Technical Notes

Basic Silver Soldering


Note: With effect from the end of 2011, Easyflo 2 and similar cadmium based products such as AG303 will no longer be available and users should consider using SF55 as an alternative. SF55 is a higher melting temperature alloy and care should be taken in joint construction to ensure full penetration.

Make it, clean it, flux it, apply heat and solder, cool it, clean and inspect it.

It is easy to produce successful silver soldered joints if a few basic rules are carried out and materials and tools chosen carefully.

  1. Joint design:- the best joint is one in which there is an overlap of the components in the form of a lap or sleeve join. This leaves a small gap between the parts to be joined whereby the silver solder can flow in by capillary action. Normally 4 times the thickness of the thinnest component is best overlap.

  2. Gap between the components:- this depends on the design of the structure and stage of construction which in turn determines the type of silver solder used. A list is given at the end with details of solders and fluxes and recommended gaps.

  3. Choice of silver solder:- generally silver solder can be of high or low temperature, free flowing or plastic flowing and can produce fine joints or ones with filleted corners. It is an alloy containing silver with additions of copper, cadmium, tin and zinc. Some structures require step soldering, a typical example of this is in a locomotive boiler where the firebox and rear tubeplate are soldered first with high melting point solder and then fitted into the boiler shell with a lower melting point solder so that the first part does not become unsoldered.

  4. Cleanliness:- is absolutely essential, all parts should be clean of grease and preferably mechanically cleaned with medium grade emery cloth or similar. For non ferrous metals and alloys i.e. copper and brass, scrub in hot water with detergent and Scotchbright pan scourers, and rinse well with hot water just prior to fluxing and assembly, oxides on these metals can be removed by pickling in a solution of citric acid. See PICKLING.

  5. Fluxing:- Flux is a material that cleans the surface of the metals being joined and covers them whilst working to prevent further oxidisation, usually supplied in the form of a powder or paste. The choice of flux is most important to match the properties of the chosen silver solder and the size of the work. The flux must melt and be active by the time the silver solder melts therefore for Easy Flo2 solder use of Easy Flo flux is required. The flux must be capable of removing oxides form the metals to be joined and special fluxes are available for certain steels and tungsten carbide tip tools. Fluxes like most things have a finite life and can become exhausted, depending on how hot and for how long they are heated. If the flux residues turn black and glassy then a flux with a higher rating should be used. If this happens part way through a job, the addition of more flux will often enable one to finish the job. Generally speaking, two fluxes are most suited to model engineers Easy Flo and Tenacity 4a. Easy Flo 2 for lower temperatures and Tenacity for higher temperatures and longer heating times. EASY-FLO flux:- normal general purpose flux for all low temperature silver solders not exceeding 800 degrees C. It will successfully flux most materials used in our hobby including stainless steel, and residues may be cleaned off by soaking in hot water.

    TENACITY 4a FLUX:- A higher temperature rated flux with longer life suited to larger boiler making, will also work with most materials used in modelling and it's residues can be removed by pickling in a mild acid. Other fluxes are available but their removal after soldering is difficult.

    FLUX APPLICATION:- flux is best mixed with water and a drop of detergent to a creamy consistency and applied by brush to joints during assembly prior to soldering. Too much flux will rarely hurt but too little can ruin the work. Inadequate fluxing prevents the capilliary action of the solder into the joint. Extra flux may be added during soldering by dipping the hot end of the solder into the dry flux powder and then transferring it to the work.

  6. Heating and soldering.(also see hearth):- The work should have its temperature raised quickly so that the flux does not become exhausted. For small work such as fitting olives on copper pipes up to 3/8" dia. or small tee pieces and boiler fittings a typical domestic DIY type blowlamp using disposable gas cartridges like the TAYMAR with a nozzle 7/8" dia is adequate. Small boilers up to 2" dia X 5" long approx. may be soldered using two of these. For larger work it is best to invest in a large propane gas bottle from Calor gas or Shell along with a Sievert neck tube burner. These consist of a handle to which different size burners can be fitted, the advantage being that the neck tube burner draws it's combustion air in well away from the flame enabling the burner to work in confined spaces such as the firebox. Normally they are supplied with a long flexible hose for connecting complete with a burst hose protector/flash back arrestor for connecting to the gas bottle.

    It is very difficult to be precise about the size of burner required for a particular job as conditions vary considerably i.e. inside or outside with a gale blowing. Briefly for boilers up to about 5"dia x 10" high a 1.25" burner will suffice, for 5" gauge loco boilers use a 2.5" burner and for really big boilers get 2 of the 2.5" burners. Oxy Actylene is to be avoided unless one is really skilled in it's use, even then only a soft white flame is used to avoid burning the parent metals. Brass fittings tend to melt instantly with the use of this type of heat source. The main drawback to oxy acetylene is it's high concentration of heat, the larger propane flame gives a better spread of heat.

    As a guide to temperature the state of the flux or the colour of the metal may be used. As the temperature rises the flux becomes clear and fluid and runs over the joint area, the solder rod may then be applied and it should melt and run into the joint. If it does not run keep heating and after a few minutes try again, continue heating and applying solder along the joint until it is clearly all soldered. The colour of the metal also provides a guide to its temperature but it is important to always work under the same lighting conditions. I find that subdued lighting inside or in the shade outside is best. If sun shines on even red hot metal it will appear black and cause confusion. The metal should glow a dull cherry red for Easy Flo 2 solder and up to bright red, almost orange, for high temperature Silver Flo 24.

    If using a higher viscosity solder follow the solder stick along with the flame as it is applied. As soon as the joint is completed heating should be discontinued and the work allowed to cool naturally until at room temperature whereupon it may be transferred to the pickle bath. Quenching from the hot state is unnecessary and can be dangerous as steam generates in hollow parts of the work and can be ejected at high velocity, if acid is being used things are worse as fumes are often given off as well. Quenching also produces thermal shock causing uneven stresses and may ultimately damage to the work.

  7. The hearth:- a soldering hearth can be made from insulating building blocks, these are available at reasonable cost from builders merchants under the trade names Celcon or Thermalite. See sketch enclosed. They do not spoil or shatter under heating and may be easily cut or carved to hold parts whilst soldering and are usually much cheaper than fire bricks. Do not use bricks from old storage radiators as these are designed to absorb heat and you can waste a lot of time heating them up instead of the job. Insulating blocks quickly heat up on the surface and glow red thus adding to the heat input into the work. Standing spare blocks around the work will improve the heating time. Also allow space to park the blowlamp till it cools down.

  8. Pickling or cleaning of residues:- after the soldering process it is necessary to clean the work thoroughly, this is best achieved by completely immersing the work overnight in either cold water, or in a mild acid for Tenacity flux. I use citric acid (lemon juice), which is available as a powder to be mixed with water from Boots the chemist at about £1 for 3.50z, enough for several gallons of pickle. After pickling rinse copiously with clean cold running water, small areas of flux remaining are easily removed with Scotchbright or a piece of sharpened wood to get into comers. Check from both sides where possible that all joints are soldered correctly, if not it is usually possible to re flux the work and repeat the process after cleaning.

    Some people recommend sulphuric acid for pickling but it is not really necessary although it can speed up the process a little. Pickling is best done in plastic containers such as polythene lunch boxes for very small jobs to large polythene dustbins or small cattle troughs for large boilers.

    Burnt and blackened residues are best removed by acid pickling but if work is required quickly water up to 60 degrees C can be used to effect removal of Easy Flo flux within 15 to 20 minutes.

  9. Health and safety:- common sense is the best thing here, never breath fumes from any source particularly if using cadmium containing alloys, use good ventilation and stand back from the work not over it. Flux can irritate the skin and prolonged contact should be avoided. The heat from soldering a large boiler can be overpowering and exhausting so be aware and be careful. Some people wear a so-called survival blanket as an apron when working or even a sheet of aluminium foil to reflect the heat. Don’t be tempted to touch anything even with gloves on unless you are sure it really is cold, always lay down sticks of solder with the hot end away from you on the hearth.
Silver solders
Solder Temperature Joint gap Comments
Easy Flo 608-617 .001” Universal general purpose solder very free flowing contains cadmium. Lowest temperature. Very small fillets.
Silver Flo 55 630-660 .001” Cadmium free solder nearest to EF2, free flowing, produces modest fillets ideal for fabrication.
Silver Flo 24 740-800 .001-.002 High temperature used as first step in step soldering. Larger fillets for copper to copper & steel
Silver Flo 40 650-710 .001-.010 Modest fillets as SF55

Silver solders are normally supplied in the form of a rod 1.5mm(1/16") dia by 600mm long. We also supply Easy flo2 in the form of .5mm wire for small fittings etc. For some applications all three solders above are available ready mixed with flux in a 30 gramme syringe so that parts may be assembled then simply heated.

Flux Cleaning
Easy Flo 550-800 cold or hot water universal general purpose flux for most M.E. applications
Tenacity 4A 600-850 mild acid pickle
hot water rinse
for high temperatures and longer working life

Flux is normally supplied as a powder in 250 gramme pots.

Make it, clean it, flux it, apply heat and solder, cool it, clean and inspect it.