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The additional marking for pencil hardness allows determination of the slate pencil hardness, irrespective of the pencil state and the level of its being rubbed out.
The additional marking is provided by means of stripes (or other kinds of signs) along the pencil surface.
The stripe length and thickness depend on the slate pencil hardness. The softer the slate pencil, the thicker and shorter the stripe (the lesser number of the turns). This is clear and intuitive as softer slate pencils draw bolder lines and rub out sooner.
The marking is easily readable when pencils are in a pencil holder or in any position on a table.
Donetsk State College of Technology and Design