Copper colors

[Kkjand]

I received several wheat pennies in bulk,  and I pulled several out.  There coloring and marks are interesting and beautiful.  I would like to learn more about how and why.

[Just Bob]

It is called "wood-grain" toning,

Here is an excerpt from an article by NGC's David Lange, originally in The Numismatist,  quoted here from "The E-sylum":

A link to the E-sylum page, with more info from Tom Delorey, Q. David Bowers, and others, can be found HERE

Here's an excerpt from Dave's article: For the period described, however, �S� Mint cents are seldom seen untoned. The only issues commonly encountered in that condition are the widely hoarded 1909-S cents, both with and without the designer�s initials �V.D.B.� Subsequent dates through the mid-1920s are typically toned to various degrees, though many have survived with partial mint color. Examples having just light toning often display a pattern of tan or light brown streaks across obverse and reverse, the so-called �woodgrain� pattern. This resulted from impurities in the alloy or concentrations of pure copper that did not properly blend with the 5% tin and zinc added to it. When these less than perfect ingots were rolled into strip, from which blanks would later be punched, the concentrations were flattened and stretched into the patterns seen on the finished coins. Invisible when first struck, these flaws appeared only after the coin was exposed to atmospheric agents that caused the copper concentrations to tone more quickly than the properly mixed portions of the planchet. Woodgrain toning is commonly seen on �S� Mint cents through 1923-24, after which time it is encountered only occasionally.