Lightfastness is a key issue for the serious artist. It is well known that, over a period of time, colours will fade or change, particularly when exposed to environmental factors such as light, heat, dust, humifity etc. That's why Derwent continually test the lightfast rating to give you the best possible performance for each colour.
The test we use at Derwent is the ISO 105 Blue Wool test, which was originally developed for the textile industry. A scale of 1-8 is obtained, with values of 6 or more being considered GOOD and will last in excess of 100 years in gallery conditions.