ISO 1833-6:2007 specifies a method, using a mixture of formic acid and zinc chloride, to determine the percentage of cotton, after removal of non-fibrous matter, in textiles made of binary mixtures of viscose or some cupro, modal and lyocell fibres with cotton. If a cupro or modal or lyocell fibre is found to be present, a preliminary test is carried out to see whether it is soluble in the reagent. The method is not applicable to mixtures in which the cotton has suffered extensive chemical degradation, nor when the viscose, cupro, modal or lyocell fibre is rendered incompletely soluble by the presence of certain permanent finishes or reactive dyes that cannot be removed completely.