Spalted walnut sapwood can be just gorgeous. I turned a few walnut crotch bowls recently for a church auction and I left a bit of the (slightly) spalted sapwood from the top of the crotch. The wood was so beautiful I have many requests for more.
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I didn't do anything to spalt this. The logs sat in the bed of my truck for a few weeks before I roughed out the blanks, so I assume the spalting was already there. Some of the walnut I've turned has sat on the ground for several months without any heartwood spalting, but the sapwood seems to spalt pretty quickly.