Armature makers use Borax Flux to bond many types of metal with the combination of silver solder (available here).
This product is mostly commonly used to braze stainless steel balls onto stems Stainless steel will not flow unless the surface is clean. When any stainless steel or silver item is heated a 'skin' called oxide stops the flow of solder. This jewellers flux prevents the oxide being produced by providing a coating which excludes the air, and makes the metal 'flow' This flux powder can be mixed with water to form a paste. 50 grams is enough for hundreds of soldering jobs.
Please note that you need a gas torch to melt this solder, either propane / butane or similar. An electric soldering iron is not hot enough and wont work.