What problems do dark and medullated fibre contamination cause?

Both dark and medullated fibre contamination cause problems for the manufacturer.  A single dark fibre in a white/pastel fabric will appear as a thin dark line if lying on the surface of the yarn or as a dark smudge if it lies within the yarn structure.  In fabrics that are coloured, medullated fibres give a different (often white) appearance when dyed.  This problem is generally more pronounced as the depth of colour increases.  As a rule of thumb, the commercial limit for dark fibre contamination in tops destined for a white or pastel end-use is less than 100 dark fibres per kilogram (df/kg), with lower levels required for ultra-high quality products (less than 50 df/kg).