27 CFR § 30.31 - Determination of proof.
(b)Solids content not more than 600 milligrams. Except as otherwise authorized by the appropriate TTB officer, the proof of spirits containing not more than 600 milligrams of solids per 100 milliliters of spirits shall be determined by the use of a hydrometer and thermometer in accordance with the provisions of § 30.23 except that if such spirits contain solids in excess of 400 milligrams but not in excess of 600 milligrams per 100 milliliters at gauge proof, there shall be added to the proof so determined the obscuration determined as prescribed in § 30.32.
(c)Solids content over 600 milligrams. If such spirits contain solids in excess of 600 milligrams per 100 milliliters at gauge proof, the proof shall be determined on the basis of true proof determined as follows:
(1) By the use of a hydrometer and a thermometer after the spirits have been distilled in a small laboratory still and restored to the original volume and temperature by the addition of pure water to the distillate; or
(2) By a recognized laboratory method which is equal or superior in accuracy to the distillation method.
(d)Initial proof. Except when the proof of spirits is used in making the guage prescribed in § 19.353 of this chapter or in making a gauge for determination of tax, the initial determination of proof made on the bonded premises of a distilled spirits plant for such spirits may be used whenever a subsequent gauge is required to be made at that same plant provided that no material has been added to change the proof of the spirits.