I think you are totally correct but now I'm stuck trying to find if anyone really knows about the formation of scratch marks inside a mug. Is it really the metal from the spoon depositing on the mug?

this seems plausible, as there are often darker marks left on white ceramics from spoons.

> Is it really the metal from the spoon depositing on the mug?

Probably, it's additive for sure, you can remove them quite easily: https://www.reddit.com/r/CleaningTips/comments/11z7get/thoug...

Set glazing basically is glass, you wan't scratch that very easily

Here's the safety data sheet of the product:

https://www.barkeepersfriend.com/wp-content/uploads/2014/12/...

Here are the ingredients:

Feldspar

Sodium dodecyl benzene sulfonate

Oxalic acid

Long ago I worked in a restaurant. One day the manager ordered new silverware and we were told to inspect and throw the others away. I was stuck by how much different the forks and spoons were. The forks were shorter by a noticeable length and bent in a different direction. The spoons had obvious wear marks on the bottom and the shape was thin and deformed.

Yes, it is scratched off metal (you see it also on bathroom tiles if you use a bathroom wiper with aluminum frame for cleaning the shower tiles) or from using cast aluminum pans with machined bottom on glass ceramic cooktops (you get little aluminum deposits on the glass).

It is reversible and you can "wipe" it off using acid and abrasives but it's tedious as the metal adheres really good to the ceramic.