Q. Why would clear glass show a greenish glow under black light?

Uranium glass can appear green under UV.

American brilliant cut glass, which was manufactured roughly between 1875 and 1925 in the US will emit a green glow once exposed to a black light at 365 NM. A longer wavelength does not display the emission from the item. American brilliant cut class is clear generally, but when placed in a darkened room and exposed, it can be quite vivid. After World War II, cut glass was no longer cut by hand. Instead, it was cut on machines that have diamond teeth. Those items do not glow green when exposed. , you might see a pink or orange color being emitted. There are a number of articles on the Internet about this and glass collectors are very familiar with the issue. American, brilliant glass is roughly 50% lead which contributes to its brilliance. More modern glass post world world two does not have this high of concentration. Uranium glass also can glow green but I don’t believe at American brilliant glass contain uranium. I’d be happy to send you some photos of the phenomena I will be in Ithaca later this summer. Is your museum open? I’ve been to Corning and I thought that was fascinating. I can only imagine what your institution offers. Thank you.

Uranium glass is usually green in ordinary light, but it is a pale green. It may even look yellow-green ,like vaseline, in which case it’s sometimes called “vaseline glass”. However , it will fluoresce a bright green in UV light. Some clear glass will also fluoresce green under UV light even though it has no uranium in it because manganese was used as a clarifier. This likely means the glass was manufactured before 1920 because selenium had replaced manganese as a clarifier by then. But a few glass makers continued to use manganese for a decade or two longer. Generally though I assume manganese glass to be pre-WWII. So the short answer to your question is: Yes, if a piece of glass is clear and it fluoresces green under UV light, it is likely old. My business is acquiring and selling antiques. I am not an expert in making glass, so bear that in mind.