Scota4570,
The old way of making hard silver solder was to use two parts sterling silver and one part cartridge brass. The cartridge brass has Zinc in it of course. Zn helps with flow and provides some protection from oxidation while under heat. I suspect to make softer or harder solder, one could merely adjust the ratio of sterling to cartridge brass. Extra Hard would be four parts sterling to one part cartridge brass by weight, for instance. That would melt at a higher temperature and have less of a grey color cast to it. That makes the solder joints stronger and harder to see with assembled sterling pieces.
I expect cartridge brass was quite easy to get - spent cartridges, and every jeweler had sterling scrap on hand. Melt it together, pour an ingot, roll it out, and there is fresh sheet solder. Skipping steps of course.
I can probably dig around and find formulas if you really need something specific. But it is readily available for purchase so hardly worth the bother unless you want it in multiple ounces quantity or in a different form factor than the usual thin sheet or thin wire.
Gerald
