire....

The English steel we could disdain
Secure in valor's station
But English gold has been our bane
Such a parcel of rogues in a nation.

-Robert Burns

Great response thq . Investigating the Robert Burns poem on the Internet led me to the Scottish (Caledonian) "Darien Scheme". A fascinating piece of history that I was not aware of. It also highlights the "economic skulduggery" that occurred during the 17th Century. This has a parallel to the nefarious activities of the "China Trade" and my 1806 Class 2 English Contemporary Counterfeit 8 Real
Thanks for stimulating this research
Thanks also for your feedback Albert.

Squire