[Code of Federal Regulations]
[Title 27, Volume 1]
[Revised as of April 1, 2003]
From the U.S. Government Printing Office via GPO Access
[CITE: 27CFR30.32]

[Page 740-741]
 
            TITLE 27--ALCOHOL, TOBACCO PRODUCTS AND FIREARMS
 
 CHAPTER I--ALCOHOL AND TOBACCO TAX AND TRADE BUREAU, DEPARTMENT OF THE 
                                TREASURY
 
PART 30--GAUGING MANUAL--Table of Contents
 
                      Subpart D--Gauging Procedures
 
Sec. 30.32  Determination of proof obscuration.

    (a) General. Proof obscuration of spirits containing more than 400 
but not more than 600 milligrams of solids per 100 milliliters shall be 
determined by one of the following methods. The evaporation method may 
be used only for spirits in the range of 80-100 degrees at gauge proof.
    (b) Evaporation method. Evaporate the water and alcohol from a 
carefully measured 25 milliliter sample of spirits, dry the residue at 
100 degrees centigrade for 30 minutes and then weigh the residue 
precisely. Multiply the weight of the residue by 4 to determine the 
weight of solids in 100 milliliters. The resulting weight per 100 
milliliters multiplied by 4 will give the obscuration. Experience has 
shown that 0.1 gram (100 milligrams) of solids per 100 milliliters of 
spirits in the range of 80-100 degrees proof will obscure the true proof 
by 0.4 of one degree of proof. For example, if the weight of solids 
remaining after evaporation of 25 milliliters 0.125 gram, the amount of 
solids present in 100 milliliters of the spirits is 0.50 gram (4 times 
0.125). The obscuration is 4 times 0.50, which is two degrees of proof. 
This value added to the temperature corrected hydrometer reading will 
give the true proof.
    (c) Distillation method. Determine the apparent proof and 
temperature of the sample of spirits and then distill a carefully 
measured sample in a small laboratory still, and collect a quantity of 
the distillate, 1 or 2 milliliters less than the original sample. The 
distillate is adjusted to the original temperature and restored to the 
original volume by addition of distilled water. The proof of the 
restored distillate is then determined by use of a precision hydrometer 
and thermometer in accordance with the provisions of Sec. 13.23 to the 
nearest 0.1 degree of proof. The difference between the proof so 
determined and the apparent proof of the undistilled sample is the 
obscuration; or
    (d) Pycnometer method. Determine the specific gravity of the 
undistilled sample, distill and restore the samples as provided in 
paragraph (c) of this section and determine the specific gravity of the 
restored distillate by means of a pycnometer. The specific gravities so 
obtained will be converted to degrees of proof by interpolation of Table 
6 to the nearest 0.1 degree of proof. The difference in proof so 
obtained is the obscuration.

(Sec. 201, Pub. L. 85-859, 72 Stat. 1358, as amended (26 U.S.C. 5204))

[T.D. ATF-198, 50 FR 8535, Mar. 1, 1985, as amended by T.D. ATF-381, 61 
FR 37004, July 16, 1996]

[[Page 741]]

                        Determination of Quantity