Recently we had the pleasure of working on a custom order where the client picked colors from our sample box. She stopped by to see the work in progress and was surprised to see us using "clear" glass where she had specified transparent canary yellow. This led to a discussion on striking, which I thought would be fun to share.
Since we use only Bullseye glass, I turned to their experts for an explanation. Here goes:
"Some Bullseye glasses appear pale or colorless in the cold sheet form, but “strike” or mature to a much different color when fired. Striking colors can vary, depending on temperature, atmosphere, and heat history."
So what does that mean? Here is a visual example. This shows how the original color of the sheet glass (which is what we start with) changes as it is exposed to different and increasingly hotter temperatures.
In addition to changing based on the temperature at which the glass is fired, the number of times a piece is fired - its heat history - may also cause the color to continue maturing, making it deeper and richer. This is one more factor we take into account when we are designing both our functional glass pieces and especially our artwork, both of which may be fired multiple times.