The wrought iron and very low carbon steel components of the laminate dissolve at different rates when exposed to acids
The ancients "pickled" metals and pattern welded barrels using citrus juice and vinegar
Later hydrochloric, nitric and sulfuric acid
Some refinishers have found, much to their customer's displeasure, that ferric chloride at too great a concentration or applied for too long can etch so the surface of the barrels are roughened
I think these are Joe's barrels

[Linked Image from photos.smugmug.com]

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