Clinkers refer to the solidified residue left after burning coal or wood. There are a number of synonyms that can be used to describe clinkers, such as cinders, slag, ash, scoria, and clots. Cinders are the fine and powdery residue left after combustion, while slag is the melted residue that solidifies as it cools. Ash, on the other hand, is the residue left behind after the combustion of organic matter, and scoria is the slag that forms when lava cools. Finally, clots refer to the lumps or chunks of solidified residue that are too large to be considered cinders. Using these synonyms can add variety and nuance to writing.