The art of silver soldering, by Mark Ward

Basic Silver Soldering Techniques

By Mark Ward, Anglia Locksmiths, Cambridge, UK

(Admin Note: throughout Locks And Security Magazine, clicking on an image will bring up a larger version. For this article, the images are very large to show Mark’s expertise and for clarity.)

To produce a successful silver soldered joint , a few basic rules need to be carried out and the materials and tools chosen carefully.

Mark's chosen example: Lengthening a key to suit a rimlock application

Joint design:- this is where you make an overlap of the components in the form of a lap or sleeve join and is by far the best way of joining two materials . This leaves a small gap between the parts to be joined where the silver solder can then flow in by capillary action.

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 to be used.

The parts are aligned with the required gap for the joint

Choice of silver solder:- generally silver solder can be of high or low temperature, free 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.

Cleanliness:- is absolutely essential, all parts should be clean of grease and preferably cleaned with a medium grade emery cloth. For non ferrous metals and alloys i.e. copper and brass, scrub in hot water with detergent and pan scourers, and then 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.

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 if using Easy Flo2 solder use Easy Flo flux. 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 all that is required.

Easy Flo 2 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 including stainless steel, and residues may be cleaned off by soaking in hot water.

Flux Application:- flux is best mixed with water and a drop of detergent to a creamy consistency and then 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 capillary 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.

The flux has been applied to the gap

Heating and soldering:- The work should have its temperature raised quickly so that the flux does not become exhausted. For small work a typical domestic DIY type blowlamp using disposable gas cartridges like the Taymar with a nozzle 7/8″ dia is adequate. For larger work it is best to invest in a large propane gas bottle 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.

Heating the work. Note the non-combustible base being used.

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. 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. 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.

The camera flash wipes out the glow from the metal. Without the flash, CCD and CMOS sensors tend to pick up a lot more infrared, making the part appear to "glow" more than to the eye.

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 the work.

The joint after cooling. Penty of metal, and it has "wet" well.

The hearth:- a soldering hearth can be made from insulating building blocks ie Celcon or Thermalite, which are available at a reasonable cost from any builders merchants. 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. 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 put the blowlamp down when finished soldering to cool down.

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. Lemon juice can be used and is available as a powder from most pharmacists and when mixed with water 3.50z, is 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.

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.

Our new, longer key after cleaning. The joint is good, and the key should last for years.

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. 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.

Easy flo is in the form of .5mm wire for small fittings etc. It is a universal general purpose flux for most model engineering applications.

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


About the author

Mark Ward is a locksmith’s locksmith. He is currently passionate about bump keys, and is constantly working on new designs which overcome the new anti-bump features of many brands.

He has been in the Locksmith Industry for the last 25 years and runs his late Father’s retail shop “Anglia Locksmiths” based in Wisbech, Cambridgeshire which has been established since 1964.

Recently he has been teaching the art of “Bumping ” at various locksmith conventions around the country.

His interests within the trade are collecting old tools, locks & keys, and outside the trade he collects rocks & minerals. Also in his free time he is an active member of the Cambridgeshire Search and Rescue Team (Camsar) assisting the Police and Ambulance Services in the area.

Anglia Locksmiths

13 Church Terrace
Wisbech
Cambs
PE13 1BL

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