Vaseline glass, originally known as uranium glass, is made with uranium salt---usually uranium dioxide---as the colorant. This causes the green glow when uranium glass is exposed to ultraviolet (UV) light. UV light can be helpful in identifying true uranium glass from other types of Vaseline glass. Uranium glass has been produced since the 1830s when it was discovered that uranium salts provided an effective method of coloring glass yellow to yellow-green. At this time, the radioactive effects of uranium were unknown, and remained undiscovered until 1896 when Henri Becquerel exposed a photographic plate to uranium salts. Production of uranium glass was curtailed during World War II when governments restricted use of uranium by anyone but the military. Only a few companies still produce limited quantities of uranium glass.
The uranium glass type of Vaseline glass glows when exposed to ultraviolet light because the electrons in the outer shell of the uranium atoms become excited by exposure to the photons of the UV light. When they drop from this excited state it releases energy in the form of a photon which is seen by the human eye as a green glowing light. This green glow is very slightly visible even in daylight because daylight includes light from the UV spectrum.
The only type of Vaseline glass that glows green is the type made with uranium dioxide or other uranium salts. The term "Vaseline glass" has been in use since the 1920s and has come to be used for any transparent glass of the classic yellow to yellow green color. There are also other colors of glass in which uranium has been used, including green, amber, pink, blue to blue-green, white and gray, and the best way to test for this is with a UV light and a Geiger counter. The presence of uranium will always cause the glass to glow green under UV light. However, other substances such as lead, can affect the appearance and strength of the green glow of uranium glass under UV light.
Due to concerns about radiation exposure from use or collection of uranium glass, extensive testing has been done to determine the risks. Current results have determined that the exposure from normal use is not much more than what would happen naturally with everyday background radiation. This does not include use that involves engraving, cutting, or grinding the glass, which could result in exposure to dust containing uranium that might cause a health hazard. Using UV light testing can help determine if the glass you plan to use was made using uranium.