The above is a short edited extract from my DVD series:
"How to build a Model Steam Launch"
The Text below is taken from my E-Book:
The Complete Guide To Miniature Steam
Many beginners often view the process of silver soldering as a bit of a "black art" - it isn't, but three simple rules do need to be followed in order to achieve success.
1) Cleanliness of the components.
2) Correct flux/silver solder combination.
3) Sufficient heat
It is not possible to successfully silver solder any parts that have already been soft soldered - the two processes do not mix.
To silver solder brass pipe unions to copper pipe you will require:
A suitable gas blowtorch:
The self contained small D.I.Y gas blowtorches for sale in most D.I.Y stores will suffice for pipe union soldering at least up to quarter of an inch diameter copper pipe. if you anticipate doing a lot of silver soldering, consider a larger blowtorch setup & a commercial "Calor" Propane gas cylinder.
Silver solder:
For normal silver soldering, "Easyflo number 2" silver solder is recommended with "Easyflo" flux, which comes as a white powder. Some types of silver solder alloys require much greater heat & different fluxes, these are not recommended for simple pipe soldering.
A suitable surface to support the parts being soldered is necessary: A piece of firebrick is very good, or alternatively a small piece of stainless steel or cast iron fire grate held in a vice, like the type used in model steam locomotives.
Using the flux:
"Easyflo" flux is in the form of a white powder; this needs to be mixed with water until the mixture takes on a thick cream consistency. Some people add a drop of washing up liquid to the mixture, to make it "wetter", this is not really necessary for simple copper pipe silver soldering. The flux does not want to be "watery & runny".
Process:
Clean the end of the copper pipe with emery cloth, "wet or dry" sandpaper, wire wool or a scourer like "Scotchbrite".
Firstly, do not forget to put the union nut in place on the pipe, the right way round & preferably as far away from the heat source as you can get it. Apply a small quantity of the liquid flux to the end of the pipe - do not get any inside the pipe. Now, push the pipe nipple onto the pipe. Light your blowlamp & apply the point of the flame to the copper pipe just behind the brass nipple. If you apply too much heat to the nipple itself directly, it may melt! When the pipe just starts to glow dull red, watch the flux & when the flux takes on a "watery" almost transparent appearance & "spreads" a little, keep the heat applied & touch the stick of silver solder to the joint between the nipple & pipe. The silver solder will "flash" around the joint by capillary action. Turn off the blowlamp at this stage & do not apply any more silver solder.
When the pipe has cooled to a black colour, quench it in water. This "thermal shock" will remove some of the oxidization. Do not quench the work while it is still red hot!
If you are silver soldering a copper steam boiler - on no account immerse it in the acid pickle bath until it is cool - if you do, you risk serious injury by being drenched in hot acid & futhermore the rapid contraction of the boiler will probably crack the silver soldered joint anyway!
If you need to silver solder a nipple on the other end of the pipe, first thoroughly clean up the joint area as described above & repeat the process - don't forget to put the other union nut on the pipe too - the right way round! Use a scourer to initially clean up the finished pipe, some of the oxidized flux can be very glass like, remove this before final polishing takes place. Polish the pipe with a suitable electric polishing spindle, or if you don't have one of these, use metal polish & "elbow grease". The copper pipe is also now much softer than it was, the heating & quenching process causes the metal to be "annealed" so be careful that you don't accidentally bend it out of shape when polishing.
In the above description, I have not mentioned the need for an acid bath, as for pipe soldering it is not essential to clean up the parts in acid, in fact the acid will discolour the union nuts. However if you move onto more ambitious silver soldering projects, an acid bath is essential to remove flux residue & oxidization. There you have it. Not that difficult is it? But like anything else in life, the more you do it, the better it gets.