I was wondering about the texture of this coin. Any suggestions? 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.
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.
I believe the technical term outside numismatics is "integranular corrosion". The word "corrosion" doesn't look pleasant in an auction catalog so numismatists started calling it "crystallization". https://en.wikipedia.org/wiki/Intergranular_corrosion
I do not fully agree with your definition, it is exactly what @Ed Snible mentions, it is intergranular corrosion, which doesn't necessarily only has to be on the impurities such as Cu. Also it is no crystallization at all, I do not know why we even use that term in Ancient coins in general, it is false. When cooling down the metal, the grain boundaries of the metal crystals are already set. What happens over the 2000 years in the ground, is that the conditions of the environment (heat, water, oxygen, UV, salts etc) reveal the grain structures by etching of the grain boundaries i.e. intergranular corrosion. Atoms at the boundaries of crystalline grains and bounderies will be in locally higher energy states and this is why metallic crystal grains can be revealed. It really is false to call structural failures "crystallization", just because we are seeing the surface of the grain boundaries. Due to the intergranular corrosion, the surface is just revealing what was already there, but people generally do not know that almost all metals are polycrystalline in the first place. I have done an extensive research project of Metallurgy regarding this topic at a university before I stepped in the commercial world. It was then intended for electrical properties (grain boundaries of the metals limit electrical conduction, and could be compared using impedance measurements).
