Some Wood’s Rosa Americana coins have blistered, bubbly surfaces … they were minted that way.

Looking at the coin below, one probably would think it is grossly environmentally disfigured. But I don't think the bubbly surface is due to environment. References below support the idea that coins with that bubbly effect, left the mint that way.

Below, an example of the blistered, bubbly surface found on some of Wood’s Rosa Americana coins. My poor, dear Rosa. I bought this coin because I thought that the bubbly appearance was an example of Wood’s difficulty controlling his minting process.



The following 2 references well explain the bubbly appearance of the posted coin.
(Text in curly brackets {} are my additions to the author’s content, which is in italics.)

1) From Walter Breen’s Complete Encyclopedia of US and Colonial Coins, 1988:
The patent represented Wood as the inventor of a new alloy in which the {Rosa Americana} coins were to be struck. Woods coinage metal … is a form of brass under the euphemism “Bath metal” = 75% copper, 24.7% (zinc), and 0.3% (silver). …
{Breen continues …}
the blanks were heated before striking, presumably to soften them enough to minimize die breakage. This explains why many uncirculated survivors are discolored, showing local porosity and bulges from trapped gas bubbles.

2) And from the archives of the British Numismatic Society’s British Numismatic Journal (BNJ) http://www.britnumsoc.org/publications/Digital BNJ/pdfs/1903_BNJ_1_14.pdf
THE COINAGE OF WILLIAM WOOD FOR THE AMERICAN COLONIES, 1722 - 1733.
By PH1L1P NELSON, M.D., member of the Numismatic Society of London
Published by Brighton, W. C. Weight, 1903.
The blanks for these coins were heated prior to being struck by the die, which was elevated to some height and then allowed to descend, and this no doubt accounts for the many specimens which appear blistered, as though they had been subjected to the action of fire.


Notice that Breen says that many “uncirculated” (Rosa Americana) coins are discolored, and show bulges. So the coins were minted with the bubbly defect.

I think these bubbly coins are due to Wood’s failure to fully understand or control his unusual metal and minting process. I presume that some factors of incorrect final temperature before striking, properties of the individual planchets, or impurities in the metal … sometimes led to, as Breen states, porosity (if gas escaped from the coin), or bulges (i.e. blisters/bubbles, if gas was trapped). Wood’s bath metal was only used on the Rosa Americana coinage. His later Hibernia coinage was struck in copper. I don’t see the bubbly condition on Wood’s Hibernia which supports the idea that Wood’s bath metal and/or his minting process caused the bubbly appearance.

This bubbly appearance is not uncommon among Rosa Americana coins. Here are some other examples of Rosas with similar appearances (not my coins).



On this next coin, some of the bubbles are broken revealing the void created by the trapped gas.



And one photo of the porosity that Breen noted. There is extensive porosity from 8:00 to 12:00.

 

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I'm not a metallurgist, nor have any experience with this metal system, but it looks like there's some funky stuff happening for 75Cu/25Zn above approx 575C. Maybe reheating planchets prior to striking could result in some phase transformation/segregation if they got above this temp. I would guess that any inhomogeneities would be more likely to come from the original alloying, but that's just my unsupported speculation. Also, I don't think reheating planchets would cause gas bubbles unless they were present in the original mixed alloy.

I think it's entirely possible that this could be a reasonable explanation for the appearance of the coins you pictured

I know there are some people with metallurgy backgrounds floating around on CT, so maybe they will chime in.


( ref:https://www.copper.org/publications/newsletters/innovations/2006/03/neptunes_daughters.html Other than the phase diagram, nothing else appears to be pertinent)

 

That would be marvelous if we could get some understanding of why the bubbles occured.

 

I made a phase diagram rookie mistake and was commenting on a composition that was 75% Zinc. The Bath Metal is 75% Copper and 25% zinc, which is on the left side of the diagram. The alloy looks pretty stable in that region to well over 900C.

Based on this, I would think the bubbles and raised areas are more likely to be due to inconsistencies in the initial melt than due to reheating of the planchets prior to striking.

 

Syd Martin's book on the Rosa Americana coins has a section on metallurgical tests done on sample coins to determine composition as well as the frequently seen bubbling. The coins were all found to be much higher in zinc than they were purported to be and there was no trace of silver. There was also traces of lead in the coins. The coins were made on cast blanks instead of cold rolled as the Hibernias were, causing the grain structure to be different and making the coins brittle. The bubbling was found to be caused by corrosion or dezincification of the copper zinc coins. The corrosion seems to have been caused by a high sulfur content in the coins, most likely from the molds used to cast the blanks. The bubbling could have appeared as soon as a few months after coining. This is a very simplified version of the metallurgy described in the book, but it is the basics of it.

 

Wonderful information. Thanks for the detail. .... I guess I was wrong when I said "I'm sure that the content will be new to you." in my Hibernia post.

 

It really is a good book, and I would highly recommend it. While the bubbly looking Rosa coins did not leave the mint like that, they very well could have developed the bubbles shortly afterward.

 

I plan to get the book either by library loan or purchase. I assume you have the book.

There is another thing about my Rosa that I have been wondering about.
The coin edge has a screw thread-like pattern see photos. It's around the entire periphery of the coin. Have you seen this in the book? Maybe they are lines from the planchet mold??

 

Could it be laminations? Due to inhomogeneities in the planchet?? (I haven't read the book @l.cutler refers to)

 

I don't think the lines are laminations. I previously thought it could be laminations, and thought it could be the source of air trapped in the planchets that cause the bubbling. But I convinced myself that flat laminations, at an angle as such, would: have a reversed thread on the opposite side of the coin, and there would be reverses of the thread from left to right on 2 opposite sides of the coin where the line goes briefly horizontal.

 

Interesting. Could it be some artifact from a screw press?

I have some experience in material science/chemistry, but no real background in US Colonial, so please keep that in mind when reviewing my comments

 

This is common on these coins. I never gave it any thought until now. Until I find out I'm going to GUESS the edges were filed to get the weight correct; however, the ones I've seen are more uniform than those in your image which should rule out my guess!

 

Could it be some artifact from a screw press? I can't see how this could be. If you are talking about the pressing of the designs into the coin, well the dies can't turn, I should think. If you are talking about a press to punch planchets, @l.cutler said the planchets were cast.

I wondered if the lines are tool cut marks from when the mold was made. But then, that would make it difficult to remove the planchet from the mold unless the mold was in 2 pieces.

 

I have looked at a population of 1 Rosa Americana coin edge. When you say this is common on these coins, are you referring to Rosa Americana coins or colonials of the period?

I can can imagine your suggestion producing the result though.

 

I will have to check, but I am pretty sure I saw a piece in the book about grinding the edge seam off of the cast blanks. I will have to get my Rosas out and give them a good going over and check the edges, haven't visited them in a while! I am so glad you started this thread, Rosas get very little discussion here.

 

I spent a little time with Martin's book, and my unfortunately small Rosa collection. I have two 1722 halfpennies, three 1722 pennies, one 1723 penny, one 1722 two pence piece and one 1723 two pence piece. According to the book the planchets were either hand filed or ground to clean up the edges. The most common seen are long grooves parallel to the edge most likely done with some sort of grinder, and less often seen are horizontally across the edge more like reeding apparently done with a hand file. Half of mine have some sort of file marks showing, three have the longer parallel grinding marks and one has the short horizontal marks, with the reeding like filing on one of the 1722 pennies. The coin pictured here with the diagonal marks is more like the longer grinder marks, just not completely parallel with the coins surfaces.

 

Thanks again for the great description.

 

Some metals and alloys shrink when they cool, which would allow them to fall out of the mold.

 

No. The coins were broadstruck, so there wouldn't have been a collar coming in contact with the edges.