And Other Fluorescent Finds
Photography and text by Sara Caruso
Updated from Echoes of LBI – 2018 Spring into Summer Edition
Many of us remember the brightly colored vintage glassware displayed by our grandmothers. With colors of bright lime green, stunning reds and yellows, and electric turquoise, the pieces seemed to glow with a vibrant hue.
Little did we, or grandma, know that with the help of an ultraviolet or UV flashlight this oddly radiant glassware actually might glow. In fact, several types of UV reactive glass fluoresce under ultraviolet light and can help us peer into the past. To some sea glass collectors, taking a UV flashlight, sometimes called a black-light flashlight to the beach at night may sound strange, but it can be the best way to find some fluorescent sea glass. By understanding some of the history and additives that made these glowing glass jewels, one can better understand these rare treasures.
In 1903, Robert Williams Wood invented Woods Glass, an optical filter made with barium-sodium silicate glass. The purpose of Wood’s glass filter was to block visible light and allow ultraviolet and infrared light to pass through. Though initially used in communications during World War I, Wood’s Glass was used to create ultraviolet or blacklight lightbulbs.
Ultraviolet light is measured in nanometers (nm), which is a unit of length equal to one billionth of a meter used to measure the wavelength of light. Higher nanometers indicate longer wavelengths and lower energy. Shorter wavelengths or lower nm are more energetic. Some materials like manganese are more energetic under a 365nm than a 395nm.
The two most common types of UV flashlights recommended to collectors of glassware are 365nm and 395nm. How intense and what color UV glass fluoresces depends on the type and amount of reactive additive was used when the glass was manufactured. One of the most popular forms of UV reactive glass, uranium glass, was first created in 1880.
Most sea glass collectors have a jar of green sea glass, but some of those pieces might be hiding a secret. Uranium glass, also known as Vaseline glass due to its color resembling petroleum jelly, became popular as decorative Depression glass through the 1930s and early 1940s.
Uranium was used in a wide variety of glass items by the 1930s. It took on the form of kitchenware, utensils, marbles, decorative tile, lamp bases, fish bowls, ashtrays, and even false teeth. When the United States entered WWII, the country’s uranium stock was put towards the war effort, but the fad of Depression glass had already begun to wear off. Uranium would not reappear in glass manufacturing until the late 1950s as depleted uranium sources were sold to American glassmakers like Fenton, Mosser, and Degenhart, among others. During manufacturing, uranium oxide, in the form of diuranate, was added to glass for coloration before melting. Antique uranium glass is usually a bright lime green or yellow tint in regular light. Uranium was rarely used in other colors. Even without a UV light, all uranium glass seems to glow a bit on its own. However, it is not the only green, fluorescent glass, however, which has led to confusion.
Lavender sea glass is one of the most popular and sought after colors. In most cases, it starts off as clear and changes to lavender after several years of sunlight exposure. But did you ever wonder why? The sand used in early glassmaking contained a lot of iron impurities which gave the glass hues of green or aqua. In the early days of glassmaking, manganese dioxide, also known as glassmaker’s soap, was added to glass to decolorize the final product. While glass made with manganese was usually clear, it can also be found in some light blue and pink glass.
Though scientists have debated how manganese decolorizes glass, the general consensus is that an ion of oxygen is taken by the iron from the manganese. This tints the glass in such a way that the color wavelengths visually cancel out and shows as white light. The amount of manganese in the glass determines how dark a piece will turn with sunlight exposure. Adding larger amounts during manufacturing would tint the glass deep purple. In the past, people placed clear glass vessels in bright sunny locations for extended periods of time in an effort to change the color. While the time it takes for clear manganese glass to turn lavender depends on many factors, you may be able to see which sea glass pieces in your collection are likely to change with the help of a specific UV flashlight.
Sea glass with manganese will fluoresce a yellowish green under a 365nm UV flashlight. Some especially high content manganese glass will fluoresce yellow under both 365nm and 395nm. This is the reason it is often mistaken for uranium glass, since true uranium glass glows under both, but with a much brighter neon green. If it is not a lime green or yellow tint without a UV flashlight, it is most likely to be manganese, not uranium. By the mid to late 1920s, manganese was replaced with a more cost-effective substitute.
Pink sea glass is one of the rarest colors to find despite the fact that the material used to make it —selenium — was quite common. Selenium made its debut as a decolorizer due to shortages caused by World War I. It was primarily used in bottles, jars, and consumer glass products. Like manganese, selenium is also affected by long-term sun exposure, but in this case the change is from clear to a pale peach or champagne.
Pink was popular in Depression ware, so selenium replaced arsenic as a cost effective, safer, and more available source for pink glass. Adding large amounts of selenium to glass made it turn red and was therefore used to replace gold oxide as the colorant. If you have clear or pink sea glass, you can check if it contains selenium with your UV flashlight. It should glow a pale reddish coral color.
However, if you have a piece of red or orange, or yellow sea glass that fluoresces bright orange yellow under a UV flashlight, it is likely to contain cadmium. This is most often seen in lenses for car taillights, traffic lights, and railroad and boat lanterns made prior to the 1960s. Over time, cadmium begins to degrade, especially with exposure to sunlight. In the past, some reflectors, lenses, and taillights were coated with a UV blocking chemical. This may be why some sea glass in these colors is occasionally found coated with a strange white film. The salt in seawater seems to react with this chemical to create the film, which cannot be removed without destroying the surface of the sea glass. Cadmium in glass fell out of favor due to its hazardous nature. While popular to collectors for their fluorescence, uranium, cadmium, and manganese are not the only UV fluorescent glass.
Clear sea glass that fluoresces blue under a UV flashlight, may contain antimony or lead. Leaded glass was popular from the late 17th to 20th centuries in decorative holloware and windows. Lead oxide was added to glass to reduce its viscosity and make it more fluid. In glassmaking, the molten glass must remain at a certain temperature to be workable. Lead allowed glass to remain easily manipulated even if it began to cool. It also increased the clarity and density of the glass. Sea glass pieces with lead might be thicker and noticeably heavier. Under a UV light, glass containing lead fluoresces a bright, almost blinding blue. Like manganese and selenium, antimony was also used as a decolorizer for clear glass. By comparison antimony fluoresces a much paler blue and lacks the density of leaded glass. Due to the history of these materials, one might ask if it is safe to have this glass in a collection.
There is no truth to the urban legend that UV sea glass is hazardous. UV glass ware contains infinitesimal trace amounts of uranium and is harmless. For example, the amount of uranium in a glass item is on average 2% by weight. Daily exposure to sunlight contains more radiation than you could ever receive from a shard of uranium sea glass.
To the avid sea glass collector, UV fluorescent sea glass is a special find. It is always exciting to see what glows in your collection, so get a UV flashlight, sort through your sea glass. You may find a fluorescent piece of history.
