Here's a second sinister symbol! (Argh, more alliteration! How awful! What have you done to us, Bobby Browning?) Just like the glass mask, the smoke in the lab lets us know that we're in a creepy place where bad stuff happens. Combined with the line about the devil's-smithy (3) this lets us know that the lab is a kind of underworld. It makes a perfect pair with the bright, glittering paradise that our speaker has come from.