Bar Keepers Friend cleanser can remove old coffee stains from ceramic mugs
I have some mugs that are 25 years old. I took it for granted that the coffee stains were permanent. Then I tried scrubbing the bottom with dish soap with no improvement. Next was toothpaste. I hoped the mild abrasive would help. If it can restore headlights I hoped it could restore mugs. Still, no improvement. Finally I tried Bar Keepers Friend. This has a very fine abrasive. It worked well. Here are two pictures taken before and two after cleaning.
I would also have used Bon Ami. I’m not sure I would use Ajax or Comet but please comment if you had success. Finally I would not scour the outside because I think it would remove the decoration.