The paper is examined for dirt and grit. A new piece of paper is interposed between thumb and blade and the sequence is repeated until the paper comes up clean. The proceedure is repeated for the other side. Once the blade is cleaned of all loose dirt carefully examine under strong magnification. Note any cracks, marks or plastic deformation especially around the area covered by the habaki.
Plastic deformation occurs when a stress is applied beyond a material's elastic limit.
Excess loading results in slipage of the crystals under load. This is evident on a polished metal surface as waves or ridges when lit at a low angle under high magnification. Not only does this compromise the functional characteristics of the piece but such areas are less resistant to corrosion. Sometimes a sword blade will be encountered, which has previously but not recently been cared for. The oil will over time, have congealed on the blade until it becomes a semi solid coating.