Sintered / Improperly Annealed Jefferson Nickels?

I have found darkened nickels and have heard of the term sintered planchet before.
My most common dates that I have run across with a dark, sometimes almost black color have been almost always 1954 1955 1958 and 1959 with a few early strays from the 1960's.
Sometimes these are called "Black Beauties" and I am fond of them myself.
Whether these are from some sort of blanks that are sintered or not I don't know, but it seems to be a more common thing for me anyway, to find darkened nickels from the Phila mint.
Perhaps it is an alloy mix in the sheets they get the planchets from? I just don't know.
FWIW
I did a google and found that a FlyingDutchman wrote:
Copper Wash/Sintered Planchet - Sintered or copper washed planchets are coin blanks left in the annealing furnace for an extended period of time, causing dust from previous coins to embed in the blank planchet making it darker then usual

hmm I see that fred wienberg has several older examples of sintered planchets on his page mostly from the fifties, I suppose the only way to truly tell is to send them in to get certified. Off they go to anacs I guess.
-XoG

All of these coins appear to have been discolored outside the Mint. You need to see mint luster before you even have a prayer of correctly diagnosing this type of error. While genuine examples seem to be due to excessive or prolonged exposure to heat, the sintering hypothesis has no validity. No one really understands the physical or chemical basis for this error, despite the abundance of examples.

Oh, this is a legitimate error category. Colors range through copper, brown, red, and black. The most desirable ones are those with a thick layer of copper on the surface. The "sintered plating" and "copper wash" scenarios don't hold water though. None of the many specimens I've examined with a thick copper layer have been overweight, as would be demanded by either scenario. Quite a few are significantly underweight. Added to that are those specimens where the nickel-colored areas are flaking or where composite copper-and-nickel layers are flaking off. The manner in which these planchets become covered with a layer of copper is obscure. Perhaps copper atoms are migrating to the surface. Perhaps nickel atoms are retreating to the interior, leaving the copper fraction behind. Perhaps nickel atoms are being liberated from the surface, leaving the copper fraction behind. Or perhaps copper atoms are being liberated from the surface, only to be re-deposited as a pure layer. I'm not metallurgist, but even metallurgists I've consulted with don't understand what's happening.

These days I prefer the non-specific term "improperly annealed". When someone reveals/figures out what's going on at a physical/chemical level, then a more precise term will be warranted. Anyway, to answer your question, these errors range in value from $5 to over $100, depending on thickness of the copper coating, area of coverage, and denomination.