What could black small spots like blemishes be in a fully cleaned extremely fine silver tetradrachm? I have seen spots like that in modern coins recently in a silver ww2 German coin and can't really understand what they are. They look like small spots of ink. I have searched and found about carbon spots but the discussions were only for modern coins, could this happen to ancient also, do they get worse or are stable and also do they really devalue a very beautiful coin? Thank you all in advance for your answers!
Anything that effects the desirability of a coin will often effect the price/value as well.
Humidity can cause a dormant
area of corrosion to become active again, but also might not have any effect, depending on what the corrosion was caused by and if the cause has already spent itself.
With modern coins, a black spot that is level with the surrounding surfaces and not at all raised above the surfaces, and also not showing any sign of having caused a reaction in the immediately adjacent surfaces, is often a carbon spot left over from when the
flan was annealed, prior to striking. The impurity on the surface was then struck by the die. Something like this usually doesn't change over time, even in humid conditions. 'Modern' dies were sometimes greasy when they struck coins, and the grease was imbedded in the coin surface under the high striking pressure.
Perhaps complicating the issue, black spots on coin surfaces caused by saliva droplets, etc., are also often called 'carbon spots'.
Sometimes with
ancient coins, an impurity struck into the coin is called an 'inclusion'. However, I see many more 'post striking spots' on ancients than 'as struck inclusions'.
I've removed a number of deposits from the surfaces of ancient silver coins. Sometimes the metal under the deposit has been effected, sometimes it hasn't. Generally, I've found that black deposits have a greater chance of concealing effected surface metal, than do deposits that are brown or lighter in color. But I've also found
good surfaces under black deposits.
I enlarged the image you provided with Photoshop, and it looks to me that the black spots on your coin are raised above the coins surface...and something that has happened 'post striking' (at least the large one at the forehead anyway).
There could be a lot written about this subject. The above are only a very few observations.
