Found a picture. Used to be Saw mill waste, now used for many things. Not the best firewood because it's mostly "sap wood" (the outside few inches of a many trees , it is soft & rots easy) Up side, it's thin & dries fast, & usually pretty cheap (sometimes free) Saw mill, Slabs:
WOW The fron of mine looked clean, but when I pulled it, it looked almost as bad on the chimney side. Bet you get much better performance when that ones cleaned too. LOL Good catch. You just find it like that ?
Mine was so bad I had to bypass the cat or the room filled with smoke. Do I win the ugliest cat award?