Elemental Alloy Mysteries
Many of us need a better understanding of just exactly what effect different base metal elements have on gold. Each base metal (really elements, found in the periodic table) that goes into making alloy for karating has its own effects. The silver we add to most alloys complicates the equation considerably. Not to mention the host of deoxidizer, flow enhancers, and the difficult to properly use grain refiners. In alloys for making 10kt and 14kt we always have a big copper content. In alloys for making 18kt we usually see more silver than copper. When I calculate the elements in 10kt, I find the following (Using a typical alloy mix as an example) Au 41.66% Cu 41.10% Silver and zinc make up the balance. Notice the copper nearly matches the gold content.
Copper-Cu The sole red metallic element we can use. Copper has the effect of helping every ingredient mix well. It faithfully reacts or alloys with almost anything. This is a mixed blessing. To the good, copper allows us an enormous range of behavior and color control. The down side is OXIDATION, O2 reacting with Cu to cause all kinds of trouble. The new de-ox or non-tarnishing silver mixes all eliminate or at least minimize the copper content by using other elements. When grandmas silver tarnishes, it’s really the copper content in the sterling doing the damage. When we increase the amount of copper in an alloy, the color goes to the darker/redder side. So, this is what we use to make rose/pink/red gold. Silver-Ag the next most common alloy element is silver. This element has the wonderful ability to make up for many of the difficulties copper causes. Silver softens the mix for easier setting or bending. It also lightens the color toward the green-yellow side. Silver is the most common element to make green gold. It is a “grain refiner”, that is to say in most circumstances we get smaller (better) grain structure with silver than without it. Silver is used in yellow & rose golds, and sometimes in white gold. Very little silver is used in nickel white gold, many white gold alloys contain no silver at all. Palladium white gold does use a lot of silver. Zinc-Zi If we use only gold, and silver and copper, we get less than desirable color in 10k & 14kt. Too dark for many clients. We also get high casting temperatures. The element we look to for this fix is Zinc. Notice that the most common yellow metal around us by far is brass. Brass is usually about 70% copper and 30% zinc. When an alloy is yellow it improves the color of 10kt. From one perspective, yellow alloys can be seen as a high-quality brass with silver added to keep the mix soft & the grain size reasonable. This white, soft, low melt element really saves the day in 10 & 14kt Yellow, some nickel white alloys, and even in some light rose color alloys. If overheated, zinc vaporizes off as a white smoke. This causes upward “karat creep”. Zinc does oxidize but is not nearly as troublesome in that way as copper. Nickel This very white, very high temp metal is the color ingredient in white gold. Most jewelers do not realize that nickel white alloys are really mostly copper! The nickel is the source for white color, but at a cost. It is the source of the things we dislike in white gold, like high melt temperatures and hardness. Nickel white gold is very hard compared to other colors. Due to a perceived health risk, it is not allowed in some European countries Palladium The best alternative to nickel for making white gold. Usually mixed with a lot of silver and sometimes a bit of copper (remember how copper helps elements mix?) On the up side, palladium white is very soft. On the down side- High temps (again like nickel), and relatively poor color. Nickel is the white color king I’m afraid. Plan on rhodium plating any palladium white gold
(Not Very) Secret Ingredients Most of the following ingredients are used by alloy makers to make your work easier to accomplish. Silicon-Si A deoxidizer, that allows us to re cast old gold with moderate amounts of new gold added. This stuff protects to a great degree from oxidation and from investment/gold reactions. The downside- thick viscosity in molten gold and larger grain structure than we would really like. Once again, this solves one set of problems, but creates a need for the following trace additive…. Boron-B A reputed deoxidizer, this really helps offset the thick viscosity issue. Nearly always found in alloys containing silicon to offset the above mentioned thick flow. Cobalt-Co Used in certain proprietary/patent alloys to increase hardness after heat treating. Very effective when used correctly. “Grain Refiners”
These elements are intended to cause a smaller than normal grain structure in gold. If not used exactly right, they can cause hard spots and localized discoloration. Use with care.
Iridium (Ir), Nickel, (Ni) Chromium (Cr), and others.