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First attempt at silver soldering :-(

Epic Fail! :-)

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Jerry Cashman22/04/2013 00:29:42
21 forum posts
1 photos

Well isn't this fun, lots more to learn

I've watched several UTube videos on silver soldering and from these I expected that the job would all be done in a minute or so... not the case for me.

The pieces I was trying to solder were a short (50mm) length of 32mm 20g copper tube but-joined to a piece of 1.6mm copper sheet to 'cap' the tube off (first of two such joints to make a Hero Engine).

All joining surfaces were cleaned very bright with emery and not touched there after to ensure they were clean.

Flux paste was applied (sourced from the plumbing section of local hardware shop). The solder was also got from the same place, it's labeled 5% silver solder, 2.5mm diameter rod - unfluxed.

I assembled a small hearth from fire bricks and placed the end cap sheet flat on the brick with the tube standing vertically, then played the propane torch all over the joints for several minutes before removing the flame and touching the stick... nothing! Not even the slightest hint that it was even tacky...

I continued heating for maybe 5 minutes, the firebrick close to the piece started glowing bright orange, but still the solder wouldn't melt on the workpieces... so shut it all down and went to read some more

The torch I'm using is sold specifically for this kind of job **LINK** and it was putting out an awful lot of heat...

Did I simply not wait long enough? I would have thought 5 minutes would have been ample?

cheers. Jerry

Chris Trice22/04/2013 02:18:36
1362 forum posts
9 photos
Silver solder flux or plumbers flux? The latter is useless for silver solder.
Brian in OZ22/04/2013 02:48:03
63 forum posts

Hello Jerry,

Firstly using a silver alloy to join metals is usually refered to as "brazing" and is done at much higher temperatures than "soldering" with lead/tin alloys. Secondly I am assuming you are using the appropriate gauge/type of pipe and flat copper for the job.

You mention"a piece of 1.6mm copper sheet to 'cap' the tube off"
This method is usually only used to "cap off" a piece of tube and not for what might be construed as a pressure vessal or joint, in which case you should use a proper "pipe cap" which is a pressure fitting.

For copper to copper you dont need flux when using a 5% silver alloy rod, if you are using something like No 2 Flux (borax in water) then you will only mess up the joint.

Then you say "cleaned very bright with emery "
Good, hope you did inside and outside of the joint so that the brazing alloy can run around the inside of the joint too.

"and played the propane torch all over the joints for several minutes before removing the flame and touching the stick.."
.AAAHH!!! Now this is perhaps where you are coming unstuck. Heating of the joint is good and application of the brazing rod is good BUT you should NOT take the heat away from the job, you need it to keep the job hot enough to continue melting the rod around the joint. OR do you mean you simply re-directed the flame to another part of the joint.

Either way I think I can safely say with reasonable surety that a propane torch will not provide the heat output required for silver brazing, it is usually used for "soft" soldering ie lead/tin alloys. I have not built model train boilers so some of you guys may have an "issue" with this statement.

Also the Bunnings site states that your torch " is ideal for medium joints, soft soldering, paint stripping, glazing, enamelling, jewellery, bending and tempering pipes, aluminium brazing, smelting light metals, freezing seized pipes, bolts and small forging work"

Regards Brian

Mike Poole22/04/2013 03:17:55
2743 forum posts
64 photos

the components to be silver soldered will need to reach the red heat range. Silver solders melt in the range 630 - 780 degrees which is in the red heat range. The top end of this range may be a bit ambitious for a propane torch.


Edited By Michael Poole on 22/04/2013 03:19:46

Jerry Cashman22/04/2013 04:53:03
21 forum posts
1 photos

Thanks guys, I know it's very subjective, but in general, how long would you expect to have to heat a couple of bits of 1.6mm copper before they were hot enough?

1 minute? 5? 10?

(just looking for broad generalisations here you understand)

re: flux - I used stuff which was specifically sold as 'brass and copper flux' it was a pink powder which i mixed with a little water - I also tried (the probably useless) plumbers paste flux (white paste).

I've just found a tub of Tenacity 4A flux powder and a couple of sticks of 45A silver solder alloy which claims to be 45% silver (it should be it cost enough!) which I will try tonight

Springbok22/04/2013 05:17:05
879 forum posts
34 photos

I am sure that if CupAlloys reads this thread he will give you advice


Speedy Builder522/04/2013 08:08:29
2107 forum posts
146 photos

It looks like the torch should have been big enough, did you have the torch too close to the joint. When you look at the torch flame, nearest to the nozzle there is a cone of blue gas. This area of the flame is 'cold' . Outside of this cone is the burnt gas which is 'hot', so the correct part of the flame to be using is probably about 5cm away from the nozzle. You are looking for about 750 deg C, and with suitable bricks will get this onto the joint. Just another tip, you said that the flat plate was placed onto a fire brick. It will take a lot of heat as you would have to heat the brick as well as the plate. Can you support the plate just 1cm above the brick? That way, all the heat will go to the joint. When I silver solder, I play the flame around the joint for some time bringing the whole area up towards the right temperature. You will see the flux melt and look like moulten glass. Gently put the silver solder rod in the flame to warm it up, but not melt and dip this into flux powder, do this several times to get the flux to stick to the rod. Now concentrate the flame onto just one area of the joint which should be at red heat and gently touch the silver solder into the flame, enough to melt a glob off the end of it and remove the solder from the flame. At the same time, move the flame around the joint. The silver solder should be flowing towards the flame - perhaps add a little more solder as you move around the joint.

It shouldn't take more than 2 mins to get your joint up to heat. Check what sort of flux you are using, because if it is for soft solder, your joint will be all black and no good.

Use the largest burner in the kit. I use propane for all my silver soldering and even use the small torches which screw onto the small gas canisters. Last week, I silver soldered some 1/4" brass lugs about 2" x 1" onto some copper tube 2.5" dia. using SilverFlo 55 - admittedly this stuff flows at 630 - 660 deg C. I used a smaller torch than yours and it took about 2 mins.

Copper EATS heat like nothing else, use your torch at maximum heat, a couple of fire bricks around the back and side of the joint, perhaps in a 'V' shape, so that the flame hits the front of the joint and licks around the back, reflected off the bricks.

Next time we hear from you, we want to hear 'Job done' Good luck.

Speedy Builder522/04/2013 08:13:01
2107 forum posts
146 photos

Just a thought - pink flux powder! My flux powder for silver solder is white, my pink stuff is for brazing (bronze rods) and melts at a much higher temperature which would be difficult with your torch on copper - pink and brazing may be OK on steel with your torch as steel is not such a good conductor of heat as copper.

OuBallie22/04/2013 08:53:58
1150 forum posts
661 photos


I remember reading somewhere that emery cloth/tape is 'oily', so not recommended for pre-cleaning a solder joint.

Is this correct or nonsense?

Geoff - Blue sky! What's gone wrong!?

JA22/04/2013 09:10:07
980 forum posts
54 photos

Just a suggestion, instead of proding the hot metal with the rod of silver solder make a ring of it and place it, when assembling the joint prior to heating, so that it will flow into the joint. This has two benefits: You don't disturb the joint and you can carry on added heat until the solder melts and then watch as it flows into the joint.


CuP Alloys22/04/2013 09:52:04
45 forum posts

Hi Jerry,

Firstly pink flux and flux paste from the plumber are NONOs when silver soldering.

Secondly copper to copper joints can be made without flux if you use a copper phosphorus alloy (CuP) alloy like the 5% silver alloy. (There's a clue there as to how the company took its' name!). Any plumbers paste would have been severely overheated, decomposed and left deposits behind ideal for stopping metal flow. As per OuBallie avoid emery cloth/ scotchbrite as they can leave deposits that can prevent metal flow. Let the flux clean the joint - it is what it is intended for. But copper phosphorus alloys melt at about 760 gegrees and you may be struggling with propane to get the joint hot enough. Use a burner genrating about 7 kw. If you want better fluidity use an alloy with 7%phosphorus and silver free. Place the work on an insulation surface eg kaolin wool or lightweight bricks.

Alternatively use a low melting range 55% silver solder and a conventional white flux powder suitable for silver soldering. Add a couple of drops of washing up liquid to the powder and then mix with water to a yoghurt consistency. Ensure a good covering.

Heat the joint generally getting it all up to temperature unti the flux melts and goes clear. BE PATIENT. After the flux has melted position the heat at 9 o'clock and apply the rod at 3 0'clock. The alloy will get its heat from the joint , melt and flow towards the heat. Result - a strong neat joint using the minimum amount of alloy. Suggest 1.0mm dia rod. Or as JA suggests make a ring but fit it inside the joint and draw the metal to the outside with the heat.

For more info go to Download the sections on "Best Bazing Practice" "Strength of Joints" and "Silver Solder Flux"




Why can't I use copper phosphorus alloys to build my boiler?

The inside of a boiler is a very corrosive environment and any sulphur present from the coal will go through the joint like a hot knife through butter.

Ian S C22/04/2013 11:43:57
7468 forum posts
230 photos

No Bogs, by the add in link he's in Austrailia, could be in NZ.

I use a similar torch, I,v used it to make little pot boilers, 1 1/2" and 2" dia with flanged ends, you need patience. For bigger jobs I'v used two torches, each from separate bottles, it's real tricky, doing it on your own, another pair of hands would be useful, even if it was just to feed in the rod when required.

Ian S C

Joseph Ramon22/04/2013 12:05:30
107 forum posts


I think yiou just aren't putting enough heat in. Don't be lily-livered, turn the torch up to full. As you approach the right heat the flux will melt and become a clear, slightly mobile liquid. Keep pouring in the heat. In subdued light you will see the copper begin to glow a dull red, now is the time to strike the solder to the work.

Even if you had feed the solder, havinga bit on the work you can watch to see when it melts helps.

If you are outside even a modest breeze will rob a lot of heat from the work, so make sure you are well sheltered.

You are much more likely to succeed in your quest is you make a proper end cap for the boiler by flanging a circular blank over a wooden former. It should be an easy but not loose fit. If blanking both ends, make sure the boiler has a pre-drilled hole somewhere to let the expanding air out!


fizzy22/04/2013 14:04:25
1732 forum posts
116 photos

As already stated, wrong flux and wrong solder. The stuff you now have should return a good result but will need plenty of heat. There should be no problem as the flux and rod are both pretty high up the temperature range. With a fierce flame your small job should be up to temperature in a minute or so, but as soon as you take the heat off you lose the temperature. I silver solder a lot, and I use the flux as an indicator. It will turn almost invisible at the right heat, all you will see is a slight yellowing of the surface it is on - then its time. Dont put the rod directly into the flame or it will melt and drop off before it gets to the job. Keep the heat on and enter the rod to the side. You only need the smallest amount for a job this side, and it should run right round the joint on its own so no need to move round with the rod.

Jerry Cashman23/04/2013 00:49:13
21 forum posts
1 photos

Succes! !

Thanks very much guys, really appreciate the advice and suggestions!

Here's what I changed (almost everything

1: I got some proper heat blocks for the hearth (I had sourced some 'heat bricks' from a BBQ company previously - they are sold to line the insides of Pizza ovens)... I think they were the wrong stuff and were absorbing heat, not reflecting it. I found some bulky white brittle heat blocks which glow bright orange very quickly when you apply the heat... they are light and easily sawn to shape with a hand saw - very cheap too!

2: Tenacity 4A flux - it now does exactly what all the books and kind advice suggests - goes powdery white as it dries then glossy clear as it melts.

3: 45% silver rods - expensive but they work

4: Patience!

Thanks again everyone, really appreciate the advice and help (and ps: I'm in Canberra, Australia - there is a model engineering club here which I am in the middle of joining, but it's great having this 24/7 resource as well

Link to pic of Hearth.

Link to pic of Joint.

PS: thanks guys who had concerns about this not being a good joint for making boilers - this is on it's way to being a Hero Engine (project 1 from the Stan Bray book) and as such isn't sealed - it always has an opening to atmosphere so will never build up much preasure! - I'll do proper flanged end plates for the next project



Edited By Jerry Cashman on 23/04/2013 00:50:36

Edited By Jerry Cashman on 23/04/2013 00:53:46

Edited By Jerry Cashman on 23/04/2013 00:57:02

NJH23/04/2013 08:33:15
2314 forum posts
139 photos

Well done Jerry - top job!


Springbok23/04/2013 09:08:26
879 forum posts
34 photos

Well done now to pickle it use citric acid the powder that home brewers use works a treat and not so dangerouse as sulphuric.


Stub Mandrel23/04/2013 20:13:40
4311 forum posts
291 photos
1 articles

Ah! The 'firebricks' were your worst enemy. The lightwieght ones are brilliant aren't they?


Ian S C24/04/2013 13:38:18
7468 forum posts
230 photos

The fire bricks absorb the heat, the light weight ones reflect it. Ian S C

fizzy24/04/2013 17:36:01
1732 forum posts
116 photos

Mine absorb heat...not much cop! Where in UK can you get the correct items?

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