Why would corrosion appear like this on a coin? Might it be the way the metal is composed that makes this "stripe" pattern?
Almost certainly it is from poor storage in an oxidizing environment. What is the composition of the coin, and how old is it?
My guess would be because the coin rested against something while it occurred. Think of textile toning - same kind of thing.
I'm trying to decide between that and the old "unevenly mixed metals, extruded in the roller to form linear areas of different composition". In this photo, it really does look like contact with some kind of corrugated paper. I gripe all the time about Zincolns, but they're perversely reverse-galvanized copper over zinc. This is brass, and I know less about brass corrosion than I'd like. Down the Google rabbit-hole again...
Regarding brass corrosion, alloys have a “parting limit,” which is the minimum concentration of the more noble metal (in this case Cu) above which the more active metal won’t corrode preferentially. In other words, below this minimum Cu concentration threshold, Zn corrodes preferentially to Cu. This preferential corrosion of alloys is called “dealloying.” For brasses, this is called “dezincification.” The parting limit for Cu/Zn is around 85% Cu, so this coin would be susceptible to dezincification. See the following links for more information about parting limits and dezincification. https://www.corrosionpedia.com/definition/856/parting-limit https://www.nace.org/Corrosion-Central/Corrosion-101/Dezincification/ As for whether or not the Zn preferentially corroded... I’m not sure. The reddish brown corrosion products don’t look like the usual Zn corrosion products (Zn oxide is white), so it’s hard to tell for sure without knowing what environment the coin was stored in.
I understand what you're trying to point out, but I'm going to ask you to think for a minute. Consider if you will that when an alloy is made it occurs with the metals in molten form. Molten metals that are stirred and mixed over and over again in an effort to make the alloy as homogeneous as possible. This results in millions and millions of tiny particles of each individual metal all mixed together in a completely random form, with particles of one adjoining particles of the other(s). Then the metal is poured into bar shape or whatever, then rolled out to final form. Now imagine, just imagine, how likely would it ever be for all of those random particles to become aligned, where the particles of one are all aligned with each other, and the particles of the other are all aligned with each other, and then both are organized into straight, parallel lines, that are also evenly spaced, and repeating. And not only that, but they are organized into this pattern on two planes, the horizontal plane as well as the vertical plane. What I'm trying to point out is that coin planchets have thickness as well as width and breadth. And for a pattern like that to happen - everything would have to somehow become organized and lined up just so. Does anybody even remotely think that something like that is likely to happen ? I mean I suppose it's possible, but how probable is it ? Things like this are what have always made me question the bad alloy theory being used as an explanation for coins like this - where everything is lined up just so. I mean if the pattern were random, broken up into different shapes and sizes, not perfectly straight and parallel, and evenly spaced, and repeating - well then yeah, I could easily go with the bad alloy theory being the cause. But when they look like this - unh uh, I aint buying it. And none of what I'm saying has anything to do with what you're talking about. Sure, what you're talking about can happen, does happen. But it's not the explanation for a coin like this. And Jeff, for your benefit, this is an example pf me trying to point out to you that you're not the only one who exercises skepticism on a regular basis
Appears to have rested on or against a piece of cloth; possibly impregnated with some mildly acidic residue over a period of time. Do both sides of this coin show this lightly-etched appearance? In metal detecting, one comes across all kinds of weird environmental blemishes; typically just on one side of the coin. One common type of marking is when you unearth a "pocket drop" and the coins are somewhat corroded/discoloured, but brighter and even shiny where they have lain, stacked on top of each other; so one side of your quarter is brown all over and the other side has a dime-sized bright spot from the coin that was lying against it.
