Soldering refers to the joining of two pieces of metal of high melting point using a molten metal of a lower melting point. Commonly the pieces to be joined will be copper, brass, iron, or similar material, which have melting points above 1000 degrees Fahrenheit, and the soldering alloy will be tin or lead based, with a melting point in the 400 degree range. Soft soldering is useful in many cases where it helps form a tight seal against leakage, but it does not make a strong joint; the soldering alloy is weaker than the metals it joins, so the joint has to be designed to be mechanically strong. Common applications for soft soldering are sealing copper pipes together, making small containers, and making electrical connections. In the case of pipe, one pipe might screw or press into another, giving the joint some mechanical strength, and the solder holds the joint together and keeps it leak tight. In the case of wire, the wires would be twisted together or otherwise secured, and the solder would keep the wires together and help provide good electrical contact. In a container made from metal sheet and soldered together, the sheet could be riveted as well for strength, or the sheets could be overlapped so that the joint has lots of surface area, giving it greater strength.
Standard disclaimer: This project and others available on this site uses heat, tools, chemicals, and open flames. You are responsible for taking all applicable safety precautions, and applying common sense. Since I have no control over how you do things in your own shop, or what precautions you choose to use or not use, and I'm not there to hold your hand, it's not my fault if you hurt yourself.
You'll need the materials to be joined. In this tutorial I'll be using some copper pipe, some brass fittings, a few pieces of copper wire, and some copper sheet. You'll need soldering alloy. This comes in various different varieties. One common solder alloy is lead with some tin added to make an approximately eutectic alloy. Another is tin with a small amount of antimony which serves to harden it. The lead based alloy is commonly used for soldering wires, and the tin alloy is commonly used for soldering pipes for water. In the past, these would be used interchangeably, but lead alloys should not be used in contact with surfaces that will contact water or food for consumption due to the risk of lead poisoning. There are many other soldering alloys, including many exotic alloys with lower melting points, higher mechanical strength, etc. but for this I'll be sticking with the basics and presenting the lead and tin based solders. Finally, you'll need a flux. Flux for soft soldering is a paste of resins and acidic materials. The resin melts and keeps oxygen from the joint, while the acid removes any oxides that are already present. Pine resin or 'rosin' is a common base for this, and the acid is commonly zinc chloride or ammonium chloride. Some solder is made in the form of a tube of alloy with a core of flux, so that little or no additional flux is needed. This is common for electrical solders.
You'll need a source of heat. Depending on the work to be done, this could be an electrically heated soldering iron, a blowtorch, an alcohol lamp flame, or even a candle. The temperatures that are needed are not hard to achieve, but the heat source has to be capable of heating the joint to be soldered, and for pipe and sheetmetal work it has to be able to provide that heat at a rate faster than it can be dissipated into the surrounding material. The larger the joint, the more heat you'll need. We'll be using an electrical soldering iron, a blowtorch, and a candle. In the past, the electric iron would not have been available, and an iron with a large copper head would have been used in its place to transfer heat from a lamp or charcoal stove to the workpiece. We'll also need something to clean the metal to be soldered. For this we can use steel wool, sandpaper, a wire brush, or a pot scrubber. Solder will not wet the metal through a layer of oxide, and the ability of the flux to clean and protect the joint is limited, so we need to remove as much oxide as possible from the joint before beginning work.
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