The Science of Gold

Gold has long been a great jewellery-making commodity for its inherent value and malleability, especially in the watchmaking era, before the invention of stainless steel, where it simply made sense to use gold rather than brass for better colour and corrosion properties. While it is possible to craft a piece of jewellery in pure gold, it is usually forged with other metals to form an alloy that will result in a better balance between desired hardness and colour.

For that purpose, a ternary system is used in metallurgy as the basis of coloured gold: through the manipulation of gold (Au), copper (Cu) and silver (Ag), the gold alloy can be made to a specific hue and saturation. Copper is an effective red dyeing agent that gives a warm hue to the gold; silver, on the other hand, is a bleaching agent used for its white colour to dilute and desaturate the colour. In order to classify these alloys, specifically referring to the concentration of gold relative to the other components, the gold industry uses the karat measurement, with a base proportion out of 24. For example, 24 karat, being the maximum, means all 24 parts of the alloy are gold, while 18 karat indicates that 18 out of 24 parts are gold, and so on. The remaining elements are primarily copper and silver, although there are other elements used, depending on the creativity of metallurgists and the desired result, which we will examine later. We should also note that there is a minimum karatage required for an alloy to legally be considered gold, which varies depending on the country; for example, in France, anything less than 18K cannot be called gold, but must be referred to as a gold alloy. As if that were not complex enough, many European countries have adopted a millesimal fineness index, which indicates how much gold a particular alloy contains in part per thousand; 18K gold is therefore also stamped with 750, the millesimal symbol being omitted.

When it comes to the colour itself, the Deutsche Institut für Normung (German Institute for Standardisation) established the DIN 8238 standard in 1966 to provide a reference point for gold foundries and manufacturers across Europe to unify their production. The colour code was generated based on the comparison of physical gold panels by the naked eye, which was subjective and often inaccurate, because lower karatage gold panels tended to discolour and tarnish with time. It was later matched by the International Organization for Standardization with the ISO 8654 standard in 1987. It adapted the CIELAB (the International Commission on Illumination) method in the measurement of colour, using a spectrophotometer to generate coordinates that describe and position a colour within a three-dimensional colour spectrum. The absence of the human eye intervention resolved the problem of objectivity and mathematical accuracy from the superseded standard, and therefore more consistent to serve as a reference to manufacturers. It remains generally in use today, and was updated as recently as 2019, to expand the range of gold colours considered and how they’re to be measured.

Provenance & Application

The strong interest of civilisation towards gold dates back to antiquity, where goldsmiths were recorded forging copper and silver with gold in producing jewellery and currency. And such fascination will naturally migrate to timekeeping devices such as sundials, clepsydras and eventually watches.

Constant Girard, the Swiss watchmaker and one of the founders of Girard-Perregaux, was known to be fond of gold, especially pink gold. His application of coloured gold can be found not only in exterior components, in pocket watch cases, dials, hands and hour markers, it also extended to the movement parts. One of his masterpieces, the “Tourbillon sous Trois Ponts d’Or”, can be found with three identical, parallel and symmetrical pink gold bridges, which held the hour and minute hands, barrel, and the tourbillon, an original and groundbreaking design. It was later awarded a gold medal at the Paris Universal Exhibition in 1889, and remains a signature of Girard-Perregaux, even in today’s watches.

Another historical piece would be one of the most complicated pocket watches ever made, so much so, that it took around 44 years to complete despite its development and manufacturing process stretched across the years of the French Revolution: it is the No.160 Grande Complication, or the Perpétuelle, designed by Abraham Louis Breguet. It was a commission timepiece for the Queen of France, Marie Antoinette, and it stated that gold should be used in place of other metals in any part where it was feasible. Unfortunately, Breguet would not witness the completion of the pocket watch in his lifetime, and the task was taken up by Breguet’s son, Antoine-Louis, and it consequently remained a Breguet family asset until it was sold in 1887. The Perpétuelle consisted of every complication known of its time: a total of 23 including a minute repeater, perpetual calendar, equation of time, bimetallic thermometer, secondes d’un coup (dead-beat seconds), to name just a few. The movement was crafted with the finest material in exceptional finishing, housed inside a plain gold case, with plates and wheels also made with gold visible under the rock crystal dial.

In 2005, the Perpétuelle was set to be recreated by the modern Breguet watchmakers (the original was stolen in 1983 from The Museum for Islamic Art and was not recovered until December 2007), commissioned by the late Nicolas G. Hayek, the founder and chairman of The Swatch Group, as an exact replica of the original No. 160. The new Perpétuelle, reference No. 1160, was given a commensurate degree of exquisite finishing, all plates, bridges and bars, as well as every moving part of the calendar and repeater mechanisms, are made of wood-polished pink gold. The project went on a research journey in recovering the original technical drawings and the styling details of the movement, with the aid of Breguet Museum Archive and Musée des Arts et Métiers, the watch was unveiled during the 2008 BaselWorld.

Modern technological advances have allowed the introduction of innovative materials that are lighter, and less subject to friction, such that watchmakers now have more material options for achieving better chronometric performance. Yet, some still prefer gold parts in their movements, mostly base plates and bridges, for aesthetic reasons. Prominent examples would be François Paul Journe for his consistent use of rose gold in plates of his movements (as a differentiating factor from the rest of the industry,) and skeletonised rose gold plates and decorative engraving on some Parmigiani Fleurier references.

Looking into the proprietary alloys

The copper element within the ternary composition of rose gold (as mentioned earlier), contributes to the charming red colour found in the alloy. It does come with a demerit, as seen in many vintage jewellery and watches, as it suffers from oxidation. Unlike bronze, where oxidation creates a more favourable greenish patina, the same chemical reaction on red, pink, or rose gold alloys results in an undesirable effect, as the alloy tarnishes and loses its red tone over time, returning to the colour of yellow gold. However, it only happens to the surface of the alloy that has contact with sweat, oxygen, moisture, and chlorine. This means that it is possible to restore the original colour underneath simply by polishing, but it also means that it requires case material to be sacrificed.

To address the problem, metallurgists have come up with contemporary solutions. As discussed earlier, the ISO and DIN standards that were established were purely as references to unify production, but brands have used a certain latitude as room for creativity and exploration in customising their gold alloys. From here, the proprietary alloy in the market is divided into two major fields of exploration: one path is to fine tune a stable and accurate colour (usually varying between the 3N to 5N range), another path is to achieve better structural integrity (e.g. strength, hardness), or sometimes both.

Fine Tuning Colours

Given the advantage of having an in-house foundry, Rolex developed one of the first proprietary alloys in the industry. In 2005, they debuted a rose gold alloy, named Everose gold. Developed based on an 18 karat gold and copper, it is also fused with a precise 2% platinum to lock down the colour, preventing the surface layer from discolouring when in contact with substances that can possibly oxidise the copper within. With over 15 years since the production of Everose watches, it is showing promising results in retaining a stable warm, rosy colour in the alloy.

Panerai has a proprietary gold alloy they’ve dubbed Goldtech, which also has added platinum to prevent the rose gold alloy from oxidation. Their research and development department openly revealed the formula of the alloy (although it is most definitely patented). Composed of 75% gold, 24% copper and 0.4% platinum, Panerai describes the alloy as having a “unique tone and an extreme degree of mechanical and oxidation resistance”.

Hublot also developed a proprietary alloy, King Gold, based on the addition of platinum, in which the ratio among all three formulas is the highest, at 5%. While the remaining ingredients remain a commercial secret, having the highest percentage of platinum should, in theory, result in an alloy with the least red hue out of all three above.

Le Grand Rose gold from Jaeger-LeCoultre and Sedna gold from Omega both add palladium in their formula to protect the alloy against oxidation, employing the same principle of adding platinum into a copper-fused alloy, they both possess a similar bleaching property, hence it leads to a similar result on the colour. However, no information regarding formulas was disclosed, likely a safety barrier to the companies’ important intellectual assets.

Achieving Better Structural Integrity

A. Lange & Söhne developed their proprietary alloy dubbed honey gold. It was named after its colour, which is reminiscent of the sweet liquid produced by bees. In the metal, it sits in an ambiguous hue between yellow and silver and is slightly desaturated. As alluring as the colour might be, the alloy was not initially developed for an aesthetic reason, but rather to improve the structural integrity so that watch cases can be better resistant to dents and scratches. Composed of gold, copper, and zinc, the honey gold alloy is twice as hard as yellow gold and harder than platinum, but as the increase in hardness results in a lower workability of the alloy, it takes more time to manufacture watch cases out of the material. The challenging part for Lange is finding a foundry capable of fabricating the alloy and matching the minimum order quantity required, hence only a limited amount of honey gold watches are produced every year.

There is also another proprietary alloy under Hublot, Magic Gold, which is exclusively produced within their foundry in Nyon. It took two years for the research and development team to figure out how to cast the material in-house. The difficulty lies upon the substance that composes the alloy, other than the fundamental 75% gold, the remaining 25% is boron carbide, one of the hardest known materials in the form of ceramic powder. It melts only at extremely high temperatures that a casting mould would find impossible to sustain, so the ordinary heat and forge method is not feasible in this case. According to Hublot, the powder is first compressed in a pressure machine under a cold environment, so it is transformed into a solid form. They then use gas pressure to combine the 24k liquid gold with the boron carbide solid, from which it became the final raw material. The result is a scratch-proof (not to be mistaken for scratch-resistant) material for their Magic Gold watch case.

Final Thoughts

It is great to see brands putting the effort to hone the various coloured golds, although there are only a few percentage differences within the compound of each alloy, by comparing the performance, or patina of the alloy over time can certainly be interesting. It is also worth noting that the cost of precious metals is also a factor in deciding the market trend, similar to how silver and white gold became prominent following the gold price inflation from 1972 to 1973. So we might see substitutions given the finite resources on Earth, or even the rise of synthetic precious metals. Therefore as tangential as some alloys might sound, it might just be the innovation we need to invent new composite materials.