The wood I used was already starting to split, as it was destined for the wood burner. I chose pieces that had enough, just enough to work with. Originally had no intention of carving a kuksa until it dawned on me that the wood was spalted. So I took the plunge. At each stage of the carve I put the cup into a nylon bag with shavings that I'd removed that day. I think that helps with drying it out slowly.
Sorry, forgot to say that none of the 3 that I've carved from the same pile of firewood split any further than when started.