02 Nov Red gold. Features and common problems
The demand for red gold has strongly increased in recent years, especially in the countries of the Far East. The processing of this variety of gold can present some pitfalls, let’s see how to deal with it.
A bit of history
Historically highly valued in Russia, red gold was made famous by the jeweler Peter Carl Fabergé, who used it in the production of some of his famous eggs in the second half of the 19th century.
In central Europe it was brought to the fore by the Cartier maison, which inserted a red gold ring into its famous Trinity Ring model in 1924.
What is red gold?
All the metals in the periodic table have a more or less light gray color, with the exception of two elements, gold, with its characteristic yellow, and copper, which is red.
Silver is also a noteworthy metal, because it is the one that more than any other is able to reflect visible light and its color is close to white.
Precious alloys are mostly composed of a combination of gold, copper and silver: by varying their ratio, numerous shades of color can be obtained, from green to intense yellow. Red gold is obtained when the amount of copper is very high.
The hardness of red gold
Gold and copper are completely miscible with each other, as are water and alcohol, as long as the temperature remains above about 410 °C. If the gold and copper alloy is allowed to cool slowly below the threshold of 410 ° C, the ability of the two metals to mix is reduced and it is probable that particular compounds, called “intermetallics”, are formed, which have very high hardness, and very low machinability: the metal becomes very difficult to draw or laminate and can easily break.
When are intermetallic compounds formed?
In addition to the aforementioned temperature, the possibility that intermetallic compounds form depends on the ratio between copper and gold: the more a master alloy contains high percentages of copper, the higher the probability of the formation of intermetallic compounds. The ratio of gold to copper also depends on the carat, the most delicate carat is undoubtedly 18k red gold.
Why is 18K red gold so fragile?
In 18K red gold the ratio of gold to copper in number of atoms is very close to 50%, this creates the perfect conditions for the formation of the AuCu compound: if the alloy is cooled slowly below 410 ° C, large portions of metal can turn into intermetallic compounds, making the semi-finished product very brittle and hard.
What can be done to reduce this problem?
Some precautions must be taken:
- The most important is to cool the metal quickly, both in the melting and in the
- annealing phase, to a temperature of about 500 ° C, this helps to maintain workability, for example in the case of intermediate annealing.
- Use master alloys in which the percentage of copper does not exceed 80%, this however at the expense of the intensity of the color.
- Use master alloys with a high content of grain refiners: these will allow the gold to maintain a good crystalline structure despite the frequent and demanding annealing that the metal will undergo and will help reduce fragility.
And what about investment casting?
In the case of investment casting with red gold, the same recommendations apply, but quickly cooling the flasks once extracted from the casting machine could be a problem if wax setting of stones is used. In this case, in fact, we have two opposing needs: on the one hand the metal which must be cooled quickly, on the other the stones, which do not withstand sudden changes in temperature. To overcome this problem, you can use a trick: once the flask has been extracted from the casting machine, it is overturned and partially immersed in water, keeping only the button in contact with the water. In this way, the main sprue will quickly cool the pieces, thanks to the good thermal conductivity of gold, while the stones, which are only partially in contact with the gold, will cool more slowly.
Red gold can present some pitfalls, but with the right precautions it can be easily worked. In our catalog you will find master alloys capable of producing gold of an intense red color and easy to process.