There is still considerable contention about the precise definition of a melanin,
which has tended to reflect the in-tractability of many black or dark natural pigments toward analysis,
thus the following statement by Jacobson and Millot (1953) quoted by Prota (1992),
“although chemical
tests specific for melanins are nonexistent, it is possible to
identify, at least provisionally, a naturally occurring brown
or black pigment as a melanin, provided that it shows certain
characteristics, namely resistance to solvents, bleaching
when subjected to the action of oxidants, and the capacity to
reduce directly ammoniacal solutions of silver nitrate”