Why some metals are better than others: a reactivity guide

Buffy Acacia

Every watch owner has their own opinion on wear and tear. For some, scratches and patina are sacred for telling the story of a watch’s life. For others, keeping a watch in pristine condition is the ultimate sign of respect for its service over years on the wrist. But, no matter how much care you take, watches do undergo change just by virtue of existing. Reactivity has been something of a hot topic lately, especially regarding the rising popularity of titanium as a watch case material. Yes, it may be highly biocompatible and regularly used in surgical implants, but why is that, and how is that relevant to watch cases?

vacheron constantin overseas tourbillon titanium wrist 2 — The Vacheron Constantin Overseas Tourbillon Titanium.

When we wear a watch, we tend to think of the case as something pretty much inert. If it looks like metal, feels like metal, and doesn’t immediately burst into flames like caesium, then it’s non-reactive, right? Well, reactivity isn’t always obvious. The vast majority of metals do in fact chemically react quickly with oxygen in the air, but the oxide layer is incredibly thin and not really visible. It’s actually that oxide layer that acts as a protective coating, preventing the metal from reacting further. In the case of stainless steel, it’s the chromium additive that helps to protect the majority of the steel from rusting as we would normally expect of an iron-based alloy. Still, the oxide can be removed by the acidity of sweat, harsh chemicals in cleaning products, seawater, and so on. So what does that mean for the longevity of wristwatches?

Stainless steel and titanium

Nivada Grenchen F77 Titanium Meteroite Dial 2 — The Nivada Grenchen F77 Titanium Meteroite, available from the Time+Tide Shop.

There are some things we know for certain, and other things we’re still waiting to find out. Since titanium now appears in almost every watch catalogue in the world, let’s begin there. Titanium is actually a highly reactive metal, but its oxide layer provides fantastic resistance to acidic corrosion. That’s why it’s considered biocompatible, and of course, it doesn’t contain nickel like stainless steel, which some people can be allergic to. However, titanium watches haven’t been around for anywhere near as long as steel watches.

1980s Citizen x 8 chronometer — A 1980s Citizen X-8 Chronometer made of titanium. Image courtesy of Horbiter.

The first titanium wristwatch was the Citizen X-8 Chronometer which was launched in 1970. But while titanium oxide is resistant to acids, it continues to oxidise over its lifetime. The initial oxide thickness is approximately 1-2 nanometers, growing to ~25nm after four years. If you look at vintage titanium watches from the ‘70s and ‘80s, you can see a slight fuzziness or matte effect on the case which may have originally been bright and shiny. A quick polish will restore it in no time, but case polishing is deeply frowned upon by most enthusiasts and collectors. It’s hard to know for certain what a titanium case will look like after 100 years.

1940s steel JLC reverso — A 1940s Jaeger-LeCoultre Reverso made from stainless steel, displaying heavy pitting. Image courtesy of Christie’s.

Conversely, stainless steel has been used as a common watch case material since the 1910s, when nickel or chrome-plated brass was still being phased out. Stainless steel watches from the ‘20s and ‘30s still look great, as the chromium oxide remains shiny throughout its lifetime and prevents orange or black corrosion from forming. However, as areas of the oxide get worn off and re-form over time, stainless steel can still be gradually eaten away. That’s how you end up with pitting, which looks like tiny dimples appearing all over a case. They can’t be polished out easily like surface scratches or titanium’s oxidisation, but they do make a case look unmistakably antique. Specialty alloys such as 904L stainless steel as used by Rolex have higher corrosion resistance and do not get pitted as much as the standard 316L recipe.

Gold and silver

1920 pocket watch 18k gold patek philippe — A 1920 Patek Philippe pocket watch in 18k yellow gold. Image courtesy of Christie’s.

Precious metals are more valuable than some others, but that’s because of several factors. In ancient times they were easy to melt and shape into coins and jewellery – and they were rare enough to control worth without being impossible to find. Most importantly though, they didn’t rust or corrode like iron and copper did. Gold is the classic example, being the second-least reactive metal we know of, as well as the backbone of the global economy for thousands of years. Even when it’s alloyed with more reactive elements to give it better strength and a higher hardness, it will almost always look perfect throughout its existence (barring scratches of course).

Boucheron sterling silver case and braclelet 1890 — An elaborate Boucheron case and bracelet made of sterling silver, c.1890. Image courtesy of Christie’s.

Silver has only recently started seeing use in wristwatches again, but it was a staple of the pocket watch era. It’s a common misconception that silver tarnishes in the air over time because it is actually very stable in air. It’s sulphur compounds that give it that dark, even black oxidisation, but that too creates a protective coating. Frequent wear of silver can actually prevent tarnishing because friction will act as subtle polishing, and any tarnish that does form is easy to remove. It may not be anywhere near as valuable as gold, but it still has some material worth and historical romance which makes it compelling.

Platinum and tantalum

cartier tonneau cintree platinum — A Cartier Tonneau Cintrée in platinum, c.1912. Image courtesy of Christie’s.

Admittedly, I was once a bit of a hater of platinum and white gold alloys. Having now expanded my understanding of precious metals and metallurgy in general, I recognise that their purposes and strengths have nothing to do with being distinguishable from stainless steel. Platinum is especially fascinating, being worth less per gram than pure gold yet often commanding a price premium in jewellery and watchmaking. Platinum is the least reactive metal of them all, forming a slim layer of platinum dioxide that will protect it from everything but hot, aggressive acids. Even though it’s a relatively soft metal, scratches on platinum have a ‘healing’ effect that makes them harder to see than some other materials. Many platinum watches from the 1920s, like plenty of Cartier models, look like they could have been manufactured a week ago.

1990s Audemars Piguet Royal Oak Championship Tantalum — A 1990s Audemars Piguet Royal Oak Championship in tantalum. Image courtesy of Christie’s.

Tantalum, while not technically considered a precious metal, is generally treated like one in watchmaking. It has almost no reactivity in temperatures below 150°C, but its greatest quality is its hardness of 860-1,200 Vickers. That’s up there with chemically hardened stainless steel and even ceramic, offering great scratch resistance. It also looks fantastic because of its dark blue tint, which is fairly unique in watchmaking. The main reason for its high price tag is how difficult it is to manufacture, but it seems like some microbrands such as Zelos have begun to experiment with it on a regular (if limited) basis.

Non-metal reactivities

tissot prx powermatic 80 carbon wrist — The Tissot PRX Powermatic 8 Carbon.

It would be a huge oversight to ignore all of the other materials that watch cases are made of these days, like plastic, carbon fibre, ceramic and sapphire. Their reactivities aren’t as simple or as predictable as metals and alloys, except for ceramic and sapphire which are totally inert as well as being very difficult to scratch. The biggest factor when considering the wear of a non-metal case is actually the effect of UV light and sunshine. Exposure to harsh UV light can cause colour fading in most materials, although modern-day ceramics and sapphires claim to have a great deal of colour stability.

Swatch Watch — Swatch made plastic watches cool in the 80s.

When it comes to plastic cases, UV not only causes colour fading but makes cases brittle and prone to cracking. It may be stating the obvious to say that plastic watches aren’t built to last, but given how many luxury brands have adopted recycled plastic in an environmentally conscious move, it’s worth keeping in mind. Carbon fibre itself is completely impervious to UV, however it gets mixed with resin in manufacturing processes. Resin is also a type of plastic, and it can also become yellowed or brittle after excessive exposure to UV light. For that reason, many carbon fibre composites use additives or other protective coatings to counteract the effects of UV, but that may not be the case for all watch manufacturers.

The verdict

The ceramic-cased IWC Pilot’s Watch Chronograph 41 TOP GUN Mojave Desert.

When you visit a watch store with cash burning a hole in your pocket, chances are you’re not thinking decades ahead. Whether you’re contemplating a watch’s resale value or its chances of becoming a family heirloom, considering its material properties could impact your decision-making significantly. I will always believe that stainless steel is a modern miracle because something so cheap can look incredible for so long, but it can’t compete with something like platinum. Nothing lasts forever, but with an understanding of reactivity and corrosion resistance, you’ll know exactly how your watch will age alongside you.