To enhance the whiteness of fabrics optical whiteners are also used but exposition to sunlight degrades them. Optical whiteners absorb invisible ultraviolet light, emitting visible light and thus increasing the total reflectance of the white surface, giving a luminous appearance. Ultramarine Blue works differently by absorbing the yellow light, avoiding its reflection.
Ultramarine Blue is caught mechanically in the fibre structure, meaning that a physical introduction in the fabric is done, while the optical whitener has a surface incorporation. Because of the lowest particle size of the Ultramarine Blue and its water affinity it can be eliminated with further washings, forming a little structure with repeated washes and rinses. For this reason and for its high light resistance Ultramarine Blue is better than optical whiteners for this application, and moreover they tend to degenerate optically, when they are exposed to light, producing by themselves yellow hues.
There is a synergy between optical whiteners and Ultramarine Blues since both complement their actions and give good results. They are two products totally compatible that join their whitening effects and cover a wider spectrum under solar and artificial light.
The same principle that explains the use of Ultramarine Blue in detergents is applicable to the use of such pigment in laundry bar soaps. In the latter Ultramarine Blue is incorporated in a typically blue-colored bar soap which is used to wash fabrics. The friction of the bar soap onto the textile: on one side, it cleans the fabric thanks to the surfactants it contains; on the other side it allows small Ultramarine Blue particles to be mechanically captured by the fibre structure and absorb yellow wavelengths.