I used Lennox brand lead free solder (from Lowes-Home Depot and etc building supply stores) for my preliminary castings.

It is 95+% tin, no lead, melts with a propane torch. Lead free solder melts around 500°F.

Though I haven't tried it yet, I did buy a jewler's metallugry book. You can mix silver and tin, but the melting point of the alloy goes up with increasing silver content.

Likewise I will someday fool with alloys of nickel and siver, but I am going to need a bellows to get a fire hot enough to melt that stuff. I doubt construction paper and masking tape will be adquate for making forms at those temperatures, and you have to start worryng about your blades losing temper when you are pouring liquids at 2000°F.

You can get the silver cheapish, pre 1964 US coins are trading at about 25x face value, currently a well worn 1964 dime would be about US$2.50 based on the (90%) silver content. Ask your dealer about "junk silver" in inventory.