I have recently been using a Yoder offset smoker. I have developed an issue with residue in the firebox. The interior of the firebox has a normal black color, but when I try to wipe it down I get smears of a brown residue that wasn't visible prior to wiping. This problem is only in the firebox - the cooking chamber is seasoned and has not has any issues. Im thinking it is possibly creosote, but am unsure. Is there a good way to remove this? Water somewhat works, but I am reluctant to wet down the firebox due to rust concerns. Any help would be appreciated.