Carbonization uses sulfuric acid or similar treatments to char and remove vegetable matter (seeds, burrs, grass) embedded in the fleece, which is essential for producing clean worsted tops from heavily contaminated clips. Degreased-but-not-carbonized wool still contains vegetable matter and requires either carbonization or careful blending to manage contamination in the final yarn. Processors must factor in the additional carbonizing step — and its associated fiber weight loss and cost — when pricing degreased non-carbonized wool.

Residual grease (or 'residual fat') content in scoured wool is typically measured as a percentage of oven-dry fiber weight using solvent extraction methods, with international standards such as IWTO test methods commonly referenced in trade contracts. Higher residual grease increases the risk of processing problems in subsequent dyeing or combing and reduces the effective clean fiber yield. Most commercial contracts specify a maximum residual grease percentage, and disputes over out-of-specification deliveries are a recurring issue in the scoured wool trade.