Short answer "Yixing purple sand"
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The explanation is the four characters "Yixing Zisha", not a person's name. These four characters are in small seal script. The pictures above are all upside down and relatively blurry. Turn the picture upside down and trace the words to see clearly:
Click "right up and down, left up and down" to read.
Dictionary Online Calligraphy Dictionary----Yixing Zisha: