Why do Indian Head Cent proof coins exhibit wild toning?

If you have never collected Indian Head Cent proof coins there are certain basics that you should become familiar with. First, Indian Head Cent proof coins are made differently than regular issue mint coins issued for circulation. Each coin was struck on a polished planchet, and generally they were struck at a slower production rate with higher striking force, often with two blows from the press. Thus, the details of the coins are fully present, and the fields are fully mirrored. These coins represent the epitome of the engravers art - they look like what the engraver wanted the coins to look like. All of the details of the Indian Princesses' feathers are present, all of the diamonds in the ribbon are complete, the details on the reverse wreath are there, including the fine details of the acorns and the veins of the leaves. When compared to the average regular issue coins, proof Indian Head Cents are little works of art - they were intended for connoisseur collectors and sold to them by the Philadelphia Mint separate from the regular issue coins. Until the late 20th century, with rare exceptions, all Indian Head Cent proof coins were struck only by the Philadelphia mint. They could be purchased individually, but they were generally purchased as part of a set of the minor proof coins - 1CN through 5CN pieces.

The second notable feature about proof Indian Head Cents is that they tone differently from regular issue cents. There are two separate metal compositions for the Indian Head Cent proofs. From 1859 to 1864, the composition was copper-nickel, (88% CU, 12% NI) which, when untoned, is a silvery white color. Commencing in 1864, the metal was changed to Bronze, a composition of copper, tin and zinc (95% CU, 5% tin & zinc). From the mint these coins were often wrapped in tissue paper to protect them from handling marks and fingerprints - as they were not released into general circulation, this tissue paper caused them to acquire vibrant colors not seen on regular issue coins. On the bronze coins the colors range from mahogany cherry reds, to violets, indigo blues, greens, and even blushes of pink, yellow and gold. Very few of the mintages of Proof Indian Head Cents have survived with their original untoned color intact. Further, the variations of untoned examples, when found, range from a mellow pink copper color, to golden yellows and cherry reds - these are the coins that the grading services label as "red", but often they are not red at all, but variations of the slightly different bronze alloys used by the mint during the 45 year production of the "copper" mintage of these coins.