Can you scrub clay off a mug?

Is this safe to drink from? (clay, mug, glazed)?

• So I made a mug and it is glazed. I used green-ware clay, I'm not sure if it's safe to drink from. The mug ended up having bubbles from the glaze. So there are small hole where fired green-ware clay is showing. I guess it's because it popped when it was getting fired in the kiln room. So is it safe to drink from it?

• Answer:

  The only way to know if it is safe to drink from, is to know the chemicals and their amounts in the glaze. These days lead is not usually part of a glaze for functional ware, especially if it was fired at a school. So it is probably safe. However there are sometimes glazes that are soft and leach out heavy metals (chemicals in the glaze, usually the colorants). The best way to know if there is a problem is to test the glaze. As far as the bubbles, if they are tiny, it is a glaze defect called pin holing. It can happen when gases are released during the firing through the glaze, and there isn't sufficient time for the glaze to smooth over and "heal." This shouldn't be a problem. If the bubbles are bigger, the size of a pencil eraser, then they are called blistering and they could be sharp and possibly cut you (see photo on link). With the clay showing through the bubbles, it could be harder to clean. So be careful when you wash your mug. I teach and have fired kilns for many years. So if this happens I fire the piece again and hold it at the end temperature to heal the blisters or pin holes. But sometimes re-firing doesn't work, especially if it is that particular glaze combination that is acting up. You may want to use your mug as a pencil holder on your desk and make a new mug if the bubbles are bad. Hope this helps.