This is based on making pottery as a hobby: Most craft potters working in earthenware use lead glazes as the colour response is more appealing than what can easilly be achieved with other low-temperature fluxes. It's also more resistant to crazing, and for earthenware to be functional it's important the glaze doesn't craze significantly as earthenware is slightly porous- if the glaze is crazed then your coffee cup will leave a damp mark on the desk.

(Lead is not used on stoneware or porcelain.)