Pharma Desiccant providing DuPont Stain for Fibre Identification Stain No.4 as Testing Solution for pharma. As per the USP 670 DuPont Pharma Solutions, fibre fillers such as cotton, rayon and polyester coils, need to be passed through the identification tests like Dupont Fibre Identification Test (testing solution). As a result, different fibres give different colour variations, with the identification test.