I have not had that much experience with silver soldering, but the issue you are having seems to be over heating the flux.

If you were working on steel your approach might work, but you can heat a 2′ long steel bar to red hot and one end, and pick it up from the other end without a glove on. (This is not recommended though).

You can’t do that with copper. If one end is red, the other is either red or not that far behind.

So in effect, you are trying to heat the whole boiler hot enough to silver solder, from a small window.

What I would do is provide background heat with something cheap and then local heat to get the part you are working on up to temperature.

Hopefully someone with more experience will be along shortly.

In my (limited) experience, insulation round all but the area you’re working on makes it much easier. And use a powerful enough burner.

On 26 October 2023 at 21:01 Andy Stopford Said:

In my (limited) experience, insulation round all but the area you’re working on makes it much easier. And use a powerful enough burner.

Same here.  I think the reason advice varies is getting the timing right depends on the heat capacity of the object being soldered and the size of the torch.  When the flux is heated it cleans oxide off the metal and then melts to keep air away so the solder can wet clean metal and form a sound joint.  Flux only works for a limited amount of time.  It starts losing it’s ability to clean oxide as soon as it warms up, and only forms a liquid seal for a short time.  The joint has to be made before the flux is exhausted, so full-speed ahead.

An underpowered torch can’t get the metal up to soldering temperature before the flux fails.  Insulation means a less powerful torch can be used, but the combination still has get the job up to temperature quickly enough, not necessarily easy.   An over powerful torch risks cooking the flux before the metal has warmed up.   It’s a balancing act requiring practice.

My small torch works best with objects roughly a cubic inch in size. Smaller means I have to be careful not to blast the flux, larger makes it hard to get to temperature quickly enough.  Mixed success with big joints, almost certainly because I spoil the flux.

In one of my old mags LBSC describes how he brazed boilers with a paraffin torch.  The job was placed on top of slow burning coke brazier: I think this provided plenty of heat to warm the job, but wasn’t allowed to get hot enough to melt silver solder or brass brazing rods.   Then LBSC started his big paraffin torch, applied flux to the pre-warmed job, and went straight in with the torch at full power.  I guess timing was important – in the short time between the flux cleaning the surface and melted air-proof layer failing.

I recall LBSC said to torch the job hot enough to be transparent before applying braze (which needs to be hotter than silver solder).   Not quite sure what he meant by that!  Old school temperatures were judged by colour on the assumption everyone knew what was meant.  They probably did a century ago, but I don’t.  I suspect braziers were operated inside a rather dim shed, not sunlight or a bright well-lit workshop.  I guess the coke took the job to about 450°C, and then the paraffin lamp boosted the joint area quickly to 600°C or hotter.

The process may require a lot of fine judgements, and I know LBSC had plenty of experience!   I’ve a notion pre-heating the job on a not-too-hot coke bed would make brazing easier, but maybe it was only done because ye olde paraffin torches were weedy compared with a big gas torch.

Dave

Dave

Hi Bill.

I’ve done several repair jobs or modifications to existing boilers just like you’re trying to do.  Your problem is getting enough localised heat into the job around just the bushes you’re trying to fit.  Sievert gas torches and blow lamps just won’t do it quick enough without heating up the whole boiler to almost soldering temperature.  Buy, beg, borrow or steal some oxy acetylene kit.  The smallest nozzle will bring an area the size of a 10p up to temperature before the rest of the boiler even knows about it.  The flame is very fine and there’s no need to play it directly onto the bush or the flux.  With a circle of solder around the bush it’ll flash to give a perfect job.

Len

One thing to always remember with oxygen /acet/propane torches is they should never be allowed to be still, always on the move!!!!!