The color of ochres is due to the selective absorption in wavelength of the light that illuminates them. Only the unabsorbed light is diffused by the material and determines its color. On the other hand, the natural ore of ocher contains only yellow goethite that must be dehydrated to give red hematite. Such principle applies to the firing of ochres, the color of which varies depending on the temperature, going from yellow at 572°F (300°C), orange and red around 1,742°F (950°C), to purple at 2,282°F (1,250°C).