Transparent Red 1122 is one of the glasses in the cadmium/selenium family. These glasses are often “strikers” or glasses that mature to their target color as a result of heatwork in the kiln. Under conditions of excessive hear, cadmium/selenium glasses are less stable than colors like cobalt blue. With these glasses, avoid firing cycles with temperatures higher than required for a full fuse, or require a process temperature soak for longer than 10-15 minutes per firing.
You can learn more about what to expect from this article: http://www.bullseyeglass.com/methods-ideas/what-to-expect-from-bullseye-glass.html
“…At Bullseye, glasses known to be fairly stable are tested by firing to a top temperature of 1500°F (815°C) and soaking for 15 minutes before annealing. Once cooled, these tests are viewed for stress through polarized light and graded accordingly. Other glasses known to be less stable are fired three times with this cycle to insure good performance under typical multiple fusing and slumping conditions, such as those used in making a simple plate…”
Learn what to expect from individual colors by checking the “About Our Glass section of the website. Choose the color, and then click on the tab for the particular form of the glass in question. In this case, visithttp://www.bullseyeglass.com/red-transparent.html
Transparent red is one of the styles that we test by firing a sample three times, to make sure that each batch is stable enough for general fusing and slumping purposes. It is one of those styles that you might want to avoid for kilncasting, which requires higher temperatures and longer soaks at process temperature.
Feel free to post your firing schedule that resulted in the opacified glass, and I am sure we can provide some feedback.