Bioburner
Moderator
If the corn is dry one has little issue with the black crud but corn has a ignition temp that is about 200 degree higher than wood. Combine that with some higher moisture(corn often sold above 13%)and your fighting a lower burn temp that start the dark gunk.Thanks for the Info...
I Imagine the black grease substance comes from sugar in the corn..
Picture is the stove setup for straight corn burning. Notice the nice gray showing proper burn.