A specimen of the textile (wool textiles in all forms) is treated in a formic acid buffer solution to which solutions of sodium dichloroisocyanurate and sodium hydrogen sulfite are added succesively, and is then rinsed and dried. The change in colour of the specimen is assessed with the grey scale. This treatment simulates the manufacturing operation in which a liquid containing or liberating active chlorine under mildly acid conditions is used for imparting shrink-resistant properties to wool in textiles.