As thou pliest thy trade in this devil's-smithy— (3)
We think Browning is giving us a kind of skeptical look at science here. Not that he's saying all science is bad, or even necessarily that what's going on here is science. Still, in this poem, the laboratory is the place where bad things happen. That location, which we see in the title, is almost immediately compared to a devil's workshop. So it's hard to imagine that this poem doesn't have something to say about the possible dangers of scientific work.
Grind away, moisten and mash up thy paste, (9)
Even though this line doesn't have much to do with the big emotional themes in the poem (jealousy, revenge, death, etc.), it's full of the kind of details that makes the whole thing come alive. In a way, the work of science, of mixing and testing in the laboratory, is like the canvas that the rest of the themes are painted on. All this grinding and mashing helps to anchor the poem, to give it a convincing sense of place.
That in the mortar—you call it a gum? (13)
Our speaker is unusually curious about how this is all going to get done. She's not just swinging by to pick up a ready-made poison. She wants to see how it's made, what the ingredients are. She even wants to learn the vocabulary, to know the names of things. This fascination with scientific instruments and materials is definitely tied up in her revenge fantasy, but we think it makes her a more interesting and multi-dimensional character, too.