This clinker is from the bark of the tree and the knots in the wood. The knots are where all the minerals collect, and bark is where the fugitive dust collects. When burned, the dust mixes with the minerals and crystallizes with the dust to form the clinker. Not a big deal, but rather the tradeoff for stick burner quality BBQ pellets. At Knotty Wood our goal is to make pellets that are stick burner quality. We recommend frequently cleaning your fire pot as this will help to prevent clinker from forming.