You can snip pieces off and put them at regular intervals around the riser, although I agre a complete ring would be neater. Clean and flux any other joints nearby and don't overheat anything, a lowish light level will help. There will always be a second chance, so go with this if you're not comfortable.
Searching for simply ‘silver solder’ has turned up a few more options, many of them from silversmiths / jewellers. This looks like the most promising: Extra Easy Silver Solder Wire 1.00mm Fully Annealed & Recycled - TS965EE It doesn’t give any specifics on composition, but the slightly higher 667-709C melt range looks comparable to Silverflo 40. Worth a go? It’s also a 1mm wire - BD’s video uses 1.5mm. I was thinking I just needed 2 loops of my 0.7mm wire to get a similar volume of filler, but yesterday’s exercise with the spirit cup made me remember that the cross-section area is driven by the square of the radius. That would be 4.5 loops which l suspect might be a bit unwieldy. -R
@IRM I'd say that's preferable to the 55% Ag. You could always give them a ring and find out the composition, if they're worth their salt they'll know what they're selling or can put you on to the supplier. Yes square law applies, but I'm also of the opinion that with good placement 1mm diameter would be more than adequate for the riser-to-vapourising-chamber repair, that volume of filler will seal the gap without issue and provide adequate reinforcement/thickining of the weak areas.
Easy melts at relatively lower value. Medium melts at relatively higher value. High melts at relatively highest value. The 1st job is done with high heat. A repair would be done with medium. This would not soften the original work. Any later repairs would be done with easy, because it would not melt either previous job. All of these need to be higher than the heat of using temperatures.
Thanks @snwcmpr. Given my very limited experience I am very wary of over heating the burner and damaging it. The melting point of brass is about 900C, so I figure the brazing used to assemble the burner was probably in the 800C range, right? Would a medium filler be around 25% silver (looking at silverflo 252 which has a melting range of 680-760C)? Edit… I forgot one question… what’s the typical operating temperature of a (paraffin) burner? -R
Looking at Thermal imaging of stove firings @presscall clocked a Primus 41 with a silent burner at about 500C and an 85 roarer at about 600C. @anlrolfe has pics however showing a 111B hitting 700C. Can I assume that a silent burner run a little cooler than roarers? -R
In normal use, the vapourisation chamber and even the spigot will stay relatively cool- barely incandescent, if even that. Mismatch of caps, underburn, cleaning burners with heat and air will all take it higher. We're looking at a balance here- the highest melting poing solder to effect a relaible repair, against risk of of a puddle of brass and bad language To me, the 'easy' filler from the jewellers looked a good compromise between melting temperature and your confidence. I'm sure you'd be fine with the elevated temperature of the 'medium', just make sure you take the time to set up, get comfortable, and get the light levels right so you can see what you're doing. If in doubt, roll up a bit of scrap brass to imitate the riser, and place it on a bit of sheet to imitate the vapourisation chamber. Practice making the joint. In fact, heat it until it melts, that will give you an idea of colour, warning signs, how to play the flame over the work....
This is what I’ll do. I have little in the way of scraps (unless ancient computer parts count), but I can get some brass tubing and sheet from the same place as the easy wire. Enough to practice rod vs wire, easy vs medium and multiple passes with different fillers. -R
A week on and I have a few bits of brass and some medium silver solder. For practice I decided to build a Tie fighter. 5/8” tubing for the fuselage, 0.81mm thick plate for the wings, and a propane torch. First pass was the medium jeweller’s solder of unknown composition (the seller never got back to me). Melting range 720-765C. Second pass was Silver-flo 302 (30% Ag) Melting range 665-755C. The distance between the two ends was 3cm. With an overlap in the melting ranges I wanted to see what happened. The first join (left above) appears unaffected, the second join, if anything is the neater of the two. Feeling ambitious I tried to upgrade to a Tie bomber: Third pass was Silver-flo 38 (38% Ag). Melting range 660-720C. This was adjacent to the first join and with the high end of the new filler’s melting range equal to the low end of the first. This one didn’t go so well. I didn’t get the filler as snuggly nestled around the tube and perhaps didn’t flux as well as I should have. I cranked up the torch to full to see if I could improve on this and succeeded in seeing what colour melting brass looks like: By the time I was finished the original join was still there, but I think it looks a bit more brittle / wrinkled: All in all a useful exercise. I think I’ll go with the medium solder for the burner repair. It seemed to melt easier than the others - perhaps because it was a flat strip rather than a 1mm wire and so had a greater surface area? -R
Yes, cleaning and fluxing it just in case it got melted would eb best practice. Spot on, good to have a trial run! The medium solder looks fine, you're right about the surface area too.
I took the leap and tried this for real: After what felt like 5 mins with the torch (but was probably 3) the solder just wasn’t flowing and the spigot was glowing as red as I was comfortable with. Is it just a matter of cleaning and fluxing again and having another go, perhaps with a little more solder for the cap in the first photo? -R
Yes, it got a final dip in my ultrasonic bath with citric acid followed by a wash with some ethanol. I think the extra mass of the burner means an even longer heating time than I was expecting and with the spigot starting to glow I bottled it. Looking at the pictures after it’s been cleaned another minute or so might well have done it. Where do I go from here? -R
Looking at this pictures some more I see the solder has mostly joined with the vaporising chamber, but not the spigot. Would that suggest insufficient fluxing? -R
I don't think cleanliness is the issue here, it looks to me like most of the join is spot on, apart from the area highlighted. Possibly, or maybe this was the coolest area so the filler did not melt enough to fully flow. As you say, clean, flux everywhere and have another go, add a tiny snip of filler rod in the gap to be sure, and direct the heat a bit more to the vapourising chamber if you're getting the riser a bit hot for comfort. Both need to be hot enough to draw the solder evenly into the gap between them. Maybe also try adding a chunk of metal (steel nut?, but avoid fumes from plating which would burn off) to the top of the riser to act as a heat sink and lower the risk of melting. It won't be perfect as the top of the riser is uneven, but better than nothing. A couple of scraps of vermiculite could help concentrate the heat in the small area which needs attention, and keep it away from elsewhere. A slight tweak of distance of torch to workpiece might give more rapid heating, although I'm sure you've found the hottest part of the flame with your experiments!
Take two, and I am very happy with the results: I ran the torch a bit hotter, things happened a bit faster, and I don’t know if that was a factor, but I felt I was better able to observe what was happening and see when the solder started to flow. I will put the stove back together over the next few days and hopefully have it running at the weekend. Thank you to @Blackdog and everybody else who helped with guidance and encouragement! -R
