Waterfowl plumages are covered by fat and oil. The fat and oil can be removed during the washing process. Therefore, the fat & oil rate is another important indicator of down quality after wash.

Method of measurement: The Soxhlet method is generally applied in all kinds of feather & down test methods standards. The technique is dissolving the fat from the plumages in some organic solvent (e.g., ether or diethyl ether). After the solvent has vaporized, weight the fat left inside, then calculate the fat & oil rate.