Coffee cup gray/black/blue line/dark brown

Please refer to the relevant usage instructions and precautions before using the microwave or dishwasher.
After prolonged use, small cracks (penetration) may appear on the surface of the vessel, but it does not affect normal use.
Containers with a matte texture may sometimes be stained with oil or color from food.
When purchasing stoneware products, there may be some penetration.