what causes parallel ridges on a coins surface?

Does anybody know what causes those parallel ridges (almost a ruffled potato chip look)? Ridges run at a 45 degree angle from the upper right to the lower left on the coin below. Can a coin's surface fold/distort when being struck? Ridges only appear in the field and not on the devices.

 

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Polishing lines on the die used to strike the coin?

Nice toning, BTW.

 

Could it be from the rollers used to make the stock for the blanks?

 

Could be that, too.

 

I would say it's on the blank before striking. When the metal flows to create the coins information, it's erased. Maybe something from polishing the blank itself? It sort of looks like a "Got Wood" coin.

 

plating issue...

 

Do they even bother with polishing copper plated blanks?

 

These are striation lines in the copper plating. They are basically the equivalent of linear plating blisters.

 

If we examined it up close, we could very well see them on the devices, but it's not uncommon for such plating lines to be absent or significantly diminished on the devices.

 

Thanks for all the ideas. Yes, I agree it's not die polishing lines.

 

This coin has a slightly similar look, but not as pronounced.

 

Except in cases of split plating(which is a separate matter altogether) it is rare to find blisters and bubbling that have broken. I would assume the reason for this is that they arise during the strike(presumably from the heat and pressures involved)-they are not on the planchet before the strike.

 

You could ask RWB

Sometimes he answers

 

RWB? Who the devil is that. Speak English, not acronyms......

 

Roger Burdette

 

Thanks Larry. Sheesh, old guys can't decipher.......