Boiled glass is a high temperature procedure. It is generally accomplished at around 1600-1700 degrees Fahrenheit. As the glass begins to break down bubbles rise from the bottom layer to the top layer. This motion pulls colors to the top surface and gives the look of circular rings of color on the surface of the glass.
I am also interested in this technique – I found only 1 reference to it online. I would like to try it, but am concerned about firing my kiln up to those temps…there is a video at the bottom of the page