My brother does pottery and recently showed me some of his recent work. He has some finishes like this. He was explaining that he will bury the pot part way in sawdust in a barrel to fire it. Before he starts it he will pour different substances around the pot to give it the color. One of the things that he uses a lot to pour around the pot is Miracle Grow fertilizer. I have some pots on order from him but as I won’t be needing any until next spring I told him there was no rush to get them.