I have had fair success in creating a reducing atmosphere in the
kiln by using carbon to absorb the free oxygen. I simply placed
graphite sheets inside the kiln, lining the bottom and sides. I use
this for annealing gold, and the metal comes out both annealed and
shiny…oxide free. The graphite will soon begin to disappear and
will need to be replaced occasionally. I got a great deal on
graphite sheets on eBay.
For very fine wire or small parts, I place them in a covered tin,
like a watch parts can, filled with powdered charcoal. Again,
everything comes out shiny and soft.
I know that for furnace soldering, most ovens use anhydrous ammonia
to create a reducing environment, but this is obviously not
something you want to try in a kiln not built for that purpose.