Manufacturing Variations in ISO Blue Wool Fading Standards under Microfading Exposure Conditions

As is the case for other accelerated light-ageing methods, microfade test results are usually rated against the responses of the International Standards Organisation ’ s Blue Wool Fading Standards, the production of which is regulated by the International Standards Organisation. The suitability and convenience of the ISO Blue Wools for this purpose have been questioned on various grounds, including wavelength sensitivity, manufacturing or batch response variations, and the e ff ect of their woven texture on systematic errors. In the present article, the e ff ect of manufacturing variations was assessed by carrying out microfade testing on samples of ISO Blue Wools from various suppliers and manufacturers spanning more than 20 years. Our results con fi rm that the fi rst three ISO Blue Wools, which are of most relevance to locating colourants at risk of signi fi cant light-fading on display in museums, had measurable batch-to-batch and/or manufacturer variations in fading rates, most signi fi cantly in the case of Blue Wool 2. Di ff erent batches were often also distinguishable by their colour and the texture of the wool fabric. It is argued that while texture-related systematic errors and manufacturing variations are inconvenient, they are not signi fi cant in light of the other potential sources of disparity between the results of accelerated exposure tests of any kind and fading at ambient light levels, and it is concluded that the advantages of the use of ISO Blue Wools as internal standards for microfade testing outweigh their disadvantages.