Heat sensitive glass is a homogeneous glass, in other words it is a single mix of glass and does not require any other mix or layer of glass to determine its colour. The colours that can be seen are developed by varying degrees of heat. Glass that has had the colour changed in this manner is referred to as “turned“.
There are only two types of glass mixes that are able to be turned. One that contains gold, which produces ruby glass, and one that contains bone-ash and tin, in combination with arsenic, which produces opalescence.
Any other shaded colours are produced as casings in association with another mix of glass.
For an explanation and comparison of shaded and heat sensitive glass click here.