A math node is not necessary to convert to gray scale. Plug a color data stream into any number, including the bump channel, and it becomes a number, I.e. gray.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)