Purple opaque glass is a tool of the Devil. With the least provocation, its surface breaks down from glassy to crystalline, a process known as devitrification. The surface of the glass goes from shiny and purple to chalky and white. You can put the glass in a reducing flame to burn off the devitrification, but that has its own problems. You lose that lovely warm purple color and instead get a sooty grey-blue effect that bleeds over the surface of your bead.
That blackness will greedily subsume whatever color you put next to it. (The white devitrified spot will do that, too, looking like a ring of salt crystals in a dried-out tide pool. The black just spreads like an oil slick and contaminates its neighbors.) The quirkiness of purple opaque glass can lead to some very nifty effects if it’s what you’re going for, but a lot of the time you just have to concede the point and admit you’re not going to get the shade of purple you wanted. This whorl is a fine example. I’d made a fine twisty of purple opaque glass and transparent rose to apply to this bead. When I took the bead out of the flame to shape it, it cooled enough that the purple devitrified upon being reintroduced to the heat. I cranked the heat and was able to get rid of the white spreading scum, but the purple didn’t go back to its original color. The effect is interestingly organic – it looks like a fossil, or a spinal column – but it’s not what I was going for. On the underside of the same whorl, you can see the twisty as I saw it in those brief moments before it turned black.
Devitrification can be prevented by keeping the glass very hot right up until you’re done working it. Taking a purple bead in and out of the flame is a guaranteed way to mess it up. Unfortunately, so is using most shaping tools and marvers. Sometimes I use devitrification as an intentional tool, as when I’m making mermaid pendants. In that case, the mottled white/purple looks appropriately organic and fishy on the tails. But overall, I much prefer working with clear purples. They’re much better behaved.