Radioactivity in consumer goods, particularly glass, often surprises people who discover vintage items. While most associate radioactivity with nuclear power or weapons, certain historical manufacturing processes intentionally incorporated radioactive elements for aesthetic reasons. This was primarily done to achieve vibrant colors or unique visual effects in glass and ceramic glazes. Understanding identification methods is important for collectors and homeowners concerned about detecting these elements.
Historical Sources of Radioactivity in Glass
The primary element responsible for radioactivity in historical glass is uranium, used as uranium oxide to create uranium glass or Vaseline glass. Glassmakers began incorporating uranium as a coloring agent in the early 19th century, with its popularity peaking from the 1880s through the 1920s. The percentage of uranium oxide typically ranges from trace amounts up to about two percent by weight, though some rare pieces contain as much as twenty-five percent.
The addition of uranium oxide imparts a distinctive yellow or yellow-green color to the glass, leading to the nickname “Vaseline glass.” The use of uranium in glass and ceramic glazes was largely curtailed in the United States and Europe around the 1940s, when the element was redirected for military purposes during World War II.
Other radioactive elements were used less frequently in ceramics and glass. Thorium, often found as an impurity in cerium used as a colorant, can make some yellow glassware radioactive. Additionally, certain ceramic glazes, such as the orange-red color found on pre-1970s dinnerware, utilized uranium to achieve their distinctive hue. In all these cases, the radioactive material was added for its coloring or opacifying properties.
Visual Identification Methods
The most straightforward, non-instrumental method for identifying uranium glass is by observing its fluorescence under ultraviolet (UV) light. Uranium glass contains uranyl ions (UO₂²⁺) which absorb UV energy and re-emit it as visible light. When exposed to a UV source, the glass will glow with a bright, unmistakable neon or apple-green color.
The type of UV light used can influence the accuracy of the identification. Uranium glass fluoresces under both long-wave UV (365 nm) and near-UV (395 nm) light. Using a 395 nm light is often preferred because other elements, such as manganese found in non-radioactive antique glass, may fluoresce a pale green under the 365 nm wavelength, potentially causing a false positive. The vibrant, distinct green glow under either UV light is a reliable indicator of uranium content. This visual method confirms the presence of uranium but does not measure the actual level of radiation being emitted.
Instrumental Detection and Measurement
The definitive method for assessing radioactivity involves using a radiation detection instrument, such as a Geiger-Müller counter or a scintillation detector. These devices quantify the amount of ionizing radiation emitted, typically reporting the measurement in counts per minute (CPM) or as an absorbed dose rate in microsieverts per hour (μSv/hr). The first step is establishing the background radiation level of the testing area, usually in the range of 20 to 40 CPM or approximately 0.05 to 0.12 μSv/hr.
A Geiger counter reading for uranium glass will be noticeably elevated above the background level, confirming the item’s radioactivity. For instance, while background radiation might be 32 CPM, a small uranium glass bowl can register in the hundreds or even thousands of CPM. When testing, the detector should be placed in close proximity to the object. This ensures accurate measurement of the low-penetration alpha and beta particles, which are the main emissions from uranium-containing materials.
The CPM reading is specific to the type of detector used, and converting it to a dose rate in μSv/hr is often an estimation for the mixed radiation field of uranium. Since the radiation from uranium glass is primarily low-energy alpha and beta particles, these are easily stopped by the glass and the detector housing. Consequently, the measured external dose rate is usually low. The actual radiation dose received from handling typical uranium glass is considered minimal, comparable to everyday activities like flying in an airplane.
Safe Handling and Storage
While the radiation dose from most uranium glass is low, simple precautions minimize exposure. The primary risk is not external exposure, but the potential for internal contamination if the glass were chipped, powdered, or ingested. Due to this risk, it is recommended not to use uranium or thorium-containing glassware or ceramics for eating or drinking.
For storage, the most effective safety measure is distance and containment. Items should be stored away from areas where people spend the majority of their time, such as bedrooms or kitchens. Displaying the glass in a closed cabinet or behind a glass or plastic display case provides an additional layer of safety. This physical barrier is sufficient to block the low-penetration alpha and beta radiation that make up the bulk of the emissions.
When handling the glass, minimizing contact time and washing hands afterward are prudent measures, particularly for pieces frequently moved or examined. If uranium glass is broken, it should be disposed of carefully to prevent inhalation or ingestion of particles. For most individual consumer items, standard municipal waste disposal is acceptable, as the low level of radioactivity does not require specialized hazardous waste treatment.