PURPOSE: To dissolve and remove ferrous materials thoroughly in a short time by dipping the hardly machinable materials adhered with the ferrous materials in nitric acid contg. Rochelle salt and boiling the same.

CONSTITUTION: The hardly machinable materials on which part of the ferrous materials of cutting tools consisting of ferrous materials are adhered by heating in cutting of the hardly machinable materials by said cutting tools are dipped and boiled in a solvent prepd. by adding Rochelle salt into nitric acid kept at 10W30% concn., whereby the ferrous materials adhered thereon are dissolved away thoroughly in one time of the treatment. The Rochelle salt acts to accelerate the reaction of the ferrous materials and the nirtic acid by dissolving the scale consisting of the insoluble oxide produced during the dissolution of the ferrous materials such as W, Mo, or the like by the nitric acid and the amt. of said salt to be added is sufficient with about 50W150g to 1l nitric acid.