What is the cause of this texture?

Discussion in 'Ancient Coins' started by Roerbakmix, Feb 22, 2020.

  1. Roerbakmix

    Roerbakmix Well-Known Member

    I was wondering about the texture of this coin. Any suggestions?
    upload_2020-2-22_11-27-26.png
    ALLOBROGES CELTS, Southern Gaul. Denomination: AR quanarius, minted 61-40 BC.
    Obverse: DVRNACOS, head of Athena to the right, wearing winged helmet.
    Revers: Helmeted warrior on horseback, holding spear downwards, wearing cape or wings. Below AVSCRO.
    Catalogue: DT 3161, LT 5762.
    Weight: 1.86 gram, diameter 13x14 mm.
    Details: a beautifully toned example, struck on excellent metal. Extremely fine, struck on a relatively small flan.
     
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  3. lordmarcovan

    lordmarcovan Eclectic & odd Moderator

    Porosity from time in the ground? Dunno. Handsome little coin, though.
     
  4. shanxi

    shanxi Well-Known Member

    Normally this effect is called crystallization, which is half right half wrong. Wrong because the silver was already crystalline before the structure was formed, right because it is partly based on an improvement of the crystals.

    You have to know that copper and silver can be mixed homogenously at high temperatures in the melt, but they try to separate at low temperatures and do not form defined mixed crystals.

    So when copper is melted together with silver, a homogeneous liquid is formed. When cooled down quickly, the two metals remain "mixed" for the time being. This results in "contaminated" crystals of two metals that do not like each other at low temperatures.

    During the so called "crystallization", impurities such as copper are excluded. Mostly a closed silver layer forms on the surface of the coin, under this layer you find the structure recognizable on your coin. If the copper (between the silver areas) is dissolved out by oxidation, the structure can be seen more clearly. Inside, however, you can usually still see silver and copper phases if you do cross sections.

    The process is very slow, it can be accelerated by temperature, but then normally not such nice structures are formed. A short-lived fire, as is sometimes assumed, cannot replace 2000 years.
     
    Last edited: Feb 22, 2020
  5. Ed Snible

    Ed Snible Well-Known Member

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