Not haveing recently read McCreights instructions, I was surprised
by the thought of mixing Ammonium chloride into the process. I can't
quite see how that would help. It's normal use (for me, at least) is
as a refining flux, where it works by converting the baser metals to
chlorides, which are generally insoluable and collect in slag,
leaving purer melted metal with fewer undesireable impurities..
Somehow, that doesn't seem like an effect I want to happen while
applying neillo, considering that lead is one such base metal that
would react with it... I recall watching the late Phillip Fike do it,
and he applied it either by spreading the dry powder or just feeding
in a stick, like soldering with heavy wire solder or plumbers solder.
The main point was the channels to be inlaid were well coated with a
liberal amount of Handy paste flux. No air anywhere near the metal.
The niello flowed nicely under the flux, just like solder would do,
though less liquid perhaps. Too much was applied, overfilling the
channels, and then the surface filed or abraded to a proper level
again. Then the work was gently heated and the niello surface
burnished with a standard steel burnisher.
Hope that helps.