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Sintered Planchet Mint Error

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Sintered planchets or plating is now known as improper annealing because it is thought that during the annealing process pure metal molecules, as in copper, migrate to the surface of the coin and form layers. Add to this some heat and oxygen and coins can become reddish, black, and spotted from oxidization of the purer metal.
Also See Improperly Annealing
Modern improperly annealed planchets are worth $35 to $120 but early dates and denominations like an Eisenhower Dollar can be worth $400. Value depends on the grade of the coin as well and alloy.

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Image courtesy of Heritage Auctions