while this may be a one off piece, it is not silver fumed. This piece could have been made several ways, hard to say without seeing it in person. The first way it could be made is with a reducing color. the second way it could have been made is to actually blow the glass into a piece of metal(would have to be silver or aluminum or something), the third way is the glass could have been electroplated, the last way is with silver nitrate(usually on the inside though so i am guessing no on this) Silver fume does not actually make glass silver like that on the surface. My best guess is either the first or third way, but it is so hard to say with out seeing it.