Often, the lid on whatever jar in the centre of the pot under the gauge comes out looking as through it’s been through a war zone.
It will happen both to standard single-use metal lids, and to the resusable Tattler lids.
It cleans off the Tattlers with the wipe of a damp cloth; it can permanently tarnish a metal lid but as they are use and toss anyway, it doesn’t matter.
It does not affect the quality of the seal, or the product inside the jar.
Still, we were curious why it happened. We wrote to the maker of Presto pressure canners and received this answer,
Our Home Economist has seen this before and feels it’s the plating from the gauge. She agrees it’s not harmful, but it’s sure unattractive. She said this will subside over time.”1
It hasn’t been our experience that it has subsided, but again, it’s only a minor cosmetic issue.
Presto Customer Service to Randal Oulton. 2 September 2016 ↩