Let’s get into the nitty-gritty of the fundamentals of melting glass. Most common glasses are called Soda-Lyme glass. This means that the glass is made of sand (60%), soda (20%) and Lyme (20%). Sand melts at around 3000 degrees, so adding soda and Lyme as fluxes will both lower the melting point and add a brittleness to the glass. Here is where the green comes in.

Most sand in the world has iron in it. When melted that iron leaches into the glass as green!