WatchPro Salon

Exploding the science behind black watches


By Giles Ellis

Black watches are very much in vogue, but there is a lot more to creating a dark timepiece than meets the eye. Schofield founder Giles Ellis gets his tech hat on to explore the science behind black watches.

Navigating the details on modern case materials, coatings and finishes can be a confusing undertaking. Ambiguous science with misleading figures all contribute to the marketing nonsense that spews from our beloved and somewhat hyperbolic industry.

One specific area in which this is prevalent is the growing trend for black watches. Most manufacturers will offer black variants in their collections because black watches have recently been inaugurated into the halls of luxury, and quite rightly too, because black has been cool since the 14th century.

Perhaps this is a recent development because there were no readily available black materials or coatings until the mid 1970s, other than oxidising or anodising metals, plastics, powder coat or paint, and most of these don’t sit comfortably beside platinum, gold or stainless steel in a chart of premium materials.

Black means a surface that absorbs light and does not reflect it. It will have an albedo of 0 (fully reflecting surfaces have an albedo of 1). Black is the absence of colour.

In 2008 researchers at Rensselaer Polytechnic Institute created the blackest material ever, reflecting just 0.045% of incident light from carbon nanotubes stood on end. This is 1/30th of the light reflected by the current standard for blackness.

But when it comes to watches we are limited to the materials we use because of either machining, safety and cost constraints. So when I say black, I also mean dark grey and sometimes with hues of brown, blue or green. This is exciting because these subtleties bring differentiation and make our black watches special.

As a designer of watches I look at all the material surface characteristics – colour, texture, slickness, temperature and sheen, finding ways to exploit these attributes favourably. Let’s look more closely at the materials and the language used to describe them in an attempt to make an informed decision on whether the material can work for us.

When considering a watch case, texture is generally either rough or smooth depending on the surface irregularities. We should also consider waviness and whether or not the case has been engraved like the amazing Linde Werdelin Octopus Tattoo or Magrette Moko.

Slickness is synonymous with smoothness but not exclusively so. A watch case can have a rough surface finish and a DLC coating. Aluminium magnesium boride (BAM) has a record low co-efficient of friction (COF) of 0.02, DLC of 0.05 and Teflon in third place at around 0.4. These three surfaces are the only ones a gecko cannot stick to. Steel by comparison has a COF of 0.8.

Temperature is interesting because different materials feel warm or cold to touch because of varying degrees of heat conduction. Steel will take on changes in ambient temperature much quicker than carbon fibre for example.

Sheen is the complex qualities of reflected light and colour. Tantalum, for example, will look black in artificial light but like blue steel in sunlight. Metal case finishes are generally limited to polished, micro-bille (bead blasted) and linished (brushed) and like paint finishes the sheen of these would equate to gloss, matt and eggshell.

All of these factors contribute to the tactile qualities of a watch. It is, after all, an object you handle every day and because of this I would place as much emphasis on the visual as the tactile.

The physical properties of materials and coatings mainly only affect the longevity of the finish, which would include hardness, scratch resistance and toughness.

Hardness can be measured in a few ways but we will concern ourselves mainly with Vickers (HV). This test can be done on most materials and involves impressing a pyramidal diamond (the load) into a surface and measuring the plastic deformation of the surface. To put this into perspective, stainless steel (316L) is between 150 HV to 500 HV. Sapphire Crystal is around 2500 HV. This is relevant because it indicates how dent resistant your watch is.

Scratch resistance is different to hardness because it involves shear and load, whereas hardness is just load. Basically scratch testing is measured by drawing a point (load) over (shearing) a surface at pressure and measuring depth and displacement of the substrate.

We all know sapphire crystals to be scratch proof, right? Not quite, they are scratch resistant. All materials can be scratched. Glass, for example, we know to be hard but it can be scratched by a dog’s claw even though glass is harder than a dog’s claw.
Toughness is the ability of a material to absorb energy and deform not fracture, dent not break.

The ideal material would be one that is luxurious, exotic, durable and tough, has beauty and depth of standardised blackness with a subtle hue depending on the light, and has tactility and heft that goes beyond expectations. A material that is just something you cannot live without.

I think there are five practicable surfaces that have these desirable qualities – one metal, one ceramic, one composite and two coatings. They are fairly accessible with all in use across many brands. With all there is a compromise, with most it is cost.

The unexpected cost of repair and re-finishing of these materials could make the purchase of a black watch much more expensive.

Tantalum is a rare transition metal, bluish grey and very dense. In sunlight it is truly wonderful. Most of it comes from Australia and one of its main uses is in the electronics industry in electrolytic capacitors. It has a hardness of around 1900HV and is very scratch resistant. It is not easy to machine and the dust can combust spontaneously on exposure to air.

The rarity and difficulty in machining makes this an expensive material for use in watch cases. My favourite watches that use tantalum, and show the metal’s qualities, would be the Hublot Big Bang Tantalum and the Panerai Tantalum. I know both well and they look fantastic. Other brands using Tantalum are Helson, Montblanc, Audamars Piguet and Urwerk.

Zirconium dioxide is the most common ceramic used in watches. There are others and ceramic technology moves fast with companies concocting their own recipes, like Hublot’s Magic Gold and Rado’s Silicon Nitride.

Zirconium dioxide has a density similar to aluminium and is very scratch resistant with a hardness in excess of 2,000 HV, but it has a low fracture toughness. It is this last fact that has caused some concern with consumers over the risk of the case shattering or splitting in two. It is the Achilles heel in what would be an almost perfect material.

Ceramic watches are sintered, which is a way of making a solid object from powders. This is done by moulding and heating. IWC introduced us to it and Rado, the kings of ceramic watches, mastered it in the late 1980s. Chanel then widened its appeal in 2000 with the introduction of the J12. Looking at alternatives I would consider the IWC Topgun Double Chronograph or a Jaeger LeCoultre Master Compressor Chrono Ceramic.

Carbon fibre is generally known to be associated with Formula One racing and sports equipment. It has a very high strength-to-weight ratio. It has not really caught on in mainstream watch case production for good reason. It just does not have the homogenous density to be machined or moulded to complex shapes with small tolerances. It also lacks toughness and has a relatively low surface hardness. It is included here because a few companies use it well by experimenting with the ingredients that make the composite.

The Audemars Piguet Royal Oak Concepto Carbon, Hublot’s All Black Carbon, The Bell and Ross Carbon and the Ellicott Mach One Skymaster are watches that use more exotic mixtures. This is an expensive process and one that is close to Schofield with the development of our own material for the forthcoming Blacklamp watch. We have developed a trademarked material called Morta, which is a special carbon fibre that is incredibly dense, making it really tough, hard and scratch resistant, negating all of the downsides associated with the material. After all, we don’t need it to be lightweight as the right amount of heft is so important in luxury, unless you are Richard Mille.

Borstal is a textured black-grey material reflecting light with a sparkle like waves on the sea. It is moulded one watch billet at a time which gives each piece a unique textural grain.

PVD (Physical Vapor Deposition) is a group of coating methods that deposit thin film coatings, usually between 5 and 10 microns thick. It should be said that DLC (Diamond Like Carbon) is a material that is applied with the PVD process.
The watch industry, however, has split the two with PVD generally being regarded as the inferior coating. It is not always inferior, but is often cheaper.

Coating watches successfully depends on a particular ‘art’. It takes experienced technicians to combat the quirks of the equipment and variations in the environment, target, compounds, processes and cleanliness to guarantee consistent results.

PVD deposits a layer of material by condensation of vapour in a high temperature and vacuum environment. It is often applied by line of sight meaning it is sprayed on. If it is angular shapes being sprayed then sometimes the crevices won’t get coated. The other issue is that PVD, when applied at high temperatures, can anneal the steel target and make it softer therefore more prone to denting. What is the point of having a super hard coating if the supporting structure is soft? If this is so, then the coating is nothing more that cosmetic.

The coating compounds are made up of various elements such as carbides, nitrides, borides and silicides; confusingly these coatings are most often hard ceramics. PVD has had a reputation of peeling or flaking but this is occasional and is not symptomatic to the method. Coatings applied with PVD range in hardness from 1,000 to 4,000 HV. PVD is ubiquitous in the marketplace, an example of a brand that uses particularly good PVD is Romain Jerome.

DLC, as the name suggests, is just that. Like PVD it can be done well or cheaply and badly. The diamond like structure of DLC can be more or less diamond like depending again on variations in the process – cleaning, compounds, catalysts, temperature, vacuum, etc. Really good DLC (like we use) is not only very expensive and time consuming to apply but is harder than most other PVD coatings. With a surface hardness of more than 4,500 HV it is extremely scratch resistant. DLC like this has very few hydrogen atoms ensuring the carbon atoms bond strongly with each other in what is known as amorphous sp3 bonded carbon.

DLC is mainly used as a tribological coating, for example in engine parts or cutting tools to reduce wear and friction. Even your Gillette razor blades are treated this way. When handled it gives an unusual greasy, slippery feel. Colour of DLC coatings used for watch cases range through dark grey to jet black. The Schofield Signalman DLC is black with subtle brown tones when viewed in daylight. We bead blast the steel prior to coating to give a matt finish which makes the case look darker. It also work-hardens the steel, improving its hardness. Other companies using DLC to great effect are Linde Werdelin, Ulysee Nardin and Cartier. There are also companies like Bamford that custom black your steel watch as an after market service.

There are, of course, other exotic materials in usage like Arcap and Alusic and in development like using femto pulsed lasers to blacken any metal. When designing watches a decision has to be made based on a materials cost, scientific pros and cons and whether it fits with the projects aesthetic nature. As consumers we can engage further with the things we touch through knowledge. For me it’s about integrity in design that includes the science but also art.

Giles Ellis is the founder and designer of English timepiece brand Schofield Watch Company. This article originally appeared in the April 2013 issue of WatchPro




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