It is a crack or fissure in the enamel coating on the cup, not indicative of deep structural flaws.
Your cup is unlikely to fail in the sense of completely breaking due to the craze in the glaze.
On the other hand, they will stain over time, and be unsightly, and hard to wash out.
Why do ceramic mugs crack?
Lead can leach into mugs when acidic drinks are used when you heat the coffee mug in the microwave or because of lip or rim release when sipping. This becomes more pronounced when ceramic mugs are chipped, old, worn or the glaze is cracked.
How do you fix a crack in a coffee mug?
Rebuild your broken coffee cup handle.
Apply glue to one of the broken pieces, then press the matching piece to it. Press firmly and hold for 60 seconds to get a nice strong bond going. Repeat until you have a completed mug handle.
Is it bad to drink from a cracked mug?
Although not proven to be dangerous in finished form, a crack, chip or scratch along the inside or the lip portion of a plastic mug can emit trace amounts of plastic substances, such as bisphenol A, or even flake off fragments into the liquid, making the mug unsafe to use.
Why do plates crack?
Although it doesn’t happen very often, microwave turntable/cooking trays do sometimes break. A few common reasons this may happen include small cracks or chips in the plate or uneven heating of the glass. Most microwave trays or plates are made of tempered glass, but they still have their limit.