I used to work in a solder factory. We made the entire gamut of products from solder up through welding wire.
1) We used tin in with copper. Copper melts at a much higher temp. I never heard of tin burning off. The components included tin, antimony, copper and silver.
2) We used magnetic flux to stir our alloy as it was melting. However, the actual casting was done in a bottom-pour arrangement in a separate vessel with no stirring. A pour might last an hour.
3) The only time I heard of alloy being lost to the air was when we had an accident on the platform and an entire batch washed out. We estimated that 800 lbs of alloy vaporized instantaneously and got carried out through the ventilation. No deaths. 1 injury-- a guy lept from the platform to save himself from being scalded to death and missed the railing on the other side of the pit.
A lot of folks collect sprue and such and wait until the end of the pouring session to remelt it. One thing I do, is keep my temps up a bit and pour the sprue and scrap back in to my melter periodically. It makes for a more homogenous pour. I'll pour maybe 10 castings, knocking the sprue into a separate metal pot, and then throw the contents of the sprue pot back into the melter and keep going.
