Do you have to glaze clay before you use it?

Clay pipe with glaze, can I use it to smoke?

• So I made a clay pipe in art and class and was wondering if I could use it to smoke, it's was made with clay that was fired in a killed, I'm not sure of the exact type though, the only major issue is that near where the flame hits the hookah there is glaze, it's colored black if that makes any difference and I'm sure an open flame will hit glaze, I've burned it before and it doesn't seem as though it burns or gives off fumes, but I could be wrong, I got all this clay from and glaze from a school, but I sort of made it on the side, never asked the teacher about specifics, I should have. Is it safe to use? or will the glaze be toxic? will a regular lighter even burn the glaze?

• Answer:

  The best thing to do is ask the teacher if all the glazes are good for functional ware -- pottery you can eat or drink from. If their glazes are all good, then your pipe will be okay. These days most schools do not have glazes with lead in them (lead is toxic). However sometimes, there are glazes that have a high percentage of Barium Carbonate, higher than recommended for functional ware for food. And there are some glazes that have softer bases, and are very high in heavy metals that could leach out. Both these situations are not good. As far as heating the glaze with a lighter, that is not a problem since it has been fired at a much higher temperature already.