Many years ago, I was teaching a kiln-carving class at my retail studio in Austin. During class, students designed, cut, and assembled the fiber paper for their projects before setting their creations on the kiln shelf. Each student left me a note with the color they wanted for their glass.
The plan was that I would cut their glass, set it on top of the fiber, and fire the projects. Except, on this particular day, I only got through about half the glass before the studio became especially busy with retail and open-studio customers. I was exhausted at the end of the day and, forgetting that half the projects had no glass on the fiber paper, I closed the lid and started the kiln.
When I opened the kiln the next day, I stared dumbfounded at the projects with no glass. Where had it gone? What new devilry had the kiln-gods visited upon my studio? And then I remembered.
Donning my respirator—an absolute necessity whenever handling fired fiber paper—I removed the completed projects and, very carefully, placed the glass on the now pre-fired fiber paper, closed the kiln, and fired the second batch.
The next day I took the remaining pieces out of the kiln, cleaned everything, and laid them out to review. What I saw forever changed how I fire glass. The projects from the second firing, on the pre-fired fiber paper, were dramatically brighter with greater clarity and sparkle. The projects from the first firing, on the unfired fiber paper, had a dullness I had never noticed before. Subtle on its own—but side-by-side, the difference was stark. And once I thought about it, the reason was obvious.
Almost all ceramic fiber products—fiber paper, fiber blanket, fiber board—use organic binders to strengthen the material and reduce airborne particles. Those binders burn away when fired. And when binders burn away, they create smoke: airborne carbon particles. Some of that smoke settles on the glass, effectively undoing all that careful cleaning you did before putting the glass in the kiln.
One of the most distinctive characteristics of glass is its non-crystalline (amorphous) molecular structure. Devitrification—literally “to become unlike glass”—is the crystallization of glass molecules. Crystallization requires a “seed” on which to start growing (nucleation). The reason cleaning glass is such an important step in reducing devitrification is that we are removing potential seeds from the glass.
By prefiring fiber paper (30 minutes at 1000°F/538°C), we remove the organic binders before putting the glass in the kiln. The faint haze on the kiln-carvings fired on unfired fiber paper was light devitrification.
An important evolution in my own studio practice has been to eliminate ceramic fiber products and shelf paper as much as possible, for both the above reasons and health reasons. This isn’t to say I never use them, only that I limit their use to when they’re truly necessary. And when fiber paper is required, I often prefire it for optimal results.
