The ability of a pigment to resist fading. Until independent standards began to be adopted by many manufacturers in the late 20th century there was much misinformation. Vested interests caused some paints to be claimed to be permanent when they were not. ASTM and the Wool Scale have dramatically improved the situation. Color ‘dethroned’ in this process in recent times include the Madders and Alizarin Crimson. Most but not all Earth and mineral pigments are very reliable but organic reds and yellows have always been troublesome. ASTM 1 is generally considered to be the equivalent of 100 years of museum light without significant change or better, while ASTM 3 is equivalent to only 25 years which is not as good as some photocopy inks. ASTM 2 is intermediate. In practice the best Earth colors, Ultramarine, Vermilion and White Lead show little change after 500 years.
