Bill,
I've heard that spalt can spread, after it dries, but I tried to make my own spalted wood and it didn't work. One thing you should know is that the flame turns brown, or green after a time. At least that's what I've heard. I have a nice curly box elder chunk, with green colored spalting. It's waxed and had a twin, but it cracked into big pieces, as well as molded into my desk top, but that's another story.
Rob