First let me say that I truly value the opinions and expertise I have found on this site. Some issues with coins present more of a learning curve than others. I was doing more research and I discovered that PCGS
one used the term on slabbed coins "sintered planchet" to describe this type of condition as an error. The term was incorrectly used and they have since replaced the term with "Improperly Annealed". From my understanding this occurs when copper dust adheres to the surface of the planchet during the heating and annealing process. The slabbed coins I have seen labeled as such exhibit the same qualities as this nickel, especially the blotchy appearance on the reverse of this coin. Now I have yet to find another example on a Liberty nickel
but I see this on Jeffersons, clad quarters and clad dimes as well. On the clad coins it appears to be more of a copper toning, often referred to as "Copper Washed" but on the nickel coins it is very close to what I am seeing here and labeled "Improperly Annealed". I would think that if it were environmental damage, there would be corrosion or other such problems which I am not seeing here. Your thoughts?