When handling antiques, dangerous compounds can surprisingly show up in everyday objects. It is up to antique dealers, historians, and museum professionals to identify these compounds when they show up and to handle them safely. One such instance is with uranium-infused glass. Uranium-infused glass was used for decorations, tableware, and cooking utensils. First created in the 1830s, radioactive glassware was produced mainly between the mid 1830s and mid 1930s, when WWII tensions caused many countries to tighten production of items that contained radioactive materials. Interestingly, new uranium glass is still being produced today, although very few manufacturers are left. The Portage County Historical Society has a variety of uranium glassware pieces in the collection, all currently on display.
How do professionals identify radioactive glassware?
Uranium glass is made by adding uranium to powdered glass before it is melted and shaped. The final product has a fluorescent bright green tint throughout the clear glass, and glows beautifully under a blacklight. When heat-sensitive chemicals are added to the powdered mixture, it makes the melted glass turn opaque, usually a milky-white color, and the item still glows green under a blacklight.
The first way a professional will identify if radioactive materials are present in the glass is by checking the name of the product. Usually a donor or seller will know something about the object, and some items will even come in their original packaging. If the items are called custard glass, uranium glass, vaseline glass, canary glass, or depression glass, they are worth further investigation. The professional cannot judge these items by color alone. Just because the glass is green or milky-white doesn’t mean it contains radioactive materials. These glass types became popular enough that dupes were also produced, which use iron oxide, chrome oxide, cerium oxide, tin oxide, or arsenic to produce a green color, and tin oxide mixed with antimony and arsenic can produce a nearly-identical milky white glass color.
Popular types of Uranium glass:
Uranium glass: The overarching name for all types of glass that contain uranium oxide, both transparent and opaque. To be properly called uranium glass, the uranium content should be at least 2%.
Canary glass: The first name given to uranium glassware, first used in the early 1840s. It got the name because of the distinctive yellow color it got from uranium oxide. The name fell out of style and was replaced with “Vaseline glass”. These items are transparent and glow green under blacklight.
Vaseline glass: transparent yellow or transparent yellow-green glass, glows green. The name “Vaseline glass” became popular because the color is reminiscent of petroleum jelly.
Depression glass: Transparent yellow or transparent yellow-green glass, glows green if uranium oxide was included. Contains other colorants to “up” the green color, like iron oxide. Less popular with collectors because it is “tainted”. This glass was cheaper to produce, and many families discarded it after WWII and moved on to collectable china.
Custard glass: Opaque, milky white glass. Glows green. True, old custard glass often has a “fiery red opalescence” around the rims when viewed against a light. Glass this color that is not uranium glass is called “milk glass”.
Burmese glass: Opaque, pink-white glass. Glows green. Most Burmese glass was treated with acid once set, and has a matte finish, but not all. Modern remakes have resorted to sand-blasting the glass to achieve the matte finish, which leads to a rougher texture.
Jadeite glass: Opaque, green-white glass. Glows green. Was created by adding green glass scraps to custard glass. Often called “Jade glass” in advertisements, or misspelled as “Jadite”. Three companies in the United States produced Jadeite glass between the 1930s and 1970s and all marked the bottom of their pieces with proprietary stamps. This is very popular, with many modern companies still making versions of this glass. Even vintage versions may not be uranium glass, because companies could mix green glass and milk glass to get a very similar hue, without the costly uranium.
If the name of an item doesn’t give the professional a hint, they will often use a blacklight to check the items. Nearly all radioactive glass items will glow under blacklight. The level of radiation present does not correlate with how brightly the item glows; it all depends on the chemical mixture used in the glass. Further complicating things, not all uranium glass will glow under a blacklight! Rarely found, there are clear-glass items that contain uranium to attain a light green or yellow tint that do not glow under blacklight. (In general, for this reason and the regular inclusion of arsenic and lead in other glass colorants, it is not recommended to use antique glassware for anything other than display). If an item glows, it may be hazardous without containing uranium. Some uranium glass “dupes” included thorium impurities in the various oxide colorants, and thorium also glows under a blacklight.
If an item glows, it may be worth checking the radiation level with a Geiger counter. There were no set standards for uranium inclusion in glass, and while most variants aimed for 2% or higher uranium content, some manufacturer’s recipes called for a content as high as 25%.
What about the Portage County Historical Society and the set that is currently on display?
Currently on display in the Engford family’s house at Heritage Park in Plover is a table set of uranium glassware and serving items, enough to comfortably be used by a family of four. There are a few uranium glass cooking items around the kitchen as well, such as the citrus juicers on the counter. This set was tested using a blacklight and the items glow green. In our accession documents (the documentation filled out when items are donated), we do have multiple records of depression glass being donated, a glass orange squeezer, and “table settings”. Because of our documents and the style of the glass on display, we can guess that these items are all uranium glass, but most are depression glass, meaning they contain other metals to increase how green they look during the day.
There were no uranium glass producers in Wisconsin, with the closest producer being in Ohio. It is likely that this glassware made its way to Wisconsin through immigration from Europe, where uranium glass was widely popular, and inter-state trade with the east coast and Ohio where these manufacturers were located.
From 1900-1930, The Wisconsin State Journal and Stevens Point Journal both have advertisements for scientific demonstrations including radium and uranium, and both have numerous articles explaining who physicist and chemist Marie Curie was and how her discovery is being used around the world. Advertisements and news articles told readers that applications of radium to a tumor site would remove cancer, and application to a birthmark would remove the blemish completely. As early as 1886, Wisconsin newspapers ran ads for imported Burmese glass for sale in Baraboo and Appleton, so we know that European trade was bringing uranium glass pieces to our area.
Our set of uranium glassware is beautiful, shining a bright green in the sunlight that streams through the kitchen window in the Engford family house. At night, as the sun sets, you can occasionally catch a glimpse of a ghostly glow from these objects when the twilight hits just right (a trick that some glass collectors use at outdoor flea markets to find uranium glassware without a UV lamp, but it takes a trained eye). Would it have been a danger to the people who used it? To an extent, yes. Experiments on uranium glassware have shown that hot or cold food items, left in the dishes for a period of 24 hours, do collect some radiation particles. But the more regularly they were used and washed, the less radiation is imbued in each serving of food or the dishwater. In short, the radiation lingers around the items, and the first few uses always have had the highest doses. But even these doses were low. Lower than the atmospheric dose of radiation that we all receive by being outside on a sunny day. For collectors of uranium glass, storing the glass in a blacklight enabled cabinet is a great way to showcase these items safely. Just be certain to turn the blacklight off regularly; the UVA beams that make uranium glass glow are the same UVA beams that cause skin cancer.
This summer, when regular open hours return to Heritage Park in Plover, you can view our uranium glass in person!
Citations:
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