Hi Jan-Peter, I have no personal experience with niello, but it is
high on my list of experimental priorities. It is used most often in
Asia. The oldest known receipe is due to Plinius who lived 23-79 A.D.
and it is free of lead, unlike a host of later receipes. He used: 3
parts fine silver, 1 part red copper, 3 parts sulphur. This is a
translation of what he wrote down about his technique. 'One melts
(heats) silver with one third part Cyprian copper in an earthen
melting pot covered with a lid, which has been closed off with clay,
until the lid starts to rise. Silver can also be coloured black with
egg yolk, but such colouring can be subsequently removed using chalk
and vinegar." Adding sulphur to the metal alloy has two effects: the
formation of silver sulphate with its saturated back colour, and it
enables you to process the alloy as an enamel that is quite elastic.
Just a historical note that I retrieved from a book (in Dutch).
Good luck, Philip Quanjer