That is just enamels on the surface that have not been completely fused. Maybe an enamel made to leave that effect.
Ach, again this confusing use of "enamels"! I have to say, even if
this is a correct use of the term "enamel," I don't see why it's necessary when most people think of something much different when the see the word. What do you mean by enamel here?
I'm thinking it probably is fine frit, just not fused, as Frank suggests. The piece was probably rolled in it when it was still hot enough to pick it up, but not hot enough to melt it. I think the "bumpy" stuff you're thinking of, Anne, may be overshot glass, where the frit is applied and melted. The fact that it's uneven and the lines look like they have more texture in them argues against sandblasting. Thanks for splitting this thread off - makes sense! I was out of town for a few days, but now I'm back, and will put some examples of surface treatments in the gallery in the next couple days.