When the thermal paper is placed in an environment above 70°C, the thermal coating begins to change color.
After the thermal paper is heated, the leuco dye reacts with the developer to produce color, so when the thermal paper is used to receive the signal on the fax machine or print directly with the thermal printer, the graphics and text will be displayed. Since there are many kinds of leuco dyes, the colors of the written writings are different, such as blue, purple, black and so on.
