

What is causing the black speckles on the foot?
Hello! I’m pretty new to pottery and made this piece recently. The clay itself didn’t have any noticeable flecks, and after both bisque and glaze firing the foot appeared a solid tan-ish color. I started using this for coffee and these dark speckles appeared. I’m not sure if they are some type of mineral deposits, mold, mildew, or (?). It has no smell and is not changed by scrubbing, bleach soak, etc. It has been microwaved a few times without any issues. The inside is completely glazed. Any feedback is much appreciated, thank you!! 🙏🏻