Uranium glass is a distinctive type of vintage glassware, first produced in the 1830s. It was manufactured by adding a small amount of uranium oxide to the molten mixture, primarily for color and visual effect. Widely used for decorative items and household objects until production largely halted in the 1940s, the uranium content (typically up to two percent by weight) gives the glass its characteristic appearance. Identifying a genuine piece requires focusing on its most remarkable property, rather than just its color.
The Bright Green Glow
The most reliable method for identifying uranium glass is observing its reaction to ultraviolet (UV) light, commonly known as a blacklight. When exposed to UV radiation, the glass emits a brilliant, unmistakable fluorescent glow that is typically a vibrant lime or yellow-green. This phenomenon is a direct result of the uranium ions within the glass matrix absorbing the invisible UV energy and then re-emitting it as visible light. The specific chemical component responsible is the hexavalent uranyl ion ($\text{UO}_2^{2+}$), which is excited by the UV light and fluoresces at approximately 534 nanometers, falling squarely in the green part of the visible spectrum.
The intensity of this glow can vary depending on the concentration of uranium oxide used, though most pieces contain less than two percent. Even a small amount of uranium is enough to produce the signature effect, which is why the fluorescence test is considered conclusive. Collectors often use portable UV flashlights operating at either 365 nanometers or 395 nanometers. While both wavelengths cause the glass to fluoresce, the 365 nm light is sometimes preferred as it can produce a slightly more intense reaction. The presence of this intense, neon-like green light is the greatest indicator that a piece contains uranium.
What to Look for in Daylight
While the UV test is the only way to be certain, the appearance of uranium glass under normal lighting offers a strong preliminary clue. The color typically ranges from a pale, translucent yellowish-green to a deeper, more opaque green, resulting from the uranium oxide acting as a colorant.
Many transparent pieces possess a subtle, almost oily sheen in daylight. This characteristic led to the popular, though less precise, term “Vaseline glass” for the pale yellow-green variety. Uranium glass was manufactured in various forms, including transparent, translucent, and opaque varieties like milky white or yellow custard glass. Relying solely on color is insufficient, as other colorants can produce similar shades. The daylight appearance should only narrow down potential candidates before the definitive blacklight test.
Tools and Safety Concerns
The primary tool required for identifying uranium glass is a portable UV flashlight, often referred to as a blacklight. These devices are inexpensive and readily available, making them an accessible tool for collectors and enthusiasts. A UV light source is far more practical and reliable for identification than attempting to judge the glass’s color or age.
A more specialized tool sometimes used by collectors is a sensitive Geiger counter, which can detect the low levels of radiation emitted by the glass. The uranium content makes it technically radioactive, but the levels are extremely low. The radiation emitted is generally considered negligible and safe for handling and display in a home environment. Exposure is often lower than that from common household electronics. The main precaution advised is to avoid using chipped or damaged uranium glass for food or drink, preventing the ingestion of small fragments.
Common Look-Alikes
Several other types of glass can be confused with uranium glass, making the UV test essential for accurate identification. The term “Vaseline glass” is often used interchangeably with uranium glass, but it is a specific subset referring only to the transparent, pale yellowish-green pieces. Therefore, all Vaseline glass is uranium glass, but not all uranium glass is Vaseline glass.
Another common look-alike is manganese glass, frequently used as a decolorizer in clear glass production. Manganese glass also fluoresces under UV light, but its glow is typically a softer, less intense green, yellow, pink, or orange hue. A key difference is that manganese glass often glows best under 365 nm UV light and may not react to 395 nm light, whereas uranium glass glows brightly under both. Regular green glass, such as that colored with iron, will not exhibit any fluorescence under a blacklight.