Brazing Silver Soldering

Silver soldering and brazing are two techniques similar to soldering, but using materials that melt at a higher temperature and which have a much greater strength. Silver soldering uses solder containing significant amounts of silver. The term "brazing" comes from the use of brass alloys, but many different kinds of alloys are available for brazing. The temperature at which silver solder melts is much higher than ordinary solder, but lower than brazing alloys.

The temperatures used in silver soldering and brazing are so high that special heat-blocking glasses or safety goggles should be worn. There is enough infra-red energy in red-hot metal to damage your eyes.

Silver soldering is generally used where great strength is required in a joint (like the flange at the bottom of a column), and, like regular soldering, relies on capillary action to draw the solder into the joint. This means that parts to be joined need to fit tightly together. Silver soldering needs a different flux because of the much higher temperatures. A traditional silver soldering flux is borax, which requires skill to use. There are many fluxes available that are easier to use and quite effective, but many of these fluxes contain fluoric acids and must handled cautiously.

A propane flame is barely able to successfully silver solder small parts, and will not succeed with large components. You will need a MAPP gas torch, or a brazing set with an oxygen tank to generate the temperatures needed. Another useful technique is to conserve heat by building a small "hearth", which can be as simple as four bricks - two together form a base, and the other two on their long sides form walls enclosing a corner. Place the parts to be silver soldered in the corner and much of the heat you apply will build up, allowing the parts to reach the needed temperature. At the proper temperature, copper will be red hot, and brass will just be beginning to glow - within a few degrees of melting!

The "secret" of successful silver soldering is to make the surfaces to be joined as snug a fit as possible - the tighter the better. As with ordinary soldering, clean the surfaces first with a wire brush and emery paper, then paint with the flux. Now join the parts together tightly. If you can see a gap, chances are that it's too wide. After reaching the required temperature, the silver solder wire will melt when it touches the hot metal. If you've done everything right, then the molten solder will quickly disappear, sucked by capillary action between the surfaces to be joined. Once the job has cooled down, clean all the surfaces thoroughly to remove the last traces of flux.

Brazing uses similar techniques, but the alloys used do not become as thin when melted as solders. Brazing alloys have the advantage of being able to fill large gaps or even to be "built up" on surfaces. Brazing alloys are usually sold in thin, stiff rods, which are often coated with the appropriate flux. Brazed joints are very strong, and can also join dissimilar metals.

If you have any doubts whatsoever about your ability to handle objects at these high temperatures, do yourself a favor and find a skilled friend or a workshop to do the job for you.

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