To add to Roger's question: I had an unembossed Canadian beehive with what I called surface milk. I'm no expert. I called it that because that's what Ken Gardner called it when I bought it from him. Chris McClelland has it now. The picture actually only shows about half of it. It swooped down from the crown, looped around the skirt and then swooped back up to the crown again. There are three distinct parallel "stripes" that are rough-textured recesses in the glass.

The reason I know it's not a scratch or an erosion is that it continues on the surface through the wire groove. It seems like it must have been a viscous substance that was on the surface of the molten glass, or something that spilled into the mold. I suppose the molten blob could've been scraped by something on its way to the mold and the molding process didn't manage to smooth it out. The surface of this insulator is otherwise as smooth and shiny as they come.

View the large version on this one.