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A toasty HOT welcome to


Part built boiler
PYRTE cylinder block and bed plate
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Loco's, road or hobby steam - whatever takes your fancy?

Silver Soldering - Silver Brazing

(Parts of this are taken from a section of the freely downloadable PYRTE boiler construction in the left column under PYRTE, and it is recommended you take a look to give you the fuller picture of how you go about silver soldering, although much of it can be disregarded if you are not following the PYRTE build)

How to Silver Solder copper pipe (steel and brass)

A small section of the Building PYRTE instructions on how to silver solder copper or brass with a stainless steel section added at the bottom.

Silver solder, when applied to a really hot piece of unclean (metal) usually copper, steel or brass, will just form a blob and will not penetrate where it’s needed, and when cold it is relatively easily removed.

It needs a certain amount of heat for it to run, and it’s the running that is needed with the silver solder melting temperature of around 670 F to 700 F being in the metal itself - not the solder. (The high temperature silver solders need around a further 200 F beyond this, but this high temperature can be very awkward to achieve and maintain with a normal propane torch. The metal must be clean, shiny and scrubbed up, and it must also have flux applied where you want the solder to attach itself. What you are doing basically, with the shining and the flux, is removing any surface oxides from the metal surface.

The silver solder works by filling a gap, not as you would first imagine by encasing the two parts to be joined, and it is for this reason that the two mating parts should be a close fit.

There is a tolerance of between three and seven thousandths of an inch to play with. Any less than three thou (0.08mm) and the solder will not penetrate, and any more than seven thou (0.018mm) and the solder will just run through the joint and not form a seal.


I imagine you've just got into gurning now that you’ve read that, especially if it your first venture into silver soldering, but take heart, what follows is very easy to do…

To give you an idea of the melting temperature of silver solder and how to silver solder perhaps a little example will help.

Take a short piece of copper piping, any diameter will do, but 13mm will show you the best results, and lay it flat on a small piece of a copper plate off-cut.

What you need to do is shine both the top side of the plate and the bottom of the tube (the parts making contact) with wire wool, emery cloth of something similar and smear (brush) your flux for silver solder along the contact point.

This flux is specially made up to match the temperature needed for silver solder to run, so don't use any old flux on these joints otherwise it will just burn off and your silver solder will not bond the metals together.

With the tube resting on the plate, simply apply heat with a blow-lamp/gas torch. Once you see the water evaporate off the proposed joint the flux is left as a white powder. Keep on heating the joint until you see the powder turn into a clear liquid (the copper or brass should be just about glowing red by then) and then apply a small amount of silver solder to the joint on one side of the pipe and onto the liquid flux.

Move the flame to the other side of the pipe, again aiming your flame at the joint, and you will see the silver solder penetrating beneath the pipe towards the heat.

That's it... Job done... Kill your flame and let it cool naturally.

Owing to the small mass of metal involved, you should have this done inside two minutes, but be aware that as you work with larger materials, you may need more heat as copper radiates heat away from the source at an alarming rate. Also, if you are using steel, it may not go red before the solder and flux run into place.

We continue from the description of the build of PYRTE......

And for those of us who have no idea what 4ba is, it is a thread size at very nearly 1/8th inch diameter (3.2mm).

Using your mark round the outside of your barrel at ⅛ inch or 3mm in from the front end, drill holes on this line at approximately ninety degree intervals around the outside – the positions are not critical, so long as they are on the marked line, but avoid using the points marking the top and bottom centre lines, as these will be needed later - with a clearance size drill for four small bolts.

(I used steel 4BA nuts and bolts initially to get it all lined up, but brass 4BA studding – I had no brass 4BA bolts at the time - with steel nuts for the actual soldering, and drilled it with a 3.5mm clearance drill, which suits the flowing ability of the solder).

Brass has been used simply because when the time comes to install the smoke-box, if a drill is used and catches one of these brass bolts or studding, it should be OK, but if steel ones are used, the drill may wander if it catches one, or even break and cause endless problems.

These nuts will be removed with a file in due course, as they would obstruct the smoke-box installation later on, but for now they can be left on.

The brass bolts or studding shafts themselves will come to no harm, as they are not in contact with any water in any way; they are under no pressure and are hidden by the smoke-box when complete, so they present no problem here.

Once the holes are de-burred inside the barrel it is time to insert the ring into the end of the barrel, keeping the front edge level with the front of the barrel.

The simplest way to do this is to insert the pressure ring almost far enough inside the barrel and then sit the barrel on its front end on a flat surface and push the barrel down to the flat surface.

The pressure ring now needs marking with the same drill as was used on the barrel in one place only, through one of the holes in the barrel, and make a point of starting your first hole around half an inch (13mm) away from the cut ends of the pressure ring.

Once this is done, the ring is taken out of the barrel and drilled squarely through towards the centre of the circle, and with the burrs cleaned off the pressure ring, it can be re-inserted in the barrel and a 4BA bolt can be inserted and the nut tightened up just so that it nips the two mating surfaces together. 

By making sure the pressure ring is sitting against the inside of the barrel and lined up with the front of the boiler once again, a second hole can be marked and drilled through the pressure ring.

Continue with each hole, cleaning off the burrs to make sure the ring is tight to the inside of the barrel before the next one is marked and drilled, either clockwise or counter-clockwise, making sure they are done and bolted tight, one at a time.

You may now find the ends of the pressure rings do not meet properly, but this is not a problem, as a small, brightly cleaned sliver of fluxed copper can be inserted in the gap and soldered at the same time as the main pressure/anchor ring is soldered, once the 13 gauge front plate is in place.

Remove the pressure/anchor ring from the cylinder and lightly chamfer the inner corner a little with a fine file along the inside edge where it touches the plate, and the front edge where it touches the inside of the barrel. This is to give a lead for the silver solder to run in.

The back (pressure plate side) of the pressure ring would benefit from rubbing on a flat sheet of emery cloth to flatten it out somewhat, as once the ring is bent in a circle, the edge nearest the plate will be mal-formed owing to compression at the inside and stretching at the outside diameter of the ring.

Clean it up with glass paper/emery cloth to a good brightness on the outer contacting surface where it butts up against the inside of the barrel. It also needs to be bright where the 13-gauge front plate butts up against it, along with the front outer inch (6mm) of that plate too.

The same cleaning/brightening of the inside of the tube needs to be done, allowing inch (13mm) or more for the pressure ring and the plate, that way soldering the outside of the ring and the outside of the plate to the inside of the barrel, along with a small fillet of solder between the mating surfaces of the plate and rear of the ring.

In other words, all mating surfaces need to be clean and shiny.

Rinse off any dust particles with clean cold water and then gently heat the proposed joint to evaporate any moisture. (By not doing this rinsing and then evaporating the water off before applying the flux, many failures from poor running of the solder have occurred in the past, as silver soldering is best done in one go, rather than having to go back to reheat the joint and releasing other soldered joints through stress because of heat distortion and gravity).

Once it is cool comes the application of the flux, which comes in a white powder form. A small quantity is mixed with a few drops of clean water and stirred. The consistency should be like dairy cream and can be applied with a child’s paintbrush where it is needed on the bright metal, once the parts are separated, and then assemble them together.

It can be noted here that to do any form of soldering, the metal must be clean and bright, especially for silver solder, but it can also be noted that the grease provided by fingers touching the bright work, providing they are not oily, will be removed by the flux. Anything else will need to be removed by scrubbing, filing or wire wool.

Do not forget to coat the brass screw studding or bolts you are using to hold the pressure ring in place with flux. Any nuts do not need this treatment!

When heat is applied to the fluxed metal, the water boils off and the flux forms a white residue, which you may think needs removing. Do not remove it, as when the metal gets hot enough, the residue turns to a clear liquid and a little after that, with the heat still in use, then is the time to begin applying your solder.

Too much heating can cause the silver solder to run through the joint, possibly because of expansion, so it is imperative that the joint is close together before heating commences, that way allowing the silver solder to just fill the gap and no more.

Regarding having a soldering hearth or something of that nature, I found that a few Thermolite breeze blocks – the ones produced for insulation purposed for house building from the builders yard (Celcon is another brand name used in the UK for the same product, and there are perhaps others – but not the ones containing polystyrene) – serve the purpose admirably and can be cut to shape with an old hand saw easily.

I shaved the end of one block to a relatively tight fit for the inside of the barrel (like a piston) and used that as a support for the front plate, that way pushing it up against the bolted pressure plate/anchor ring.

The weight of the barrel with the anchor ring bolted (finger tight) in place, sat on top of the front 13-gauge plate, supported on top of the end of the piston (shaved block) held the front plate in place and allowed ample space in the joints to be available for the silver solder to be applied easily. Plus you have the added benefit of the insulation from the block, that way needing less heat to do the job.

I placed a few blocks around the upright barrel to retain the heat and began the soldering process after applying flux to all cleaned parts to be joined as it was assembled.

If there are any small gaps in your metalwork, then caulk them up with slivers of shiny copper, soaked in flux, and solder away.

One point here is that copper has a habit of radiating heat away from where it is needed, so be prepared to have a second blowlamp handy if your flux appears to remain as powder.

The heat is applied and although there is quite a mass of metal to be heated, because of the insulation, it took me around ten minutes to get the flux to run, as I made a point of directing the flame on the centre of the pressure ring in a general manner, warming up a large area of copper before I turned the gas up and concentrated it on the pressure ring on the top until I saw liquefaction of the flux, and less than a minute later I touched the silver solder to the joint.

A blob fell onto the joint and sat there, meaning the metal was not hot enough, so while I played the flame around and onto that blob of solder, I dipped the hot end of the silver solder rod into the powdered flux. The dry flux attached itself to the tip of the silver solder rod ready to be applied once the blob softened and ran through the joint.

After a little while the blob started to lose its volume – it was running in between the joints, so I then applied the tip of the silver solder to the same place and saw it run after a few seconds. It was just a case of re-dipping the hot tip of the rod in the dry flux and to continue working the flame and solder round the inside at the joint between the plate and the pressure ring until a complete ring was evident.

If you find the solder does not run too readily, you can use a piece of clean steel wire as a dragging rod to move the solder along.

If your solder is applied and it forms a pool around the joint and you see bubbles rising, then what you are seeing is the solder displacing any air within the voids caused by largish gaps in the joint(s), so keep applying the heat until the bubbles cease, regardless of how hot it feels, unless of course, you feel your solder is just running through the joint, but if this was the case, there would be no bubbles showing. This is a sure sign that you did not get your joints a close fit and the joints will need caulking with some small slivers of fluxed copper!

A seam of silver colour could be seen between the inside of the barrel and the front (uppermost) outer edge of the pressure ring, and also each nut showed signs of silver, confirming that the solder had flowed properly where it was needed, and a good seal was made.

To check whether your silver soldered joint is successful and to neutralise the chemical properties of the flux, you need to remove any remaining flux and residues, and this can be achieved by letting the warm metalwork soak in a dilute solution of acid.

Generally sulphuric acid (the type you get in car batteries) is the best option, but citric acid is more readily available (from most home-brew shops or chemists) and does virtually the same job.

An acid bath using a weak solution is best for your safety, as your hands can be immersed in it readily, although it is best to rinse them off after each immersion - I use eight 25 gram packs of acid, that’s around 8 ounces, in a 25 litre tub, say a 5 gallon wide-topped wine-brewing tub, topped up to within 5cm (a few inches) from the top, with clean, cold water - and the metal can be left in for around an hour to neutralise and dissolve the residues off.

The week acid solution needs to be rinsed off in clean water and then the joint can be inspected more thoroughly after cleaning up with wire wool or something similar.

Do not place the metal in the acid solution while it is still hot as you may well be softening parts of the barrel, which can lead to a misshapen barrel, apart from you getting splashes (sulphuric acid vapour is very corrosive to some clothing) or surrounded by hot, scalding steam in the process.

I have found that if a joint is left uncleaned for a few days after silver solder is applied, the flux residues form a white cover over the joint, which eventually turns green, and this most surely is a chemical reaction between the flux and the copper or solder. It may well be eating into the joint, making it weaker! So make a point of cleaning it off with an acid solution, for your benefit later on.

As an aside here, the more you use this acid (pickle) bath, the more you neutralise the acidity with the alkaline flux, so you will need to add more acid powder as time goes by. For the purposes of this traction engine build, the initial acid content is plenty to complete this model and perhaps a second one as well. A sure sign that the acid bath is losing its acidity is the fact that algae will begin to grow on the top once the solution is neutralised.

As an addition, How To Silver Solder Stainless Steel is covered seperately as basically the only difference is the type of flux used. Generally, most silver solder fluxes are alkaline based, whereas the flux for stainless steel is acid based and produced specifically for nickel or stainless steel.

If you specify from your supplier that you want to silver solder for stainless steel, then they should accomodate you easily.

Soaking in a bath of warm water for half an hour or so will remove the acid into the water and a final rinse in fresh water does the trick.


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