ISO 1833-5:2006 specifies a method, using sodium zincate, to determine the percentage of viscose, cupro or modal fibre, after removal of non-fibrous matter, in textiles made of binary mixtures of viscose or most of the current cupro or modal fibres and raw, scoured, kiered or bleached cotton.

Where a cupro or modal fibre is present, a preliminary test should be 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 or modal fibre is rendered incompletely soluble by the presence of certain permanent finishes or reactive dyes that cannot be removed completely.