A rubbing (frottage) is a reproduction of the texture of a surface created by placing a piece of paper or similar material over the subject and then rubbing the paper with something to deposit marks, most commonly charcoal or pencil, but also various forms of blotted and rolled ink, chalk, wax, and many other substances. For all its simplicity, the technique can be used to produce blur-free images of minuscule elevations and depressions on areas of any size in a way that can hardly be matched by even the most elaborate, state-of-the-art methods. In this way, surface elevations measuring only a few thousandths of a millimeter can be made visible.
https://en.wikipedia.org/wiki/Rubbing
https://en.wikipedia.org/wiki/Frottage_(art)
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