Authored by John Sartin
This is the second of a three-part series and it continues with important terminology for working with gold and dives into working with this metal, including soldering, annealing and casting gold.
Heat is a catalyst to the oxidation process. High heat (such as the temperatures used in soldering or annealing) causes what we know as firescale, which is a layer of oxides that forms on the surface of the metal. Most gold alloys do not oxidize as much as sterling silver when soldered or annealed properly. This makes soldering gold alloys a bit easier. The oxides that do form on the surface will generally come off in the pickle bath, and a light buffing will bring up the shine on the surface.
Since pure gold, like pure silver, does not contain copper, it will not form firescale when heated. This is also true for 22K gold because it has very low copper content.
Protect gold alloys from oxidation with a barrier flux (such as a mixture of boric acid and methyl alcohol, Stop-Ox II, Cupronil® or Firescoff) any time you heat the metal. You will also need to use a flow flux (such as Handy® Flux or My-T-Flux) at the joint when you set your solder in place.
As with all soldering operations, you need to use the right torch for the job at hand. A fuel/oxygen torch is highly recommended when working with gold. It produces a hotter flame than a fuel/air torch, and you can adjust the fuel/oxygen mixture to produce a reducing flame. Flux and a reducing flame help combat firescale. Using a torch that is not hot enough can cause the flux to break down, and oxides will begin to form not only on the surface but also in the interior of the metal. This firescale will need to be sanded, filed or ground off, which can be costly in time and in material loss.
Gold solders, like silver solders, are formulated to melt at different temperatures. As when soldering silver, you should use a step-down soldering procedure when there are multiple solder joints on the same piece, especially when the joints are close to each other. Use hard solder on the first joints, followed by medium solder and easy solder (which have progressively lower melt temperatures) on subsequent joints. This will prevent the previous joints from re-melting.
There are two types of gold solders on the market. "Plumb solder" has a gold content equivalent to its karat value. For example, 14K plumb easy solder will have at least 58.3% pure gold in its formulation. Plumb gold solders provide the best color match and retain the overall karat value of the piece of jewelry. "Repair solders" are usually not at karat value. The only time to use a repair solder is when doing repair work such as prong repair or sizing. Do not use repair solder for fabrication because it will decrease the karat value of the piece below the legal standard.
Annealing gold alloys follows the same basic process as annealing sterling silver.
Flux becomes completely molten and flows around 1100°F (593°C); this temperature is just short of the annealing temperature for most gold alloys and can be used as a visual indicator. Annealing by torch is best to do in dim light so you can see the heat color of the metal.
Annealing temperature is 1100°F (593°C); heat to a dark red.
Karat alloys and formulations within the same karat value can have significantly different melting temperatures, casting temperatures, and flask temperatures. Suppliers will provide specification sheets outlining these for each alloy.
Many alloys are formulated for specific uses. These include working alloys, open-system alloys, closed-system alloys, and universal alloys. Working alloys are used to cast ingots that will be made into sheet or wire for fabrication. Closed-system alloys are for use in a casting machine that has a closed melting chamber with a cover gas such as nitrogen, which greatly reduces oxide formation. Open-system alloys are designed to be used when the metal is melted with a torch in an open crucible.
Gold alloys will have slightly different casting properties and require different metal temperatures and flask temperatures. A design might cast beautifully in a 14K yellow alloy, but the details may not fill completely when that same design is cast in a white gold alloy because white gold has a different viscosity. Experimentation and meticulous record keeping is a must to be successful when casting multiple gold alloys.
Get tips on stone setting and finishing your gold jewelry in the third and final part of the Rio Grande Jeweler's Guide to Working with Gold. For a review on gold terms and how it compares to silver, read Part One: Characteristics.