Naturally white ostrich feathers and bleached grays, like all material
taken from the animal realm, retains even after scouring and bleaching
a more or less noticeable yellowish tint, which becomes visible after
some time even upon such as appear snow-white immediately after
scouring. The cause is, probably, that the fat which is contained in
feathers, as well as in wool and hair, and a part of which remains
after the cleaning process, is oxydized by the action of the air.
To perfectly and completely extract this remaining small amount of
fat which does not hinder the subsequent dyeing operations, is not
advisable; for, it has been observed that in that case, the feathers
become brittle, and for this reason, scouring with soap is preferable
to scouring with soda. It has been observed that colored matter
taken from the animal body in the course of time loses its brilliancy
of color and becomes dull; white feathers turn yellowish, even if
perfectly protected against dust. To prevent this alteration, the
bleached feathers are "dyed white," or rather blued or tinted; that is,
the brilliancy of the white is heightened and the faint yellowish tinge
neutralized by the application of a very light, scarcely perceptible
tint of a complementary color.
For this purpose, for instance, indigo carmine (greenish white), indigo
carmine with a very small addition of ammoniacal cochineal (reddish
white), induline or extract of indigo (bluish white), or methyl violet
6 B. (direct white), are employed, and a few drops of acid added to
the bath, either sulphuric, acetic or oxalic acid. The acid, however,
can be dispensed with, as it scarcely has anything to act upon, and
as only a diminutive amount of it is employed, the blued feathers are
not rinsed but immediately dried from the blue-bath. The additions
of dyestuff to baths must be made so small that they do not affect a
coloring of the feathers but only a faint tinting. Rather too little
dyestuff may be added, which defect can at all times be corrected by
adding a few drops more of the coloring solution, than too much. In
the latter case it would become necessary to bleach the feathers again
and go over the whole process of preparing the raw material for dyeing.