View Full Version : Copper Boiler Silver Soldering

12-10-2001, 11:24 AM
I am getting ready to silver solder a lap joint along the length of a copper boiler I am building. My question is, How long can I leave the flux in and along this joint without it going bad. I have to press in 165 copper rivets and I think that alone will take all day. Will the flux that I paint on the rivets dry out or become unusable? I will probably need to rest overnight. Brazing is new to me, please help!


kap pullen
12-10-2001, 02:33 PM
The flux is going to dry when you heat it anyway. Handy flux will dry, than melt and form a translucent blue color when near silversolder heat.
I would reflux it prior to heating up. Wire brush or sand to clean before you assemble.
You could rivet silversolder tape into the joint.
You should do some practice joints.
A copper boiler makes a super heat sink so you need lots of heat, and insulation with fire bricks or something.
Some Siver solders are toxic. Do this in a ventilated area.

Any one else have any thoughts?

I built a 1 1/2 scale tich boiler but didn't feel good sitting behind it, I used three propane torches but didn't know if I had full penetration.
I purchased a boiler from Kennion Brothers in England, just to be sure.
Mine went to the scrap yard.
We run the wheels off of that engine.
happy steaming

12-11-2001, 05:05 PM
Where can I ger Silver Solder Tape? This sounds like a great idea. I have been worried that there would be no room after all the rivets are in place for any solder to flow in.


George Hodge
12-11-2001, 09:18 PM
Wade,as long as there's flux in that joint,the silver will wick in there slicker than scat! You need to get the metal you're soldering to the right heat though! Without playing a flame directly on the joint. I believe the tighter the fit the better the joint. Follow the directions on the flux jar.

12-12-2001, 12:51 AM
I've found, by expeience, that for silver solder to work well, you have to "tin" both pieces that will be soldered. Then flux 'em, join 'em, and heat 'em as you feed in solder. As the tinned parts heat up, the melting solder will draw in the solder you apply and you end up with a really good joint. Let everything air cool, then wash in warm water to remove the flux (on steel it makes rust). This makes most efficient use of the solder, as it's expensive stuff.

Jim Hubbell
12-12-2001, 04:13 AM
On copper to copper joints I like "Silphos 15" (15% Ag). No flux needed. Flows into joint like magic. I use it on refrigeration lines and it stays put with no leaks. If possible purge with nitrogen to keep scale from forming. On other joints 50% Ag solder and proper flux is the way to go. Do a joint or two and cut them open to get idea of whats going on. Steam can get real nasty so boiler needs propper stays etc. Good luck.

12-12-2001, 08:11 AM
Get the surfaces CLEAN and properly fluxed, apply enough (but not too much) heat, and you're about 95% of the way there.

There are different alloys with different gap-filling properties. The Small Parts catalog ( http://www.smallparts.com/ ) lists several alloys and their properties. George is right -- if you have clean surfaces and flux, and the proper heat, the silver solder will wick into the joint. Some alloys are more sluggish than others in that regard, though.

Another thing: as a general rule, the solder will want to travel toward the heat, so apply the heat on the far side of the joint.

Kozo Hiroka (sp?) wrote an article about silver soldering in a back issue of Live Steam that is outstanding in its clarity and information. He's an amazing writer. I've been a technical writer for 20+ years, and it's humbling for a Japanese guy to be a better writer of technical English than I am!

Anyway, I suggest you get a copy of that article.

kap pullen
12-12-2001, 08:21 AM
There has been a debate for years whether or not silphos will deteriate in a firebox because of the sulfer in the coal gasses.
I don't know for sure but have seen one copper boiler junked blamed on this action.
The silphos does work great in other work

12-13-2001, 03:25 PM
Kap is correct in that sil-phos is not to be used on boilers. There was an article in Live Steam a couple of years ago warning about this. Read write up by Kozo Hiraoka, it is excellent. It is reprinted in several of his books as well. Pickle before and after silver soldering. Use sulfuric acid. Hydro test upon completion, and after first few steamings, retest with full hydro test.

Jim H.

12-14-2001, 11:16 AM
<font face="Verdana, Arial" size="2">Originally posted by wade_he:
Where can I ger Silver Solder Tape? This sounds like a great idea. I have been worried that there would be no room after all the rivets are in place for any solder to flow in.


The following (Sulphur Springs Steam Models) has silver solder tape as well as a small selection of silver solder formulations in small quantities.

www.sssmodels.com/ (http://www.sssmodels.com/)