Hard soldering is the art of soldering or uniting 2 metals or 2 pieces of the same metal together by means of solder that is almost as hard and infusible as the metals to be united. In some cases, the metals to be united are heated to a high degree, and their surfaces simply united without solder by means of fluxing them. This process is then termed brazing, and some of the hard soldering processes are also often termed brazing; both brazing and hard soldering are usually done in the open lire on the braziers' hearth. When soldering work of copper, iron, brass, etc, the solder generally used is a fusible brass, and the work to be soldered is prepared by filing or scraping perfectly clean the edges or parts to be united. The joints are then put in proper position, and bound securely together with binding wire or clamps; the granulated solder and powdered borax are mixed in a cup with a very little water, and spread along the joint to be united with a strip of sheet metal or a small spoon. The work is then placed upon a clear fire, and heated gradually to evaporate the water used in uniting the solder and borax, and also to drive off the water contained in the crystallized borax, which causes the borax to boil up with an appearance of froth.

If the work is heated hastily, the boiling of the borax may displace the solder, and for this reason it is better to roast or boil the borax before mixing with the solder. When the borax ceases to boil, the heat is increased; and when the metal becomes a faint red, the borax fuses quietly, like glass, and shortly after, as the heat of the metal is increased to a bright red, the solder also fuses, which is indicated by a small blue flame from the burning of the zinc. Just at this time the work should be jarred slightly by being tapped lightly with the poker or hammer, to put the solder in vibration and cause it to run into the joint. For some work it is not necessary to tap it with the poker, for the solder is absorbed into the joint and nearly disappears without tapping. In order to do good work, it is necessary to apply the heat as uniformly as possible, so as to have the solder melt uniformly. This is done by moving the work about in the fire. As soon as the work has been properly heated, and the solder has flushed, the work should be removed from the fire, and, after the solder has set, it may be cooled in cold water without injury.

Tubes to be soldered are generally secured by binding wire twisted together around the tube with the pliers. All tubes that are soldered upon the open fire are soldered from within, for if they were soldered from the outside the heat would have to be transmitted across the tube with greater risk of melting the lower part of the tube, the air in the tube being a bad conductor of heat; and it is necessary that both ends of the tube should be open, so as to watch for the melting of the solder. In soldering long tubes, the work rests upon the flat plate of the braziers' hearth, and portions equal to the length of the fire are soldered in succession. The common tubes or gas-pipes are soldered or welded from the outside. This is done by heating the tube in a long air furnace, completely surrounded by hot air, by which means the tube is heated more uniformly than in the open fire. After the tubes have been heated to the welding heat, they are taken out of the furnace, and drawn through clamps or tongs to unite the edges, and are then run through grooved rollers 2 or 3 times, and the process is complete.

The soldering or welding of iron tubes requires much less precaution in point of the heat than some of the other metals or alloys, for there is little or no risk of fusing it.

In soldering light ironwork, such as locks, hinges, etc, the work is usually covered with a thin coating of loam to prevent the iron from being scaled off by the heat. Sheet iron may be soldered at a cherry-red heat by using iron filings and pulverized borax as a solder and flux. The solder and flux are laid between the irons to be soldered, and the whole is bound together with binding wire, heated to redness, taken from the fire, and laid upon the anvil; the 2 irons are united by a stroke upon the set hammer. Steel or heavy iron may be united in the same way at a very low heat. For soldering iron, steel, and other light-coloured metals, as well as brasswork that requires to be very neatly done, the silver solder is generally used on account of its superior fusibility and combining so well with most metals, without gnawing or eating away the sharp edges of the joints. Silver solder is used a great deal in the arts, and from the sparing or careful way in which it is used, most work requires little or no finish after soldering, so that the silver solder, although expensive, is in reality the cheapest solder in the long run. For silver soldering, the solder is rolled into thin sheets and then cut into narrow strips with the shears.

The joints or edges to be united are first coated with pulverized borax, which has been previously heated or boiled to drive off the water of crystallization. The small strips of solder are then placed with forceps upon the edges or joints to be united, and the work is heated upon the braziers' hearth. The process of silver soldering upon the larger scale is essentially the same as the operation of brazing.

For hard soldering small work, such as drawing instruments, jewellery, buttons, etc., the blowpipe is almost exclusively used, and the solder employed is of the finest or best quality, such as gold or silver solder, which is always drawn into thin sheets of very fine wire, and it is sometimes pulverized or granulated by filing; but if solder is pulverized very fine, a greater degree of heat is required to fuse a minute particle of metal than to fuse a large piece.

In soldering jewellery, the jeweller usually applies the borax or other flux in solution, with a very small camel-hair brush. The solder is rolled into very thin sheets and then clipped into minute particles of any desired shape or size, which are so delicately applied to the work that it is not necessary to file or scrape off any portion of them, none being in excess. The borax or other flux used in the operation is removed by rubbing the work with a rag that has been moistened with water or dilute acids.