What causes these surface characteristics on Lincolns?

[pendragon1998]

These are two pairs of images from two different 1909 Lincolns (the first is a '09 VDB, the second is a plain '09). Both have a somewhat grainy appearance which is more apparent under magnification. I didn't put a lot into these coins, but I'm curious what's up with the surfaces. With the obverse image, my thought is wear, but why did it come out seeming so granular? Thoughts?

[physics-fan3.14]

I don't really see it with the second one, but the first one is a classic woodgrain toning. I've seen it many times before, and I have to say I really like it. I don't know what causes it, although I have heard someone say irregularities in the rolling stock. It is cool, though.

[Dcoin]

My guess would that copper does not oxidize in uniform patterns. The small differences in color give the coin its rough-looking surface.

 

I think we have several chemical engineers here, maybe they can confirm (or more likely refute my idea).

[BillJones]

It is the result of a less than perfect mixture of the copper (95%) and tin and zinc (5% combined) that make up bronze. You see this somewhat often on some 1909-S Indian cents that have the so-called “wood grain” toning. On the this piece the effect is not as pronounced, but you did pick it up here.

[pendragon1998]

Oh, don't get me wrong - I like both pieces. The 09 VDB (obverse image) was an eBay coin and the 09 (reverse image) was a CU forum BST coin.

[Keigwin]

Lincolns with character are great. I love the woodgrain of your '09.

 

As Bowers pointed out in his Guide Book, "...the planchets for the 1909 San Francisco cents and some of the 1910 cents were made from an alloy which streaked after being run through a rolling mill. After a few years these planchets developed a normal light 'wood grain' effect..."

 

Here's an S-VDB I regret selling:

 

You'll find some of these through the 20's, too. Here's a '25-D with some nice streaking:

 

Lance.

[DWLange]

The roughened texture of early Lincoln Cents and the woodgrain toning effect are two entirely different matters that may or may not be seen together.

 

The master hubs for this coin type had finely-textured fields as sculpted by Brenner. This style of modeling was in vogue at the time, and it is seen in early issues of all the new coin types introduced between 1909 and 1916. Chief Engraver Barber didn't care for it, and the Mint ultimately smoothed out the coarse fields of the nickel and the silver issues. The cent, however, retained its more subtlely roughened texture for several years, until the huge number of dies needed for the World War I era coinage of 1917-20 wore down even the master hubs and left the fields smoother by default.

 

All Lincolns made from fresh dies in the years 1909-16 show this texture, unless the coins were just weakly struck. It's more prominent on proofs only because these were struck at least twice to complete fill the dies.

 

The woodgrain toning is a result of the bronze alloy not being fully blended. When the ingots were rolled into coining strip, these irregularities became drawn out into long streaks. The streaks were invisible at the time of coining, but subsequent reaction to the atmosphere caused them to appear on finished coins.