A hydrometer scale used in the leather industries to describe the specific gravity of tanning solutions. Symbol, Bk. One degree Bk is equivalent to an increase of 0.001 in specific gravity, and the zero point of the scale (0° Bk) is at specific gravity 1.000.
Thomas C. Thorstensen.
Practical Leather Technology. 3rd ed.
Malabar, Florida: Robert E. Krieger Publishing Co., 1985.
Page 315. The author states the definitions are from Federal Test Method Standard No. 311, January 15, 1969.
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Last revised: 3 March 2010.