I've made my own silver nitrate with 99.9% pure silver, it's not a very difficult process and not that dangerous if you know what you are doing. First of all it's important to do it outside or in a fume hood. Second, wear protective glasses and gloves at all times when handling the chemicals. You don't need a bain-marie for heating up, a small bunsen burner or heating plate will do, just make sure you use a large enough beaker that is heat resistant because once the reaction starts it's exothermic and it gets really hot. You can use .925 silver but you will have contamination of Copper Nitrate (blue color), since copper nitrate does not crystallize easily you just have to wash your silver nitrate crystals a bit more till you got rid of the blue color and your fine. Don't store the crystals in anything plastic!! only use glass containers, otherwise you will be left with a gooey substance after a while.