How to identify uranium glass with a UV light (and what else glows)

What is uranium glass?

Uranium glass is glass with a small amount of uranium added to the batch as a colorant. In ordinary light it usually looks yellow to yellow-green. Under ultraviolet light it fluoresces a bright, almost electric green. That glow is the whole reason most of us got hooked.

Glassmakers in Europe were using uranium as a colorant by the 1830s, and it was most popular from roughly the 1880s through the 1920s. In the US, production largely stopped during World War II when uranium became a controlled material. Some makers came back to it after restrictions eased in the late 1950s, and a few still make decorative uranium glass today.

The UV flashlight test

The basic test is simple: in a dark room, shine a UV flashlight on the piece. An ordinary blacklight flashlight is enough to see the classic uranium glow. Uranium glass lights up green from inside the glass, evenly and brightly, and it does it the instant the light hits it. When you switch the light off, the glow stops.

Do the test in real darkness, not a bright kitchen. Hold the light a few inches away and move it around the piece, including the base and any thick spots. Thicker glass usually glows brighter because there's more of it.

365nm vs 395nm: does the flashlight matter?

For spotting the classic uranium glow, not much. Most inexpensive blacklight flashlights are 395nm, and uranium glass glows bright green under them. The catch is that they also throw a lot of visible purple light, which reflects off the glass and can tint pale pieces or drown out a faint glow. The Corning Museum of Glass warns collectors not to mistake reflected purple light from a UV lamp for fluorescence.

A 365nm flashlight, ideally one with a dark filter over the lens, gives off much less visible light, so you see more of the glass's own glow. That includes the weaker glows from other ingredients, like manganese, selenium and cadmium. Experienced collectors use 365nm lights for exactly that: reading the different glow colors to tell glass types apart. It also means more of your glass will glow under one, and not all of it is uranium, so judge the color and strength of the glow, not just whether there is one.

What a real uranium glow looks like

Real uranium glass glows a strong, vivid green. Not blue, not white, not orange, and not a faint lemonade haze. The color in daylight doesn't have to be green for the glow to be green: opaque custard glass, blue "bluranium" pieces and pink-to-yellow Burmese can all glow green under UV.

• Strong, even green throughout the glass, not just on the surface
• Brightest in thick areas like bases, stems and handles
• Glow starts and stops instantly with the light
• Opalescent edges on Vaseline opalescent pieces can glow differently from the body, which is normal

Lookalikes that fool new collectors

Plenty of glass reacts to UV a little. The trick is knowing what uranium looks like next to everything else.

Manganese glass is the big one. Manganese was used to decolorize glass, and the Corning Museum describes its glow as a weaker, lemonade-like greenish yellow. It's usually faint and wispy, and it often shows up in old clear glass that has turned a little lavender in the sun. If you have to squint and ask whether it's glowing, it's probably not uranium.

Lead glass can fluoresce an icy blue, most strongly under shortwave UV lamps. Some other additives cause their own colors too: certain cadmium and selenium glasses glow orange, red or pink under UV, which collectors love but which isn't a uranium glow. That's why our shop has a Glowy Glass filter for anything that glows, while our Uranium Glass collection is strictly uranium glass.

And color alone proves nothing. Lots of yellow and green glass has no uranium at all. That's why the Vaseline Glass Collectors' rule of thumb is simple: if it doesn't glow green, it isn't Vaseline.

Painted and coated fakes

Every so often someone tries to sell glass that's been coated with luminous or fluorescent paint. A few tells:

• Glow-in-the-dark paint keeps glowing after you turn the UV light off. Uranium glass stops immediately.
• The glow sits on the surface, often patchy, streaky or worn at the edges, instead of coming from inside the glass.
• Scratches or chips in a coating show a dark spot where real uranium glass would still glow.
• Under normal light you may see a film, a painted look, or a color that doesn't match the glass underneath.

Can you tell without a blacklight?

Not reliably. A transparent yellow-green color hints at Vaseline glass, and opaque pale yellow hints at custard, but plenty of non-uranium glass comes in those colors. A UV flashlight is the standard test, and a documented maker and pattern back it up. If you're buying online or on a live show, ask the seller to put the piece under UV for you.

What the glow can't tell you: age

Here's the part a lot of listings skip. A strong green glow tells you there's uranium in the glass. It does not prove the glass is old. Uranium glass was made again from the late 1950s on, and some decorative pieces are still made today. New pieces glow just like old ones.

To judge age you're back to the usual detective work: maker marks, mold details, wear on the base that matches the piece's age, pattern history, and knowing what reproductions of popular shapes look like. The glow tells you what's in the glass. The rest of the piece tells you when it was made.

How we test on our live shows

On our shows, anything that might be uranium goes under the UV light on camera so you see the glow before you bid, base and thick spots included. Then you get a good look at the daylight color and the condition up close, rims and feet included. Tom will probably make a joke about it glowing in the dark. Then we'll tell you what it actually is.

If you want to see it for yourself, catch us live on Whatnot and ask us to hit anything with the light.

FAQ