Silver Solder Paste

[tornitore45]

I post here thinking live steamers must be expert in all aspects of silver soldering.

Brazing a copper boiler, is there any problem using siver solder paste rather than wire. I think the paste can be placed and it will melt in place at the right temperature rather than running around so many spots to touch with the wire.

Any experiences, both positive or negative?

[gwrdriver]

Mauro,
I can't recall the mention of silver solder paste in anything I've read about silver soldering boilers going back 30+ years. This should tell us something. It is also relatively expensive and can be difficult to find, but this wouldn't necessarily make it unusable.

Looking at it from a practical standpoint (and from my experience), one of the attributes of solder paste is that it tends to deliver the least solder possible to a joint and once that solder is in place the supply is gone. Successfully silver soldering a boiler requires (among a number of other things) that sufficient solder to make a sound fully penetrated joint be available to the joint when it gets to flow temperature, however much solder that requires. This is not a time to skimp on solder and starve a joint. It's my opinion that if you rely upon solder paste alone to make the joint there will almost certainly be a place or places where more solder than is carried in the paste is needed to fill the joint. In that case if you aren't ready with the wire solder backup you could easily end up with a joint which has been starved and which cannot be cleaned sufficiently to be re-soldered.

The potential for problems would be less on a small boiler, no more than 3", where the joint clearances could be better controlled and where expansion would be minor factor. I wouldn't attempt it at all on a large boiler.

You could of course make your own solder paste and really load it up with solder, or load up a commercial paste, and that might help, but I would never want to be without some wire, just in case.

[pockets]

Mauro and Harry,
I have used a lot of paste type silver solder, in the smaller scales, before graduating to live steam. On occasion, these joints had to be undone and, without exception, I found voids in joints that had the external appearance of of sound joints. IMHO, there just isn't enough metal in the paste for use on anything but cosmetic appurtances.

Greg B.

[alanstepney]

I have only used paste in situations when the items were small and the joints close tolerence.

Like the other replies, I dont think it worth the risks (of a poor joint) or the added expense.

[tornitore45]

Thanks to all for saving me a bad experience, I will use the wire and keep the paste for unstressed small parts.