Thanks for the comments. Alas, they haven't helped! It's nothing to do with textures (this happens when I remove them, too). And it's not just Firefly. It happens when I use the Poser 4 renderer too (I'm using Poser 6, by the way). It's nothing to do with raytracing either because I haven't got that on. Anasta, I don't have an Irridance Caching option, but fiddling with the others you mention doesn't seem to help.

Incredibly, though, the Sketch render options do not have the ink stain! Which is completely useless for what I'm trying to do. Ah well...