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Why old low purity silver coins always seem to be brown or copper colored

I started reading "Cleaning and Preservation of Coins and Medals" yesterday and it answered a long time question I've had:

"Coins of inferior silver alloys containing fifty percent or more of copper are now so badly worn in some cases that they can be mistaken for copper coins, even though they were intended to represent silver coins at the time of their minting.
Why has the silver worn while the copper has become more prominent? This is the result of the handling during the manufacture of the coins. In the course of fabrication, the coins always were annealed and then pickled, at first with a tartrate-salt solution, and later with a dilute sulfuric acid. Thereby occured at one and the same time, but also intentionally, "blanching" as a result of which the acid at the surface dissolved the copper oxide due to glowing, while the silver components were left in full silvery luster.
This surface was naturally porous and was attacked more or less rapidly, depending on the percentage of silver. The darker copper then appeared"

A few of my old billon coins look exactly like this but I had never thought through why.

Comments

  • scubafuelscubafuel Posts: 1,471 ✭✭✭✭✭

    That’s interesting, thanks for posting it. I’d noticed this with some of the low quality Honduras coinage.

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  • neildrobertsonneildrobertson Posts: 1,056 ✭✭✭✭✭
    edited November 14, 2019 3:26PM

    The quote is a translation in German and isn't the way I'd say it. I'd explain the phenomenon slightly differently.

    As part of the minting process, the coins are annealed and then pickled (exposed to an acid solution). The acid solution preferentially removes copper from the surface of the coin, leaving a surface that is disproportionately silver, when compared to the rest of the coin. This results in a surface that looks silver. The selective removal of copper creates a surface that is rougher and more porous than a normally prepared and minted silver coin. It can sometimes give the appearance of being coated with a silver powder even though that wasn't what happened. When this silvery surface wears away through minor circulation you are left with a coin that looks more like copper.

  • ColinCMRColinCMR Posts: 1,482 ✭✭✭

    What type of annealing processes were used?

  • jgennjgenn Posts: 714 ✭✭✭✭✭

    I certainly understand that silver enrichment of the surface is a result of the acid treatment of the strips and blanks. However, when that wears away you are left with the billon alloy and then it will look like whatever the mixture was. That quote sort of sounds like the copper will be more visible irregardless of the proportion of silver to copper.

  • neildrobertsonneildrobertson Posts: 1,056 ✭✭✭✭✭

    @ColinCMR said:
    What type of annealing processes were used?

    I don't know. I might have to see if I can look it up. I had always assumed that they were just heated in an oven. The exact temperature, ramp up time, ramp down time and hold times can vary I guess.

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