The phenomenon of glass glowing under a “black light” is known as fluorescence, a captivating effect that transforms ordinary objects into radiant pieces. A black light is a source of ultraviolet (UV) light, which is electromagnetic radiation outside the spectrum visible to the human eye. When certain chemical additives within the glass absorb this UV energy, they become temporarily excited. They then release this energy as visible light, causing the item to glow brightly. This distinct glow helps collectors identify glassware containing specific chemical additives.
The Distinct Glow of Uranium Glass
Uranium glass is the most recognizable example of fluorescent glass, famous for its vivid, neon-green glow under UV light. This luminescence is caused by the addition of trace amounts of uranium dioxide, typically ranging from 0.1% to 2% by weight, during manufacturing. In natural light, the glass often appears a pale, transparent yellow-green, which historically led to the nickname “Vaseline glass” due to its resemblance to petroleum jelly.
Production of this glassware was popular from the mid-19th century through the 1940s, including decorative tableware, vases, and jewelry. The use of uranium in glassmaking declined sharply after World War II when governments restricted its availability for nuclear projects. This history means that most genuine uranium glass items are antiques, though some modern producers have resumed manufacture.
Other Chemical Additives That Create Fluorescence
While uranium produces the most intense green glow, several other elements added to glass can also cause fluorescence under UV light, each yielding a different color. Manganese, historically used as a decolorizer to counteract the greenish tint from iron impurities, can cause a subtle greenish-yellow glow, especially under a 365 nanometer (nm) UV light. Glass containing cadmium, often used to create yellow and orange glass, will fluoresce yellow-orange or occasionally pink under UV light.
Selenium, frequently used alongside cadmium, contributes to pink, red, and ruby-colored glass, which glows pink under ultraviolet exposure. Even lead, a component in high-quality flint glass, can produce a faint, icy-blue fluorescence if present in concentrations of 5% or more. The specific color and intensity of the glow depend on the concentration and the oxidation state of the element within the glass matrix.
The Physics of Glass Glowing Under UV Light
The mechanism behind the glow is a process called photoluminescence, specifically fluorescence, which relies on the interaction between light and the electrons of the activating elements. Ultraviolet light is a form of high-energy, short-wavelength radiation. When this UV light strikes the glass, the electrons in the chemical additives, such as uranium or manganese, absorb the energy.
This absorbed energy temporarily boosts the electrons to a higher, “excited” energy state. Because this state is unstable, the electrons quickly drop back down to their original, lower energy state. As they return, they release the excess energy by emitting a photon of light. This emitted light has less energy and a longer wavelength than the absorbed UV light, placing it within the visible spectrum, which the human eye perceives as the glow.
Practical Considerations for Collecting and Handling
For those interested in collecting fluorescent glass, a portable UV light is the definitive identification tool. Collectors often utilize two main wavelengths: 395 nm and 365 nm. The more common 395 nm light works well but emits a noticeable purple visible light that can sometimes mask a faint glow.
The 365 nm UV light is closer to the true ultraviolet spectrum and emits less visible light. This allows the fluorescence to appear more vibrant and distinct, making it the preferred choice for display and identifying glass with lower concentrations of additives.
A common concern, especially with uranium glass, is its low-level radioactivity. However, the radiation is minimal and comparable to natural background radiation, posing no significant risk from normal handling or display. Most experts advise against using uranium glass for food or drink as a precaution against ingesting fragments if the glass were to chip. For safe display, the uranium is securely locked within the glass structure.