Silver glass strikes due to silver crystal growth. When the glass is worked hot, the silver crystals dissolve, yielding a clear glass. When the glass is reheated, crystals form inside the glass. These crystal lengths grow to the same size as various wavelengths of light. The color sequence of lengthening crystals is as follows: clear, yellow, orange, red, red-purple, purple, blue, green. When glass is worked hot (“reset”) the glass looks clear. Due to ambient heat within the glass, the first stages of striking usually occur automatically, yielding yellow-orange-red, which all blend together to read as “amber” or transparent dark brown. As the glass is cooled and reheated, purples, blues, and greens are developed.
Common Issues with Striking
If the glass is “bobbed” in and out of the flame, or worked for an usually long time in a cool flame, the crystals grow to random lengths, and reflect random wavelengths of light. This reads as an off-white, tan opaque color. The short answer to avoiding muddy khaki colors is to work hot, cool, and reheat gently to strike, while avoiding “bobbing” in and out of the flame.
If the surface of the glass is worked hot, and then directly annealed, a “reset” surface layer can exist which can strike to amber in the kiln. This sometimes occurs when one strikes the glass nicely, and gives it a final fire polish before annealing.
If your striking glass stays a milky white color, it is probable that the glass has never been heated to the “reset” temperature. Heat it until the glass turns clear/transparent, or at least until the outer layer goes clear, then allow to cool until the glow is gone, then reheat gently to strike.
If the flame used to strike is too intense, or the bead is taken past the crytsal’s “reset” temperature during striking, the glass will continuously reset, yielding only a dark amber color.
I prefer to make a small (10mm) ball on the end of a clear punty to test and practice with. This conserves glass, and also relieves me of the emotional attachment and time invested in a bead or sculpture. In this way I am able to test or practice striking a glass without undue loss of time or material.