The Handyman's Pocket Book: Soldering - Soft.

This handy means of joining metals together can be used for work which is not subjected to more than moderate heat.  Since soft solder melts at somewhere in the region of 350 deg. F. it is clear that the temperature of the work must not subsequently approach this heat.  For articles which contain water soft solder is safe as a rule for general purposes, because the boiling-point of water is 212 deg. F. and water in an open vessel does not rise above this temperature.  Obviously the vessel must never be allowed to become dry.  Not all metals can be soft soldered, but the majority can be tackled.  Brass, copper, german silver, gold, silver, bronze, tool steel, and wrought iron are all safe.  Cast iron, however, does not make a strong joint, and aluminium cannot be soldered by ordinary methods.

SOLDER AND FLUX.  Solder known as "tinman's" is generally used, and is obtained in strips.  There is also cored solder which is more like thick wire in appearance, and contains its own flux, generally resin.  Various fluxes can be used, and, apart from any question of personal preference, some are more suitable for a certain job than others.  The simplest way is to obtain one of the proprietary fluxes such as Baker's Fluid or Fluxite.  The former is very effective but has a corrosive action on iron and steel.  Objects on which it is used are liable to become rusty, and steel tools in the workshop are affected if in the vicinity.  In this respect Fluxite is entirely safe.  In fact, for electrical work it is preferred as otherwise corrosive action is often set up some time after soldering and may go right through wiring.

SOLDERING IRON.  The term "soldering - iron" is a misnomer since the bit with which the soldering is done is always copper.  It can be of the simple type to be heated in a fire or over a gas ring, A, Fig. 1, or it can be electric (B).  Apart from its convenience the latter is cleaner in use.  Do not choose too small an iron because it will lose its heat quickly.  A 1.5 lb. head is a good average.

TINNING THE BIT.  When first obtained the bit will need tinning.  If of the simple kind a gas ring or bunsen burner is better for heating as it is cleaner than a fire.  You can generally tell the heat by holding the iron a couple of inches from the cheek.  It should give a comfortable warmth.  With a file or sheet of glass-paper, on a flat board rub all four sides of the tip of the bit until perfectly clean, and at once dip into the flux.  Then, holding the bit over a piece of tin plate over which some flux has been smeared (an old cocoa tin lid will do), hold the end of the stick of solder against it.  The solder will melt and will run down and a film of it will adhere to the bit as in Fig. 2.  Rub the bit back and forth in the solder, and dip it occasionally into the flux.  The tin lid should be kept as it is used later in soldering.

This tinning need never come away - in fact it will only do so it the bit is overheated.  If this occurs it should be tinned afresh.  The bit should never be allowed to become red hot.  In subsequent use always wipe the point of the bit on a piece of coarse rag before dipping into the flux.  The piece of tinplate should be kept because fresh solder can be melted on to it and be picked up on the iron as required.

One of the secrets of successful soldering is in cleaning the surface of the metal thoroughly.  If it is new metal this can generally be done with glass-paper.  A really dirty surface, however, may need the use of a file or a scraping tool.

MENDING A KETTLE.  As an example, suppose a tin kettle has developed a hole.  Scrape away all dirt for an area of about 2 in. all round the hole and finish off with glass-paper or emery paper in Fig. 3.  Cut a piece of tinplate about an inch in diameter - tinman's snips are generally used.  It it becomes bent in the process beat it flat afterwards on a flat block of wood.  Both the area around the hole and the tinplate patch have now to be tinned.  Smear both with a film of flux, and heat the iron.  Dip the latter into the flux and pick up some solder with it from the tin in which the bit was originally tinned.  (The advantage of keeping this tin is now realised.)  Rub the bit back and forth on the cleaned area of the kettle until a thin film of solder is deposited on it.  It may be necessary to pick up fresh solder - in the event of a large area having to be covered the stick of solder can be held against the bit so that it runs down.  Treat the tinplate patch in the same way.

Place the patch in position, and hold the iron above it in the middle.  The heat will pass through and melt the solder beneath, and, as the liquefied solder always tends to run towards the source of heat, it will run towards the middle.  Finally run the iron around the edge of the patch, using fresh solder, and holding a metal rod in the middle of the patch to hold it down.

The same general procedure is followed in most jobs.  The surfaces are cleaned thoroughly, smeared with flux, tinned, and the iron applied.  One useful point to keep in mind is that solder, when molten, obeys the same laws as other liquids.  It tends to run downhill.  When, therefore, you have an awkward corner to deal with you can often help by tilting the work so that the solder tends to flow where it is wanted.  Apart from this it runs in the direction of the source of heat.

You must have sufficient heat, and the metal to which the solder is applied must be as hot as the molten solder.  It is useless merely to drop molten solder on to the metal.  This is why the iron must be held on the work long enough for the heat to be transferred.  It also explains why a small iron used on a large bulk of metal is useless.  The heat is lost.

SWEATING.  Another method of soft soldering is that known as sweating.  No iron is needed, and it is useful for some classes of work.  No hard and fast rule can be laid down as to which each should be used for.  In some cases either could be used equally well.  The joining parts are cleaned as before and smeared with flux.  Each is then tinned by heating the metal in the flame of a bunsen burner or blow lamp.  As soon as it is hot a stick of solder is dipped into the flux and applied to the metal which should then run over the surface.  Once again remember the two points about the flow going downhill or towards the source of heat.  Thus, suppose a brass collar is being sweated on to a shouldered rod as at A, Fig. 4.  After tinning, the parts should be assembled and the heat applied to the thick part so that the solder tends to run in that direction where it is wanted.  When the parts are a tight fit it may easily happen that they will not go together after sweating.  There is no difficulty if the metal is heated, however, as the solder will at once flow.  Do not make unnecessarily hot as it may just burn the solder.