Cecil’s column today discusses the safety of phosphorescent materials, and with regard to radium-based phosphors, includes the [mitigated] claim that “the best information available at the moment finds no real threat in this low level radiation.”
I offer the tale of Undark and the Radium Girls for your consideration, as well as the general safety aspects of radium. I gather radium-based phosphors are no longer in general use, which is just as well; I suspect most people want to hear “there is no dangerous radioactive material in your watch,” instead of “there isn’t much dangerous radioactive material in your watch.”
There is a big difference between having a small amount of radium paint close to your body and having a much larger amount in direct contact with it, not to mention actually ingesting the stuff. Just sayin’…
The amount of radium in the watches was miniscule, and the beta decay radiation is effectively blocked by the watch body and the crystal. It takes about 6mm of aluminum to block beta particles.
The problem with the Radium Girls is that they used their lips to shape the brush, so they could paint on the little dots on the clock face. The brush contained Radium, and Radium is taken up by the body and placed in the bone instead of calcium. The embedded radium particles sit inside the bone matrix and bombard the bones with electrons. This damages the bone structure and the DNA in the bone cells.
So, if you find a watch with a radium dial, it’s probably safe as long as you don’t lick it.
They’re not available in the US, but in the rest of the world you can get glowstrips that use radioactive tritium as an energy source. They work much like the radium ones. A little glass tube contains the tritium gas; the inside of the tube is coated with phosphor. They’re usually mounted in solid plastic cases because they’re fragile.
Tritium emits fairly low-energy beta particles. They can only penetrate about 6mm of air, let alone the glass case of the tube; the tritium does slowly leak out over time, but in such tiny quantities as to be indetectable. It’s got a half life of 12 years and the cheapo key rings you can get for about $10 each tend to noticeably fade after 6 or 7 years.
Just to note: that’s a reprint of a “classic” column from 1978, so there could well be developments in the last 30 years. Is this something that should be re-addressed, d’ya think?
Absolutely; there should be new materials, new research and new applications (glow-in-the-dark tattoos for one) by now. Everybody loves toys that glow-in-the-dark, so it would be appreciated by the TM.
I don’t know about glow in the dark tattoos. I’m not sure where the ink in the tattoo lies. As far as I know all non-radioactive glow in the dark inks involve intersystem crossing from a triplet state to a singlet state. The triplet state involves two unpaired electrons, and unpaired electrons can act like free radicals. Free radicals are generally not very healthy.
I don’t know anything for sure about the safety of glow-in-the-dark inks, but I’d guess injecting it into your body isn’t a great idea. I wouldn’t have any issue with painting my face with it using external make-up.
I thought it might be worth mentioning in an update as it’s an interesting idea. I’ve heard rumors, but only seen UV reactive ink–not the same. A quick search revealed only a passing mention in a patent from 2000.
I’m not sure what you mean by “UV reactive ink”. Fluorescent ink is usually UV reactive, but it certainly isn’t new or unusual. It isn’t even glow-in-the-dark.
Personally, I’d expect that any glow-in-the-dark tattoos would have the actual glowing material encapsulated somehow, in tiny beads of some safe material. It’d decrease the image resolution, of course, but it seems like it’d make it much safer.
I’ve heard rumors of glow-in-the-dark ink, but haven’t seen any. I’ve seen UV reactive ink and that might be the source of the rumors of glow-in-the-dark ink.
Here is a site that suggests glow-in-the-dark ink, but it isn’t. These inks are fluorescent and require UV light to “glow”.
Chronos is right, that it might be possible to encapsulate a glow-in-the-dark pigment. For all
I know about tattoo inks, they may already be encapsulated.
I got the data from the Wikipedia page on tritium, which says that they’re particularly low energy particles and will only pass through 6mm of air. Is it wrong?
The first useful glowing watch markers were radium. Radium produces a more dangerous type of radiation that can penetrate many materials, and large amounts are dangerous to be near. In practical terms, as long as you don’t swallow the paint or inhale the dust, radium watch markers are relatively safe. Radium also has a long half-life, meaning that the hands will glow decades. In fact, the usual reason the glow of radium hands decreases is due to the destruction of the zinc compound rather than the radioactivity decreasing. You can see this on some 50+ year old watches where the numbers and hands appear to be coated with a thick coat of grainy charcoal-gray paint. The radium is capable of discoloring the plastic crystal over time–I’ve had several crystals with 12 brown dots an an image of where the hands stopped.
Radium was entirely discontinued in the late 1960’s, replaced by promethium for a few years, then tritium, and steadily-improving nonradioactive phosphors. Most “glowing hands” watches now use nonradioactive phosphor. I’m not sure if any still use tritium paint, most modern tritium watches seal the gas in tiny glass vials. Tritium has a 12 year half-life, but the initial glow of the vials is bright enough that even at 1/4 strength the glow is easily visible. If a watch does have tritium, it will generally be marked T-SWISS-T . (Asian manufacturers rarely used radioactive paint)
I work at Glow Inc and think I might be able to shed some light on this thread.
First, radium paint is not typically considered “glow in the dark” aka “phosphorescent”. Radium paint is also not been available for a long time and what is left is very dim or altogether dark. Tritium paint is available in viles and is very expensive.
Phosphorescent paint is made from Strontium Aluminate or Zinc Sulfide pigments. Unlike radioactive paints, it stores light and releases it slowly as a glow. It is completely non-hazardous. It is used in watches and even tattoos.
Pigments are different that dyes in that a pigment must maintain its original structure within its surrounding medium. Dyes alter the actual color of the surrounding medium. A true ink requires a dye, not a pigment.
Therefore, you will not find a “glow in the dark” ink, but can easily find glow in the dark paint. I will also note that you can mix glow powder and plastisol or other materials to make a pseudo ink.
I hope this clarifies any confusion. Check out our glow paint if you are interested.