I read this somewhere on this forum. To darken leaves you can dampen them and wrap in foil. Then bake at 220 deg. for 2 1/2 hours. I tried this and it worked. Maybe it would get darker if roasted longer but maybe not since the leaves dry out. The leaves baked very dry and were difficult to separate after they were roasted. You have to rehydrate them in the foil before trying to remove them. Thought I'd post a photo to let you see the results.