Linear plating bubbles.

[Arthur-Dent]

Has anyone figured out why these occur ?
My best guess is the same phenomenon of roller lines on copper cents.
They're still there, just plated ?

 

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[PetesPockets55]

Yes, that is my understanding of part of the cause. I don't believe any type of gas is actually causing the copper to bubble.

Contaminants from the rollers get on the zinc as it is rolled out. These can cause the plating to not adhere properly. Temperatures can make the zinc and copper expand and contract differently.

For those that are interested, a couple of years ago I found a video from Jarden Zinc, who produces the copper plated cent for the US mint to use to strike Lincoln cents. Here is the link (from Dec. 5,2016). It even shows the burnishing process for producing the mirror like finish on proof coin blanks.