The Handyman's Pocket Book: Silver Soldering.

When work is liable to be subjected to considerable heat or has to withstand greater strain than soft solder will resist, it is necessary to silver solder, or hard solder it is alternatively called.  The process is entirely different from soft soldering and calls for great heat.  The silver solder itself is obtained in thin sheets, and is cut up into strips with snips as required.  The flux used is borax.  It is in powder form and is mixed with water.  The heat can be from a blow lamp, gas blow pipe, of for very small work, a bunsen burner.  Larger work may need some form of forge or oven in which coke can be arranged so as to prevent loss of heat as far as possible.

Cleanliness is important, and this involves rubbing the joining surfaces with glass-paper or emery.  In the case of old and dirty work it may be necessary to file or scrape the surface.  If there is any form of mechanical joint such as one part fitting inside another they should fit as tightly as possible because the molten solder will flow into the smallest aperture.

FLUX.  Borax can be obtained in lump form if preferred, and is reduced to a paste by rubbing on a piece of slate, using water.  Alternatively, powdered borax can be used, this requiring only to be mixed with water.  The only other requirement is a piece of thick wire or pricker.  It is handy for picking up more borax, and it can often be used to encourage the solder to flow.

Having cleaned the joining surfaces thoroughly smear over them a layer of flux made by mixing the borax powder with water.  The purpose of the flux is to prevent the metal surfaces form oxidizing, as this would prevent the solder from adhering.  It also assists the solder to run.  If one part fits within another it is an advantage to coat the inner one with the borax paste before inserting it into the other.  It is advisable to hold the parts together in some way because when the work is heated the borax begins to bubble and may easily displace them.  Generally the simplest way is to fasten iron wire around them.  When they are together go over the joint with more borax.

Quite small work can be dealt with just as it is, being held in a vice (so long as the heated part is well away from the jaws) or laid on a piece of asbestos or old fire brick.  If you are using a bunsen burner you can often hold the work over the flame, but generally it is more convenient to have the work stationary and apply the heat to it.

Some workers put the silver solder in position before the work is heated, and in some cases it is more convenient.  An example is when a small rod is being silver soldered in a hole in a larger rod.  A narrow strip of silver solder can be bent to form a ring and put around the rod in the form of a collar as in Fig. 1.  It cannot drop down and it is in just the position where it is wanted.  This is not always practicable, however;  the bubbling up of the borax as it becomes hot may easily shift it out of position.

THE OVEN.  Large work cannot be dealt with in the open owing to the loss of heat.  It needs to be packed round, and the best way is to make a sort of oven, using an old fire bucket or some other tin, or a few fire bricks piled to a convenient shape.  The work is placed in it and packed round with coke as shown in Fig. 2.  The whole idea is to prevent heat from escaping.  Remember that unless the required degree of heat is reached the solder will not flow.

Assuming that you are using a blow lamp, light this and get it thoroughly hot before using.  For a start apply the flame to the main body of the job so as to make this really hot first.  Otherwise the heat will be conducted away from the joint and it will be impossible to make the solder run.

MAKING THE SOLDER RUN.  For a start the borax will begin to bubble and look very messy, and gradually the metal will reach a dull red heat.  This is not sufficient; a quite bright red is necessary.  The borax itself will have a glass-like, smooth appearance.  As soon as the metal is glowing bring the flame right on to the joint.  It will soon become a bright red and the silver solder can be picked up with tweezers and applied to the joint.  It will rapidly crumple around the work but will not run until the heat is reached.  You will see it do this.  It will assume a bright appearance and will run freely.  Often it can be assisted by drawing the pricker over it.  Once it runs freely it will flow into the tightest joint and adhere strongly.

Sometimes more borax may be needed, and this can be applied by dipping the pricker in water then in borax and so on to the job.  It should be noted that, although enough heat is essential, the over-heating of brass should be avoided because the melting-point of brass is not much above that of silver solder, and it may run or crumple, especially if it happens to be thin.

CLEANING.  When the solder has run around the whole joint take away the flame and leave the work until it has lost all redness.  It can then be lowered into a bath of pickle made of one part of sulphuric acid to ten or twelve parts of water.  Pour the acid drop by drop into the water.  Never reverse the procedure as otherwise the acid may spit and splash and so cause a burn.  This bath in pickle will help remove the hard borax and dirt.  Wash well afterwards.  It is not advisable to put the work into the pickle before redness has cooled out because otherwise the metal may be distorted and the joint may crack with the quick contraction.

BRAZING.  This process is practically the same as that of silver soldering, except that brazing spelter is used instead of silver solder.  This spelter is obtainable in the form of granules - rather like moist brown sugar.  and is cheaper than silver solder.  It takes more heat than silver solder, however, and for this reason it is used generally for steel rather than brass.