Was taking a closer look at this 1804 1/2 Real and the metal at the top of the reverse looks"muddy". Is this a die state situation it some other form of damage? It's a nice coin. I wish the obverse was as nice as the reverse.
Most of the coin looks environmentally damaged. Given the color and the encrustations, I'm more inclined to think buried in the ground than seawater recovery, but either are possible. However, the area the OP is asking about is the area above the crown on the reverse. To answer the OP, this is neither a die state or damage - this is a planchet issue. Metallurgy was not very advanced when these coins were struck - they often had issues with the alloy, and creating planchets from an imperfect alloy with crude technology led to a variety of problems such as you see. This is a common effect seen on many coins of this period.
It is actually from the preparation of the planchet. You get these wavy cracks when they have the edge put on, which causes these waves like miniature cracks. It is a good diagnostic to watch for on real coins.
Combination of deposits and enviro damage (I concur with the "buried in the ground" theory), and yes, likely some planchet quality issues to begin with, as well.
I’ve seen this called plata agria - “sour silver,” I guess from the idea that it results from an impurity in the alloy. (I’m posting the link mainly for the pictures; the Spanish text describes the effect as a lamination error, says it’s common and doesn’t greatly affect collector value, and that the author considers it a sign of authenticity.)
