Does ceramic leach or degrade over time?

[kenny1999]

I have a cup that I like it very much, I'm not sure about its exact composition but it should be made of ceramic. It has been used for a few years but I start to find some stain spot of pale black color in the interior of the cup but they shouldn't be stain from my drinks, they are not removable even with heavy-duty scrubber. I wonder what they are.

[MigL]

The glaze is a 'glass', but sometimes develops little hairline cracks which allow the staining of the porous substrate.

[kenny1999]

1 hour ago, MigL said:

The glaze is a 'glass', but sometimes develops little hairline cracks which allow the staining of the porous substrate.

Does it only affect the appearance? Will it leach anything into the drinks?

Edited December 25, 2022 by kenny1999

[mistermack]

The black staining might be fungal or bacterial residue, if the glaze has hairline cracks. Filling it with boiling water would sterilise it, but probably not change the appearance. And the quantities will be so small that it's unlikely that it harbours any health risk. 

[Phi for All]

8 hours ago, kenny1999 said:

I start to find some stain spot

Are you sure it's a stain, which lies on top of the ceramic and should come off with some effort, and not a small chip showing a darker clay beneath? If you use a probe (skewer, chopstick, etc), can you tell if it's concave like a hole?

8 hours ago, kenny1999 said:

of pale black color 

Pale black? Should we assume the ceramic coating is white, and these spots are darker? I have a set of red latte mugs with a white coating on the inside of the mug, and one of those has 2 small chips in the bottom inside that look like dark spots. Not sure if it was fired badly, or if I might have dropped a spoon too hard on the bottom, but they're not stains.