For many glass collectors, the only color that matters is Vaseline. That’s the catch-all word describing pressed, pattern, and blown glass in shades ranging from canary yellow to avocado green. Vaseline glass gets its oddly urinous color from radioactive uranium, which causes it to glow under a black light. Everyone who collects Vaseline glass knows it’s got uranium in it, which means everyone who comes in contact with Vaseline glass understands they’re being irradiated. It doesn’t matter whether you’re the gaffer making footed cake plates in a glass factory, the driver loading boxes of lace-edged compotes onto a truck, or the tchotchkes dealer setting out vintage Vaseline glass toothpick holders and tumblers for prospective customers—all of you are being zapped.

“If radioactivity is the thing that makes Vaseline glass cool, it’s not what makes Vaseline glass glow.”

Let’s say you’re that tchotchkes dealer’s customer, and you decide to purchase those tumblers because you think their hue will go nicely with your lemony Formica kitchen table. Well, you just bought yourself four tumblers full of radioactive beta-waves. Go ahead and fill those tumblers with orange juice or milk, then serve these wholesome beverages to your adorable children. Now you’ve exposed your innocent lambs to even more radiation, since minute traces of the uranium in the glass can leach into whatever your kids are drinking, coating their throats and stomach linings with a cool, radioactive wash. After slaking your children’s thirst, carefully rinse those tumblers by hand to absorb sponge after sponge of even greater concentrations of radioactivity.

For the record, none of this matters, not even a little bit. Yes, canary glass, uranium glass, or Vaseline glass, as it became known in the early 20th century for its similar color to petroleum jelly, emits radiation, but the amounts are tiny, infinitesimal, ridiculously small. Our bodies are subjected to many times more radiation every day. We receive a daily dose of radioactive contamination from the gamma rays that make it through our atmosphere after hurtling through outer space, from the naturally occurring radionuclides present in the ground we walk upon, from the background radiation lingering in the materials used to build the places we call our homes.

The beds we sleep in are radioactive; the lawns we sprawl out on during the dog days of summer are, too. In fact, there’s more radioactive potassium-40 inside each and every one of us than anyone could ever receive from handling, using, or just plain eyeballing a piece, display case, or entire museum full of Vaseline glass. If you are really worried about the trace amounts of radiation in Vaseline glass, you’d do better to stop putting bananas on your yogurt, to cut out all those healthy spinach salads, and to stay very far away from baked potatoes, all of which are packed with blood-pressure-lowering, radioactive potassium.

None of this matters, either, but you’ve probably figured that out by now.

Still, in our post-Hiroshima, Nagasaki, Chernobyl, and Fukushima world, radioactivity gives Vaseline glass a certain badass cachet. Some are drawn to its perceived menace so they can pat themselves on the back for not being intimidated by its unfairly toxic reputation. Others, like Dave Peterson, who co-founded Vaseline Glass Collectors, Inc., in 1998 and has written three books on the topic, gravitated to the material for more down-to-earth reasons. “It’s glass that does tricks,” he says, as full of affection for the stuff today as he was several decades ago, when he saw his first photo of a toothpick holder performing Vaseline glass’s most famous trick, glowing under a black light.

Even if radioactivity is the thing that makes Vaseline glass cool, it’s not what makes Vaseline glass glow, says Barrie Skelcher, who’s written two Vaseline glass books of his own. That may come as a surprise to many Vaseline glass collectors, who assume that radioactivity is the reason why Vaseline glass glows under ultraviolet light, confusing the cartoon depiction of radioactivity for the science.

“Vaseline glass was a victim of the ordinary light bulb!”

“It’s the chemistry of uranium that makes Vaseline glass glow, not radioactivity,” Skelcher says by phone from England, where he lives with his wife, Shirley, and 500 or so pieces of Vaseline glass in a collection that once numbered more than 1,000. “It wouldn’t make any difference whether the glass contained depleted uranium with the 235 isotope removed or natural uranium; the chemistry is identical. Uranium fluoresces under UV light.”

My kid sister agrees. Normally a sibling’s opinion on a question like this might not be especially relevant, but Naomi Marks is a Ph.D. in geology and a research scientist at the Lawrence Livermore National Laboratory in California, where she, uh, well, I don’t actually know what she does, and she probably couldn’t tell me if I asked. Let’s just say she knows enough about uranium to confirm Skelcher’s statement.

“Clearly, it’s not radioactivity that makes the glass glow,” Marks says. “If it was that radioactive, you definitely wouldn’t want it in your home! The uranium fluoresces under UV light because the UV excites the electrons above the ground state and gives off photons as the electrons transition back to the ground state.” Sure, everybody knows that. “The fluorescence is just an inherent property of the uranyl compound in the glass.” Natch.

What about Skelcher’s added detail about depleted uranium? “In depleted U,” Marks continues, lapsing into fancy-pants-scientist jargon, “the 235 is mostly, but not completely, removed. Since the fluorescence is a fundamental property of the U and has nothing to do with the isotopics, it doesn’t matter what the radioactive level of the U might be.”

So there you have it—the glow of Vaseline glass under a black light has nothing to do with radiation, as many people erroneously believe. Which is not to say that absolutely all glass that glows green under a black light has uranium in it. Other elements such as manganese can produce a similar effect, and sometimes pieces with a relatively large amount of uranium in them will glow less brightly than those with less, depending on the composition of a particular batch of glass. In general, though, if it glows green it’s Vaseline.

Skelcher learned to look for that telltale glow when he was amassing his collection during the research he conducted for his books. “I sometimes shopped at outdoor antiques fairs in open fields,” he recalls. “As the sun began to set and the twilight came up, the real pieces of Vaseline would glow during that little window of time—that’s when I would look around the field to see which stands had uranium glass.” Although less ultraviolet light reaches the surface of the Earth at twilight, its effect is more pronounced since there’s also less visible light at that hour. Thus, the stuff with uranium in it, as opposed to run-of-the-mill, uranium-free, green Depression glass, became a beacon to this sharp-eyed, Vaseline-glass hunter.

People in the 19th century probably noticed that twilight glow, too. “We’re not really sure,” Skelcher allows, “but we think that glow was considered quite attractive in those days. People’s houses didn’t have electric light all those years ago. Most would have had candles or perhaps a gas light. If they put their uranium glass on a windowsill, the glass would glow as the sun went down.”

The name of the person who first used uranium in glass has long been lost to history, but the uranium-glass creation myth generally invokes Bohemian glassmaker Josef Riedel, whose factory in what is now the Czech Republic cranked out the first production-level quantities of uranium glass in the 1830s in two colors—Annagrun (green) and Annagelb (yellow). James Powell’s Whitefriars glass company in London almost certainly beat Riedel to market by a year or so, and Skelcher says he’s even found evidence of uranium glass manufactured in England in the 1820s using radioactive rock mined in Cornwall.

Regardless of who did what first, we know that the mineral itself was identified in 1789, when German chemist Martin Heinrich Klaproth named it after our solar system’s most recently discovered planet. Back then, uranium was seen as just one more mineral to color clear silicon dioxide, the main constituent in the sand glass is made from. Chemists like Klaproth knew that cadmium turned glass yellow, cobalt made it blue, manganese produced violet shades, and certain compounds of gold went red when heated, blown, and cooled.

“When they found uranium,” says Skelcher, “they probably thought, ‘Oh, this makes a colored solution; what would happen if we put it into glass?’”

Over the years, successive glass manufacturers in Europe and the United States melted a lot of sand to find out. In the Czech Republic, Harrach Glassworks used uranium in decanters, goblets, and trays, while Riedel put Annagelb and Annagrun to work in intricately cut and layered vases and handled mugs. In England, one of Skelcher’s favorite glassmakers, Thomas Webb & Sons, began adding uranium to its glass batches in the 1840s; almost half a century later, a Webb recipe for an 1880s Topaz color called for a whopping 7.3 percent uranium by weight.

In the United States, Pennsylvania companies from McKee to Adams to Westmoreland made Vaseline glass fairy lamps, candy containers, and lidded pots. Hobbs, Brockunier & Co. and Northwood of West Virginia were known in the late 1800s for their bumpy hobnail pieces, while one of the state’s (and country’s) biggest Vaseline glass producers, Fenton, arrived in 1907. Another West Virginia giant, Fostoria, didn’t get into Vaseline until the 1920s, which it marketed briefly as Canary.

And then there was Ohio, home to the highly influential Cambridge Glass Company, whose uranium content in its Vaseline-glass recipes ranged from Thomas Webb & Sons-levels of 7 percent to as little as 1/10 of 1 percent. In general, recipes for Cambridge Vaseline hovered at the low end of that continuum, although a batch of an opaque color called Primrose called for 2.9 percent uranium by weight, which meant a batch of Primrose with 1,000-pounds of sand in it included almost 60 pounds of uranium. More typical was the recipe for a clear hue also called Topaz, like Webb’s, which contained 7/10 of 1 percent uranium by weight, or roughly 12 1/2 pounds per batch.

Those Cambridge recipes are from the 1920s and ’30s, long after uranium was discovered to be radioactive by French physicist Henri Becquerel in 1896 (he shared a Nobel Prize for his insight with Marie and Pierre Curie in 1903) but well before scientists understood how harmful radioactive materials could be to people’s health. Still, concerns for public safety, even misplaced ones, were not the reason why the popularity of Vaseline glass was already waning at the end of the 19th century and the first half of the 20th. According to Jay Glickman and Terry Fedosky, whose 1998 Yellow-Green Vaseline! remains one of the better primers on the subject, the decline of Vaseline glass had a lot to do with the picture Skelcher paints of those shadow-filled Victorian domiciles lit at twilight by shelves of glowing Vaseline glass. With the advent of electricity, such sublime moments were flooded by the glare of artificial, full-spectrum light. “Vaseline glass was a victim of the ordinary light bulb!” the authors exclaim.

By the middle of the century, uranium was deemed critical to the war effort (in the United States, that meant the Manhattan Project), which removed uranium from civilian use from 1942 until 1958. Radiation tricks, however, were still commonplace in many public places. “I remember when I was a kid in the late 1940s,” Skelcher recalls. “You could go into a shoe shop and x-ray your foot in a boot to see if it fit. No one realized back then that the radiation was doing you damage.”

X-rays are far more powerful and dangerous than the comparatively paltry alpha and beta rays found in Vaseline glass. “Every house has alpha waves in it because every house has a smoke detector,” notes Peterson, referring to fractional micrograms of americium-241 that can be found in each device. Alpha rays are weak, which is why smoke needs to come in contact with the detector to set off the alarm, and they can be blocked by a flimsy sheet of paper. Beta waves are stronger, although a single pane of glass is all it takes to deflect them, and they dissipate within 18 inches anyway. In contrast, about the only thing x-rays can’t penetrate is lead, which is why they took such good pictures of bones, even those wrapped tight in flesh and boot leather.

After restrictions on the civilian uses of uranium were eased in the 1950s, Vaseline glass made a comeback. In the United States, Fenton was one of the biggest producers until it ceased operations in 2011. Mosser Glass, also based in Cambridge, Ohio, was founded in 1964 and is still pressing Vaseline glass, making molded-glass cake stands, mixing bowls, creamers, salt-and-pepper shakers, compotes, tumblers, candlesticks, oil lamps, punch bowls, water pitchers, kittens, hens, and chicks. For Mosser, Vaseline is just another color in its extensive catalog, like Amber or Aqua, Passion Pink or Hunter Green.

Mosser’s Cambridge neighbor, Boyd’s Crystal Art Glass, which has been pressing glass since 1978, made its last piece of Vaseline glass about a year or so ago, as it winds down operations after a 36-year run. Until recently, John Boyd, who earned a masters in botany and is both the grandson and son of the firm’s father-and-son founders, was the guy responsible for adding uranium to batches of Boyd glass. In the early days, he says, Boyd’s was able to purchase raw quantities of U-308, which he says “looked like coffee grounds. You just can’t get that any more. We had to switch to uranium dioxide, which looks like iron filings. The color is a little bit different, a little bit greener.”

Boyd’s used its 15-pound allotments (the maximum amount of uranium the company was allowed to keep on hand at any given moment) to make a type of uranium glass it called Firefly. But, John says, you can use uranium to make colors other than Vaseline. “We made a color called Golden Delight, which is kind of an amber. It will fluoresce under a black light just like any uranium-containing glass. I’m pretty sure we used less than ½ of 1 percent of uranium in a batch. Cambridge Glass,” he adds, “had a color called Avocado, which had 3 percent uranium in it. You just can’t make that any more. You just can’t reproduce that color. There are too many restrictions on the use of uranium.”

The main restriction is that 15-pound limit, which, if used all at once in a 1,000-pound batch of glass, would only get the uranium content up to 1 1/2 percent. That would have been a lot of uranium for a piece of Boyd’s Vaseline glass, as a typical Boyd’s recipe shows. “Someone else would put out the 400 pounds of sand, the 150 pounds of soda,” John says, rattling off the main ingredients in a typical batch, “and then I would do the finesse, weighing out the 12 ounces of uranium dioxide.” While handling the material, John would take the sorts of precautions you might expect a worker in a glass factory to take to protect himself, but it wasn’t like he was lumbering around in a lead-lined suit covered with radioactive warning tags. “I’d wear coveralls, a respirator, and have a fan going so I’m upwind from any dust. I just tried to be aware of the hazards around me, the risk of silicosis from inhaling the silica in the sand or the damage to your lungs from breathing in the cobalt. We were dealing with some pretty caustic materials, so the coveralls stayed at the factory—you didn’t bring them home.”

In fact, uranium was often not the worst thing in a batch of Boyd’s Vaseline glass. “We used arsenic as a refining agent,” John says casually of the world’s most infamous toxic element. “Arsenic will actually go from As 2 O 3 to As 2 O 5 , meaning it’s picking up the oxygen atoms in the glass, which show up as bubbles—you generally don’t want a lot of bubbles in your glass. So there can be things like arsenic in a batch of glass that are actually a little rougher than uranium. You have to be very mindful to handle everything safely.”

Lead, of course, is another traditional ingredient in glass, as in leaded crystal. Again, referring to a Cambridge recipe, one batch from the first half of the 20th century called for 850 pounds of sand, 330 pounds of soda, 100 pounds of feldspar, 42 pounds of lime, 50 pounds of nitrate, 36 pounds of lead, 10 pounds of arsenic, 43 ounces of uranium, and 13 ounces of copper oxide. Lead was removed from household paint in 1978 because it is so harmful to children, which makes that 36 pounds of the stuff seem a good deal more menacing than a mere 2 1/2 pounds of uranium. Fortunately it takes several hours for the lead in a crystal glass to leach into, say, the wine that glass is holding, which means lead is fine for glassware but probably not a great idea for decanters if you don’t plan on drinking whatever is inside in a short period of time.

With lead, though, there’s at least a use case in which the poison can enter your bloodstream. Getting uranium into your system, says Skelcher, would actually require a fair amount of effort. When asked to be more specific, about the scenario Skelcher could think of to make the small amount of uranium in Vaseline glass harmful to one’s health would be to grind up a piece until it was a fine powder and swallow it, which, he was quick to point out “would be a daft thing to do.” But in that daft scenario, the radioactivity in the uranium would now be in your body, and those alpha and beta rays would have nowhere else to go.

Most people will probably be able to resist their urge to put Skelcher’s hyperbolic suggestion to the test, but that’s not to say fans of Vaseline glass are completely out of the woods just yet. It comes back to that trick, that fluorescing, that black-light glow people like Dave Peterson, Barrie Skelcher, and John Boyd enjoy so much. Black lights, by definition, emit nothing but ultraviolet rays, which are known to cause skin cancer (that’s why we put on sunscreen when we go outside, although now that’s supposed to be bad for us, too). Depending on its wavelength (the shorter ones are the worst), UV light can also damage the retina and cornea of the eye, which means the only truly dangerous thing about Vaseline glass is making it perform its trick. For his part, Dave Peterson plays it safe by making sure the black lights he uses emit the relatively safer, long UVA waves rather than the more harmful shorter waves that characterize UVBs or UVCs. “I’m more concerned about what black light I use than how much uranium I have in my house,” he says.

(For more information about Vaseline glass, check out Vaseline Glass Collectors, Inc., or Barrie Skelcher’s site. Peterson’s books, including Vaseline Glass: Canary to Contemporary, are available via amazon and other online sellers; Skelcher’s are available via Schiffer Books. To purchase new Vaseline glass, visit Boyd’s Crystal Art Glass or Mosser Glass.)