Has anyone ever heard of an oil based substance coming from driftwood while its being cured?

I found 2 pieces of driftwood on the shore of a local river near my house. I placed them both in my tub filled with hot water, enough to fully cover them. After a few hours i start noticing a film on the surface and it has the same look as an oil based substance would have. Its like a small amount of vegetable oil was poured into the water. If anyone can help me figure out what this is I would really appreciate it.