Uranium glass is a type of glassware defined by the inclusion of uranium compounds, typically uranium oxide, in its composition. This addition was introduced into the glass mixture before melting, primarily to achieve a desired color effect. The material was widely produced from the 1830s until the mid-20th century, though manufacturing slowed significantly after World War II due to restrictions on uranium use. While the general term is uranium glass, the most common type is frequently referred to as Vaseline glass.
The Color Under Normal Light
The most common appearance of uranium glass under standard light is a pale, transparent yellowish-green hue. This specific coloration historically earned it the popular nickname “Vaseline glass,” as the shade resembled the appearance of petroleum jelly at the time. The exact shade of the glass can vary significantly, ranging from a vibrant lemon yellow to a deeper, soft green.
This variation in color is directly related to the concentration of uranium oxide used in the manufacturing process. Most pieces contain between trace amounts and two percent uranium by weight, with higher concentrations generally leading to a more saturated yellow-green. Other elements were sometimes added to the glass formula to intentionally alter the base color, creating unique variants.
Two notable opaque variants include Custard Glass, which is a milky, pale yellow or ivory white, and Burmese Glass, which features a blend of pink and yellow tones. Despite these differences, all these variations share the defining characteristic of containing uranium oxide. The visible color alone is not a definitive test, as other glass colorants can mimic the yellow-green shade.
The Unique Fluorescent Signature
The most reliable feature of this glassware is its characteristic reaction when exposed to ultraviolet (UV) light, commonly known as a blacklight. Under UV illumination, uranium glass emits a vivid, intense lime-green glow. This fluorescence is the definitive way to identify a piece of uranium glass, regardless of its color in ambient light.
This phenomenon is not a result of radioactivity but rather a specific property of the uranium compounds sealed within the glass matrix. The uranium exists primarily in the form of uranyl ions, which are highly efficient at absorbing energy from UV photons. Once absorbed, this energy excites the electrons in the uranyl ion to a higher state.
As the excited electrons drop back down to their stable ground state, they release the stored energy as visible light. The wavelength of this re-emitted light consistently falls within the green spectrum, creating the signature fluorescent glow. The intensity of this bright green light is directly proportional to the amount of uranium present in the glass.
Collectors typically use a long-wave UV light source to trigger this reaction. Even pieces that appear almost clear, yellow, or a different color due to other additives will still display this consistent, unmistakable green fluorescence.
Uranium Content and Safety Considerations
The presence of uranium raises questions about radioactivity and safety, but uranium glass is generally considered safe for collecting and display. The uranium used in the glass is a low-level radioactive material, mostly consisting of the uranium-238 isotope, which decays by emitting alpha and beta particles. The concentration of uranium oxide is low, typically below two percent by weight.
The glass acts as an effective shield, blocking almost all of the alpha radiation and a significant portion of the beta radiation from reaching the surface. Alpha particles, which pose the greatest risk if internalized, cannot penetrate the glass matrix or the outermost layer of dead skin cells. Consequently, exposure from handling or displaying the glass is minimal and far below levels considered harmful.
Studies show that the radiation exposure from a typical collection is negligible, comparable to natural background radiation. The primary safety concern is not external exposure but the hypothetical risk of ingestion.
Experts advise against using uranium glass pieces for food or drink, especially acidic beverages. This is due to the minimal possibility of trace uranium leaching out or ingesting glass dust from chipped edges.