Both pieces must be clean, clean, clean, clean/ie; no oils - degreased completely - perhaps a few more 'cleans' to emphasise the cleanliness needed. No finger prints allowed- for a perfect solder joint. "Hard" silver is quite easy with the correct flux.
Many of us have had very good luck soldering up sights using low temp silver - 4% silver, high % of tin- no lead, I think it is. That is what I used on my 14 bore rifle's front sight in 1986. It has a low bead of Purdey's pattern and has not failed - yet. The bead is a tapered piece of brass rod, soldered to a low angled vertical rib rising up from the steel base.