Improperly Annealed Planchet
Illustrative example — errors vary coin to coin.
What it is
Before striking, planchets are annealed — heated in a furnace and cooled — to soften the metal so it strikes cleanly. If the furnace runs too hot, too long, or with the wrong atmosphere, the planchet's surface chemistry changes and the finished coin comes out an odd color: deep coppery-red, chocolate brown, or charcoal gray, evenly over the whole coin. Collectors often call the coppery ones 'sintered planchet' coins. The mint's Sacagawea dollars and some state quarters are the best-known examples. Because heat and chemicals outside the mint can mimic the look, these really need professional authentication.
How to spot it
Look for an even, unnatural color across the entire coin, both sides, with normal fully-struck design detail and normal weight. The color should look like it's in the surface, uniform and satiny, not painted on, blotchy, or concentrated in the recesses the way environmental toning usually is. Weigh and measure the coin to rule out wrong planchet, and compare to certified examples' photos. Honestly, this is one error where even experienced collectors defer to PCGS/NGC, so treat any raw example with skepticism.
What it's worth
Certified examples bring real money: improperly annealed state quarters and Sacagawea dollars typically sell for $100-$500, exceptional pieces more. Raw, uncertified examples sell for a fraction of that because heat-damaged coins are so easy to confuse with them.
Famous examples
- 'Sintered' Sacagawea Dollars
- Improperly annealed State Quarters
- Copper-colored Jefferson Nickels
Series where this error is found
Memorial Cent Roosevelt Dime State Quarters Sacagawea Native American Presidential Innovation