I have imported the excellent walnut material:
http://matrep.parastudios.de/index.php?action=view&material=84-walnut&fc=1
Appended it into my file, made a plane, put the walnut mat on the plane with default light. It rendered fine. But after closing the document and re-opening it renders much lighter than the preview. Why?
I have tried several lighting setups but nothing seems to work.