Yes, you can overheat it, and that's what usually happens when the joint isn't clean enough -- it won't flow so you heat it up a little more and it still won't flow so you try more heat, and then the flux starts burning. My old shop teacher could smell burning flux from across the shop and he'd be there giving you heck before you could get your goggles off. It didn't pay to tell him, "I cleaned it," either; that would set him right off.... I used to laugh and say, "Well, I thought I cleaned it," and start cleaning it again. That was the only way to get him off your case.
He was right, too. You think you cleaned the heck out of that joint but if the solder won't flow, you just have to clean
more heck out of it -- then it'll work. If the joint is tightly fitted (silver solder won't bridge a gap at all, it's gotta be
tight) and it really is clean -- not just "good enough;" when you think it's "good enough" it usually isn't -- but when it's tight and clean there's nothing easier than silver soldering; you just touch the torch to the joint and it flows all over, wherever you put the flame the solder follows and you have a perfect joint in seconds. All the work is in the preparation.
First I degrease the joint with solvent and then I hit it with the emory cloth until it's just as bright and shiny as can be, and then hit it with the solvent again. Emory cloth that you've been using on metal that might have been a little oily can transfer enough oil to the joint to prevent the solder from flowing. Touching the joint can get it oily from your skin even if you just washed your hands. Touching the emory cloth and then rubbing the joint with it can do it. You can't be too fanatic about cleanliness when you're silver soldering. Like my old shop teacher used to say (actually, he used to yell), "NEVER tell me 'I cleaned it'!"
-Cougar Allen :{)
