The Fowler 52-760-000-0 Hardness Tester File Set is an inexpensive, easy-to-use method for determining material hardness by sliding the file across the object's surface. Conveniently packaged in a leath­er-like case with a Velcro-lined flap, the files are approximately 0.157" in diameter and 7" in length. Six color-coded files indicate hardness from 40 to 65 HRC. Run the Black file (65 HRC) over test material. If it marks the material, it is less than 65 HRC, try the Blue file (60 HRC). If it does not mark the material, it indicates softer material between 60 and 65 HRC. If the Blue file (60 HRC) marked the material, continue testing with the Green (55 HRC), Light-Green (50 HRC), etc., until the material is not marked. Fowler, an American company, manufactures premium inspection and measurement instruments like electronic indicators, calipers, bore gages, and micrometers. The company, founded in 1946, is headquartered in Canton, MA.