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Silver soldering question

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Ned Ludd:
Hi Guys,
If the burner is too small, could it be that the flux is exhausted (time thing not a heat one) before it has had a chance to do its job. If so perhaps a change to Tenacity flux instead of Easiflo might help, once a bigger burner is obtained.
As another guide to the right heat, if the rod does not melt and "flash" as soon as you put it to the joint, it is not hot enough. As has been said, it takes an awful lot of heat to solder a boiler, even a small one.
Ned

benchmark:
I will be going shopping for burners and will look for a good propane burner or i may even get an oxy acetylene burner that oozes over 1000 degrees centigrade of heat.

Jasonb:
Be carful with oxy acetylene it is a very localised heat and you may not get good penitration also its easy to burn through the copper.

Jason

Bogstandard:
Benchmark,

The silver solder you have is ok, but lay it down as pallions or circles around the joint rather than trying to feed by hand.

Your main stumbling block is your flux. The latest, Tenacity 5, designed for prolonged heating times, is what you really require, and don't use too much, just enough to cover both joint faces is plenty. Other fluxes will exhaust their cleaning properties well before you have the job up to solder flowing temperature, just like the one you are using now.

Two points about the heat source, don't use oxy/acet unless you can control it like a real professional, get a good sized propane burner, around 3/4" to 1" will be fine for smallish boilers. We made Stews boiler for his 3.5" loco with two burners, one at 3/4" and the larger background one was somewhere around 1.25".
NEVER point the heat source directly at the flux or solder. Play it onto the metal around or under the joint, when the job gets up to temperature, the solder will flow by itself towards the point of highest heat, it is then you can come in with your hand fed rod and fill in any gaps that are left.

Once you have mastered the technique, you should never have any more problems.


Bogs

cidrontmg:
There's maybe some confusion about heat capacity and temperature. They are not the same thing. A lit match will have quite a high temperature at its "sweet spot", certainly far over 1.000 oC. It will actually melt iron - if the iron "lump" is  small enough. Test with a strand of 000# steel wool and see for yourself. But the heat capacity of a match flame is not much to write home about. It will not raise the temperature of one litre of water very rapidly.
The flames of small and large propane burners will attain +/- the same temperatures, but heating a litre of water to boiling will take a lot less time with a big burner. The two burners have different heat capacities.
In making a good silver solder joint, itīs not so much about temperature than about heat capacity. Most of the time the workpieces are very good heat conductors (copper, brass, bronze), that have to be constantly supplied with quite a lot of heat to keep them at a proper temperature. That is why a hearth would be needed. You will need a lot less heat capacity in the burner (meaning a lot less gas also, it's not free after all...) if you prevent the heat from escaping freely.
It is not enough to have a small spot at the tip of the flame over the melting point. That's a sure recipe for a failed silver solder joint. Itīs all OK when actually "gas welding" pieces together, or even brass brazing them. Silver solder behaves differently. It will be sucked into the (well prepared) small cavity of the joint by capillary force (yes, even somewhat "uphill"), but only if 1). the surface is clean (=well fluxed), 2). the cavity is small enough to create that capillary force, and 3). it is hot enough also farther off from the flame tip, so it will not begin to solidify. It's often said that silver solder will "follow the heat". That's very true, it does.
Using an oxy-acetylene flame for silver soldering is asking for trouble if you don't do it daily, or train a lot how to control and manipulate the small but hot flame. And it is hot, it will reach 3.500 °C (6.330 °F). It's not so much the ultimate attainable temperature that interests, as long as it is well above the solder melting point. The important thing is the heat capacity. It has to be in relation to what you are trying to solder. With oxy-acetylene, you are very likely to heat just a small spot, not the whole workpiece (and there is always more than one workpiece to heat), and that spot to far too high a temperature.
Yes, you can overheat when silver soldering, and make the joint (or even the whole attempt to do it) fail because of that. If you go over the top with the temperature, several bad things will happen. First, the oxidizing of the workpiece will be greatly accelerated, second, the flux will not work well or at all way beyond over its intended temperature range, and third, the solder, being an alloy, not pure silver, will start burning and evaporating some of its constituent metals, causing a change in its designed properties, and in 99.99% of cases for the worse. The metals in silver solder materials are rather low melting and even have low(ish) vapourizing temperatures, so blasting at silver solder with 3.500 °C, thereīs a real danger of creating some metal vapour by overheating the material(s). Zinc (in brass) will evaporate at 907 °C, even silver starts evaporating at 2.162 °C. Metal vapours are not a good thing to breathe even in small quantities.
Oxy-acetylene is certainly nice to have in a shop. But it's not my first (or even second) choice for doing silver soldering  :)
 :wave:

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