The Collaborative International Dictionary of English v.0.48:
Whitening \Whit"en*ing\, n. 1. The act or process of making or becoming white. [1913 Webster] 2. That which is used to render white; whiting. [R.] [1913 Webster] Whitening stone, a sharpening and polishing stone used by cutlers; also, a finishing grindstone of fine texture. [1913 Webster]