When discussing vaseline glass in the United States, collectors are referencing a type of old glass that is yellow or yellow-green in normal light and glows bright green under a blacklight. The nickname "vaseline" comes from the resemblance of some pieces to the hue of Vaseline petroleum jelly, which is light yellow in color. The yellow color of a piece of vaseline glass can actually vary to the naked eye under different light sources. But to purists, it must fluoresce bright green under a blacklight or it doesn't qualify.
Vaseline glass is also called "uranium glass," at times, and for good reason. Like all types of colored glass, minerals are added to the molten mixture during production to achieve the color. The type of mineral added to vaseline glass is actually what makes it glow.
Source of the Color and Glow
As little as 0.1 to 0.2 percent of uranium dioxide is added to the formula of glass to give it the yellow-green tint, according to Vaseline Glass Collectors Inc.
The small amount of uranium dioxide not only gives the glass its color, but it also makes it glow. When you place a piece of glass you believe to be vaseline in a dark area and illuminate it with a blacklight, it should fluoresce bright green. If it doesn't glow green, it's not vaseline glass. Some pieces of light yellow glass appear to be vaseline, but once this test is performed, that designation is thoroughly ruled out if they glow a different color (manganese causes an orange or peachy glow, for instance) or the piece doesn't glow at all.
That bit of uranium in the glass also makes it slightly radioactive. Not to worry though, the radioactivity of the glass is so minute that it doesn't pose a threat if displayed in your home.
History of Production
What we identify as vaseline glass was produced from the mid-1800s through the beginning of World War II, but its peak of popularity was from the 1880s through the 1920s. Large companies like Fenton Glass and Mosser Glass made these wares, along with some other smaller shops. The glass made by smaller companies usually isn't as easy to identify, in terms of a manufacturer, as that of the larger firms.
According to Vaseline Glass, Inc., "The government confiscated all supplies of uranium during WWII and halted all production of Vaseline Glass from approximately 1943 until the ban was lifted in Nov. 1958." From 1959 onward, glass companies began making vaseline glass again. However, because of the expense to obtain uranium dioxide, production was and is very limited.
Green Depression Glass
Authentic green Depression glass was also made with uranium dioxide, but iron oxide (what we usually call rust) was also added to the glass manufacturing process to make the glass greener in color. So, green Depression glass will also glow under a blacklight due to its uranium content, but that doesn't make it vaseline glass. Serious collectors will tell you that to be vaseline, the glass has to clearly be yellow or yellow-green first, and only look vivid green under a blacklight.
That doesn't mean that some pieces of green Depression glass haven't shown up in books about vaseline glass, or that dealers won't label them for sale that way on occasion. However, those serious about this type of glass don't believe green Depression glass falls into this category.
As a side note, Burmese glass and custard glass also glow green under a blacklight but, due to the unique look of these types of glass, they not usually confused with vaseline glass.