The depth of pencil color is different models for different writing purposes. From the time that graphite was found to be used to write paintings, pencils actually have a history of nearly five hundred years and have undergone continuous improvement. Typical pencil refills are now made of graphite powder mixed clay, and the outer layer is a protective wooden shell. According to the different mixing ratio of graphite and clay, the hardness and hardness of the refill are not the same, from the softest 10B to the hardest 6H, B is the meaning of "black", that is, the higher the number B-type refills are softer and darker, and can be used to draw sketches or fill in answer sheets. Conversely, H is Hard. The higher the H number, the harder it is and the lighter it is. It can be used for writing or contouring.

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