As a professional numismatist, I have the privilege of seeing many more coins than the typical collector, including high-grade pieces that often are beyond my personal budget. While this may seem like a potential source of frustration, it's actually a very rewarding and enlightening experience. I've made observations over a period of years that simply wouldn't have been possible if I were limited to my own resources.

One thing I've noticed while examining uncirculated, bronze coins produced by the San Francisco Mint from 1908 (when cent coinage began there) through roughly 1924 is that they have some very distinctive features. These characteristics often identify them as "S-Mint" products before one even has a chance to check the mintmark.

Though the composition of United States cents was prescribed by law, some peculiarities in alloy appear to be unique to cents made in San Francisco. When entirely untoned, S-Mint bronze cents had a very pale, brassy color unlike that of the more reddish or coppery specimens from the Philadelphia and Denver Mints. For the period described, however, such cents seldom are 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 "VDB." Subsequent dates through the mid-1920s typically are toned to various degrees, though many have survived with partial Mint color.

Barely toned examples often display tan or light-brown streaks, creating a "woodgrain" pattern. This is the result of impurities in the alloy or concentrations of pure copper that did not properly blend with the small amount (5 percent) of tin and zinc added to it. When these less-than-perfect ingots were rolled into "strip," from which blanks later would be punched, the concentrations of copper 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 the atmosphere, which caused the copper to tone more quickly than the other metals.

Woodgrain toning commonly is seen on S-Mint cents through 1923-24, after which time it is encountered only occasionally. Examination of the U.S. Mint Director's annual reports for the period in question reveals that cent planchets were alternately made in-house (at the various branch mints) and purchased from outside vendors. After the mid 1920s, the Mint gradually phased out production of both cent and nickel planchets in favor of ready-made blanks. This seems to have standardized the planchets used at all the Mint's facilities.

Though most collectors favor bronze coins that are fully "red," I find this distinctive toning quite charming. In addition, it further serves as an aid to authentication. I've never seen a 1909 cent from the Philadelphia Mint that was brassy and displayed woodgrain toning, so the presence of such distinctive features almost guarantees that a coin is a genuine San Francisco Mint product. This is true of both Indian Head and Lincoln cents.

As noted, with the exception of 1909-S and 1909-S VDB Lincolns, early S-Mint cents seldom are seen with completely original color. Most exhibit brown toning or retain just partial Mint red. One peculiarity I've noticed about all copper and bronze coins is that sharply struck pieces tend to tone more readily than weakly struck ones. This is true regardless of date or mintmark. I suspect that the degree to which the planchet is work-hardened determines its resistance to atmospheric toning.

This phenomenon is not unique to 1908-24 S-Mint cents, but it is a critical factor given their greater overall rarity. Having collected this series for years, I almost never encounter a sharply struck coin with full Mint color, while the well-struck pieces I've owned always were brown or displayed light, woodgrain toning.

Reprinted courtesy of THE NUMISMATIST, official publication of the American Numismatic Association  www.money.org.