Blue wool scale

In many applications ultramarine blue is stable to around 400 °C, violet to 280 °C and pink to 220 °C. All have excellent light fastness with a 7-8 rating (full and reduced shades) on the International Blue Wool Scale. Color fade attributed to light exposure or moderate heat is almost always caused by acid attack. Ultramarines react with all acids, and if there is sufficient acid, the pigment is completely decomposed, losing all color, to form silica, sodium and aluminum salts, sulfur, and hydrogen sulfide. Evolution of hydrogen sulfide with acids is a useful test for ultramarine. 

Another lightfastness category is the Blue Wool Scale, which includes eight levels from 1 (poor) to 8 (extremely lightfast). 

Lightfastness Post treatment Base pigment Type of preparation Based on the blue wool scale (11) As disclosed by the manufacturer... 

Wikipedia, Blue wool scale — wikipedia, the free encyclopedia, 2012. 

Attempts have been made to replace the Blue or Wool Scale by physical devices that measure the radiation of the light source. To determine the radiation of a given light source, the irradiance (in W/m2) is multiplied by the exposure time (in s). The equipment necessary to carry out such measurements is commercially available. Measurement in the UV region would be desirable. It has recently become possible to purchase an instrument which can measure spectrum-related irradiance in the important UV range. 

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