The subject is one of the greatest interest to collectors and yet, there is still widespread misunderstanding of how luminous materials actually work. Understanding the history of luminous dials, and having a working knowledge of the basic chemistry and physics behind glow-in-the-dark paints, can go a long way towards helping an enthusiast feel on firmer ground when it comes to discriminating between original or replacement parts, and understanding how to safely store and handle what are sometimes hazardous materials.

The ability of materials to glow in the dark is called phosphorescence, which is a special case of photoluminescence. Photoluminescence is the ability of some materials to emit light after exposure to light. Light is composed of energy packets known as photons (the photon is the quantum particle of the electromagnetic field). While the exact quantum mechanical description of photoluminescence is complicated, the basic idea is simple: if an electron orbiting an atom in a certain material absorbs a photon, it will be excited to a higher energy state, and when the electron “relaxes” to its ground state, it emits a photon which we see as visible light.

The two types of photoluminescence familiar to most of us are fluorescence and phosphorescence. Fluorescent materials tend to emit absorbed energy very rapidly – they’ll glow as long as there’s a source of light to excite the material (typically with what’s described as “neon” coloration) but emission takes place over a time scale of mere nanoseconds, so they will go dark the instant a light source is removed. Anyone who’s played with a UV lamp has seen materials fluoresce. These are colloquially known as “black lights,” and black light posters were a dorm room staple in the ’70s (and for all I know might still be).

Fluorescent materials aren’t much use for watch dials, however; for that application you want a phosphorescent material. Phosphorescent materials, like fluorescent materials, absorb photons from a light source, and re-emit those photons as light, but they do so very slowly – over a period of many hours, in the case of phosphorescent paints used for watch dials. This is because in phosphorescent materials, the excited electron is in a special energy state, in which the re-emission of a photon would require a “forbidden” energy transition. Despite the name, these transitions still happen, but they’re statistically unlikely and so the trapped light leaks out much more slowly than in fluorescent materials.

In order for phosphorescence to take place you need an energy source, and a so-called “phosphor” – a material that will absorb and re-emit light. Let’s look at how radium dials work.

Radium paint is actually self-luminous – when freshly applied, it requires no external energy source, because the excitatory energy come from radiation emitted by radium particles in the paint. Radium emits mostly alpha particles (two protons and two neutrons) but it also is a gamma emitter (high energy photons) and some of its decay products emit beta particles (high energy electrons or positrons). Radium by itself glows weakly, so it’s combined with a phosphor in radioluminescent paint – almost invariably, zinc sulfide, which is often combined with a “doping” metal to give a specific color.

The other problem is that radiation from radium causes zinc sulfide to break down chemically (in a way similar to the mechanism by which UV radiation in sunlight causes materials like ordinary paint, vinyls, and other plastics to deteriorate). Radium dials usually lose their ability to glow in the dark in a period ranging anywhere from a few years to several decades, but all will cease to glow at some point.

A key point to bear in mind is this: the dial is still highly radioactive. The phosphor deterioration means you can’t see a glow anymore, but radium takes thousands of years to completely decay. Radioactive materials, as they emit radiation, decay to other elements. Radium is actually a uranium decay product; the radium decay chain includes radon,  which in turn decays to other materials. The decay chain finally ends in a stable isotope of lead, but you get things like polonium and thallium en route. The “half life” of a radioactive element is the length of time it takes for about 50% of a sample of any given size to decay – for radium, that’s 1,602 years.

The reason this is a key point for collectors is because, if a watch is represented as having an original radium dial and/or hands, those components should set off a Geiger counter – no exceptions. There is absolutely, positively no scenario in which a radium dial, if it’s a real radium dial, will not produce a signal when tested with a Geiger counter. Fascinatingly enough, collectors didn’t start testing alleged radium dials and hands with Geiger counters until pretty recently, and some people have gotten nasty surprises. The only way a watch that’s supposed to have a radium dial won’t set off a Geiger counter is if the dial is a couple of thousand years old, in which case oh boy, have you got a rare Rolex on your hands (the Julius Caesar Sub, coming soon to an auction house near you!)

Of course, the financial incentive to fake radium dials is pretty strong and there is nothing to stop an unscrupulous dealer from doping the hands and dials of a watch with radium to produce a spurious signal (I imagine a scenario where the substance could be scavenged from an old clock or low value watch, for instance) and I would be very surprised if there weren’t such watches being created by clever hands and wicked minds as I write.

How does radium paint behave under a black light/UV flashlight? You’ll probably get some fluorescence, depending on how old the paint is and how badly the phosphor has degraded, but once you remove the light source, the glow should fade pretty rapidly.

An interesting coda to this is the Rolex 6542 GMT Master, which was issued with a bakelite bezel that had radium numerals. Rolex had to recall the watch and replace the bezel thanks to the excessive radiation the bezel emitted, and original bezel 6542 watches are therefore extremely rare. However, if the bezel is original it’s basically just as radioactive as the day the watch was made – an important caution for any collector who owns one.

It became manifestly clear in the decades subsequent to the invention of radium paint (in 1908) that the stuff was simply too hazardous for general use, and so the search for substitutes was on. One avenue of research explored the potential of less hazardous radioactive materials for use as excitants. Promethium emits only beta particles, and at a lower energy than radium, so it’s generally considered much safer; it also doesn’t cause phosphors to break down nearly as quickly. Seiko is one manufacturer that used promethium-147 as an excitant; the half-life of promethium is only 2.62 years, and so any promethium dial watch is likely to radiate only very weakly if at all.

Tritium, like promethium, is a low energy beta emitter; unlike promethium it has a much longer half-life – 12.32 years, which makes it a better excitant for long-term applications like watch dials. Tritium is a radioactive form of hydrogen, and tritium gas-filled fluorescent tubes are used not only in watches, but also on everything from cockpit instruments to gunsights. Two of the best known users of tritium gas tubes for watch dials are Ball and Luminox.

Tritium paint for watch dials and hands will lose its ability to photoluminesce over time, although as with promethium, degradation of the phosphor will take place more slowly, as tritium is a much weaker radiation source. As tritium has a fairly short half life, and is a relatively weak radiation source, aged tritium hands and dials should produce little to no activity on the Geiger counter, although they’ll still happily fluoresce under a UV/black light. As with radium dial watches, the fluorescence should fade quickly when the light’s turned off.

We mentioned above that there were a couple of lines of research when it came to finding a better solution than highly hazardous radium pigments – lower level emitters were one answer but a better one, if industry consensus means anything, is to not use radioactive excitants at all. To get away with this, you need a phosphor that will glow enthusiastically for hours after exposure to light – something that will act as a sort of “light bank,” emitting stored energy in a measured fashion. Enter Luminova. Luminova was invented in 1993 in Japan, by Nemoto & Co. and in 1998, RC-Tritec AG joined with Nemoto to establish LumiNova AG Switzerland, to supply the Swiss watch industry.

Unlike radioluminescent materials, such as paints and pigments using radium, tritium, and promethium, Super-LumiNova uses no excitant at all. Instead, it incorporates a material known as strontium aluminate, which is an extremely efficient phosphor that once charged, will glow very brightly initially, and with diminishing intensity for several hours thereafter. (For strontium aluminate to be an effective phosphor, it must be combined with europium, a non-toxic, non-radioactive chemical element).

Strontium aluminate is a much more efficient phosphor than zinc sulfide – it’s about ten times as bright and glows about ten times longer and the color can vary between various shades of green and blue, with blue supposedly producing the longest glow time, and green offering better brightness. The disadvantage of Super-LumiNova in comparison to radioluminescent material is of course that its brightness fades until it’s recharged by another exposure to light.Personally I’ve found that with most of the sports watches I’ve owned over the years, if my eye’s dark-adapted the watch remains legible, with some difficulty, through most of the night. Still, it is a disadvantage of the material relative to the always-on glow of radioluminescent solutions (which is part of the appeal of Luminox and Ball). You can also get some pretty high wow-factor visual effects with Super-LumiNova that have much better permanence than they would have with radioluminescent pigments; Bovet has used the material to great effect in the Récital 22 Grand Récital, as has MB&F in the Black Badger collab version of its Starfleet Machine.

Other than the fact that its luminosity diminishes over time, Super-LumiNova appears to be a near-ideal solution to the problem of nocturnal visibility. Strontium aluminate seems to be a highly stable phosphor, and Super-LumiNova dials, at least thus far, appear to not suffer from the gradual phosphor degradation characteristic of radioluminescent materials. It is adversely affected by moisture, so a high humidity climate might cause issues, but in general, Super-LumiNova and other strontium aluminate-based pigments seem likely to be with us for a long time. Exactly how long Super-LumiNova will retain its ability to glow is unclear – it seems safe to assume that sunlight can cause the material to break down eventually and if the vintage watch craze has taught us anything, it’s that nothing lasts forever; but it seems, in general, a very durable material.

I’m old enough to remember seeing radium dial watches in actual use (barely; I was very young, but anything that glows in the dark makes a great impression when you’re four) and while I miss the brilliant radiance of radium dials and hands, it’s probably just as well for all concerned that the industry got out of the business when it did. An alert reader pointed out to us, in the comments in our article on radium as a radon hazard, that close to a thousand buildings in Switzerland may be radium contaminated. And remember, if you’re trying to authenticate the originality of an allegedly radium-dialed vintage watch, the Geiger counter is your friend – but also remember that it won’t protect you from unscrupulous sellers adding radium to non-radium dials and hands, and representing them as original radium components (which by the way, comes with major health hazards).

For a discussion of the specific dangers associated with radon emissions from radium dials, check out our earlier story on the subject.