HEAT SENSITIVE COLORS
The next hurdle to cross when working with colored borosilicate glass is tackling the more sensitive colors. This group of colors is sensitive to excessive and/or aggressive, quick heat. The reason for this sensitivity is the colorant agent used in these glasses.
There are many metals such as copper, cobalt, silver, and chrome that have relatively high boiling point and do not volatilize at low temperatures. These are ideal colorants because once in the glassy state, they take aggressive heat without causing the glass to boil. There are some metal oxides however that have extremely low boiling points that even when in the glassy state they do not take heat well.
At Northstar we try to chemically minimize these boiling issues, but there is only so much technology can do to curb the nature of particular metal oxides. These colors must be worked in cool, gentle flames and heated up slowly!
In order to do so, start by gently heating the rod in the upper extremities of the flame. Pass it back and forth through the flame so it heats up gently and evenly. As it starts to warm it can be brought down closer to the torch head, where the heat is more intense. If you notice the surface start to overheat, move out of the hottest part of the flame. This is the key to preventing surface scarring.
The most challenging of the heat sensitive glass colors are the Intense Opaques:
The other major glass color family that is sensitive to heat is the NS-54 Star White based colors which include:
These glass colors are far less sensitive than the Intense Opaques, but must be heated up slowly to ensure they do not boil. Once these colors are applied and gently smoothed out they will take more heat without boiling. Refer to the reference guide for additional colors to watch for.