Uranium glass (Vaseline glass) is glass which has had uranium, usually in oxide diuranate form, added to a glass mix before melting for colouration. It creates a green/yellow tinge to the glass. The proportion usually varies from trace levels to about 2% uranium by weight, although some 20th-century pieces were made with up to 25% uranium. Most uranium glass is fairly innocuous, but some of it is quite radioactive. In fact, some of the 'hotter' specimens are dangerous to even have around, let alone work with. It's actually not the uranium that makes uranium glass dangerous but other elements found in the uranium ore. The simplest way to positively identify uranium glass is through the use of a black light (or small UV torch) and a Geiger counter (although most people don't tend to have one lying around!). A black light/UV light will make true uranium glass glow very brightly, while the Geiger counter will indicate the presence of radioactive material.