A document written many years ago as a guide to getting started with silver-soldering.
Solver-soldering is a useful technique for joining metals using silver-soldering rod and flux, to create a strong joint. The process is suitable for model boiler work and can be used to join different metals.
The silver-soldering process uses three steps.
Clean the mating surfaces
Flux the joint
Heat to red hot, and add the brazing rod**.
The first two steps are very simple.
To clean the parts, wire wool or a Scotchbrite pads can be used. Both will bring the metal up to a bright shiny finish.
To flux the joint, the best flux is a powdered form. This can be mixed with water to create a flux paste and also used as a dip for a warmed silver-soldering rod, creating a flux-coated rod for your joint. Out of the fluxes available Tenacity 5 is less easily exhausted than some, and can be used with stainless steel as well as other metals, so is a good choice for all round silver-soldering.
It is worth remembering that wherever you put the flux paste, the solder will flow to. So to keep the joint neat and to avoid additional finishing operations, be careful where you apply the flux.
**To clarify what type of brazing rod is used, there is a choice of 'Silver-flo 55' or 'Easy-flo no 2'. The Silver-flo has the advantage that it is Cadmium free and therefore less hazardous to use. However both these solders have a similar melting point and will join metals with similar performance.
The third operation requires a little more attention.
To heat the joint two things are required.
A hearth
A source of heat.
The photograph here shows a home made hearth. Only materials designed to withstand heat should be used when building a hearth for safety reasons. House bricks for example are not a good choice as they may contain moisture and can explode when heated.
This hearth used a layer of firebricks for a base. Firebricks store heat rather than reflect it, so a heat reflective layer of Vermiculite was used as the working surface. Vermiculite is very light and cork like in appearance but reflects the heat well, it is sometimes sold under the brand name of Skomolex, Folsain or Fossalcil.
The use of a hearth is essential for both fire safety and to retain enough heat to successfully braze.
The use of a hearth is essential for both fire safety and to retain enough heat to successfully braze.
When brazing ensure you have everything to hand, including a bucket of water - just in case.
For a source of heat, we must discuss the difference between temperature and heat capacity.
Some gas torches, oxyacetylene for example, produce a very hot flame in excess of 3000°C, but the volume of heat is not high. The flame is small and focused. These high temperatures could melt the parent metal if the flame is not used carefully.
For model engineering work, a gas torch will work, but it needs to have sufficient heat capacity. A simple plumber's torch will only do soft soldering. Mapp gas is a better bet or just propane, but with a nozzle of sufficient diameter to create enough heat output. The torch on the right is made by Sievert and connects to a large propane cylinder. This is adequate for most jobs and smaller nozzles can be fitted for detailed work.
When silver-soldering, only when the work is glowing red hot should you add the silver-solder rod. At this point you should also see the flux turn from a dark paste into a clear liquid. Warm the rod in the flame and dip it in the flux powder and then apply to the join and allow it to melt and flow. Aim to have the heat of the work melt the rod and not the flame. Keep heating the work as you move along the joint but place the flame nearby, rather than directly on the rod.
The image on the left shows a good joint. It has a nice fillet of silver-solder and is clean and tidy. The fillet blends into the surface of the workpiece indicating that the solder has 'flowed well' and has made a strong joint.
Once a component has been brazed, it should be left to cool before cleaning up.
Cleaning can be done by mechanical means such as Scotchbrite pads or a wire brush, but the best results come from pickling. Pickling is the process of using a dilute acid bath to clean the metal and has the advantage that it will clean internal and hard to reach places aw well.
Acid can be dangerous and difficult to dispose off, but the exception to this is citric acid. This is available in crystal form from brewer's shops and 2 teaspoons in a litre of hot water will be strong enough to pickle most parts. Citric acid will take a little longer to work, but is much safer and nicer to handle than strong acids.
Finally - if you would like more technical detail about any aspects of brazing then this book by Tubal Cain is available from the Workshop Practice Series.
It will tell you everything you need to know about equipment, solder types, fluxes, metallurgy, techniques and more.
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