Soldering is one of the most basic techniques in metalworking, but also one of the hardest to master. It can be very scary to try and wield an open flame, and the possibility of melting a piece that has a lot of work (and expensive materials) in it is always present. However, as with any process that has a certain mystique about it, understanding how and why soldering works will make us much more comfortable with it. Especially knowing why the process works will let us do some intelligent troubleshooting, rather than simply guessing when a problem arises.
Soldering is the process of joining two or more pieces of metal by using a metal alloy whose melting temperature is lower than the metals being joined. Hard soldering is also called low-temperature brazing. Soft soldering uses very low melting temperature solder alloys, usually of tin and lead, and is not commonly used in fine jewelry.
There are five basic areas that make up the soldering process: solder, flux, heat, fit, and cleanliness. If the optimum conditions are maintained in each of these five areas, the soldering process will go smoothly. It is when we begin to take shortcuts or get sloppy, that things go wrong. Knowing why something happens will let us solve the problem quickly. Even the pros sometimes have to go back to basics!
Solder is a nonferrous (without iron) metal alloy, the major percentage of which is usually the same as the metal being joined: gold, silver, copper, or brass. Gold solder is available in different colors to match various alloys. Because brass and copper solder, both also known as brazing rod, has a high melting temperature and is brittle, silver solder is usually used on these metals as well as on silver. All of the nonferrous metals (gold, silver, copper, brass, or bronze) that have a relatively high melting temperature can be soldered with either gold or silver solder. Both gold and silver solders are available in different melting temperatures and are manufactured in several forms.
Melting temperatures of solder are determined by the zinc content: the higher the zinc content, the lower the melting temperature. Zinc is what turns the lower melting temperature silver solders a yellowish-gray; to avoid conspicuous solder lines, use the highest temperature solder feasible. Pits in the solder seam are caused when the solder is overheated and the zinc burns out. Again, using a higher temperature solder (and controlling the heat) will help to prevent pitting.Forms of solder include sheet, wire, pallions (clippings or chips), and paste. Which form of solder to use is a matter of training and personal choice. I prefer to use sheet cut into small pallions, because wire solder, being round, will sometimes roll away from the force of the flame; sheet stays where you put it (usually). Paste solder, a mix of tiny bits of solder mixed with a paste flux, is used primarily by mass producers in machine soldering and is the most expensive form of solder. Knowing how much paste solder to use requires a bit of experimenting.
Why use different melting temperature solders? When fabricating a complicated piece, using different melting temperature solders will help prevent the previously soldered joints from remelting and either shifting or coming unsoldered.
Soldering on the right type of surface is also important. Using the wrong material to solder on can result in the heat being absorbed by the surface rather than heating the piece to be soldered. You should use a firebrick or a commercial soldering brick or pad which are made to reflect the heat so the part that you are soldering will stay hot enough to melt the solder.