Kiln-Formed Glass

Kiln-formed glass — usually called fused glass — is the branch of glassworking that shapes cold sheet material with heat in a kiln rather than at a furnace or a torch. It is the most accessible route into glass, which is why it is so widely taught, and it is also the one where a single invisible mistake destroys the work weeks later. Understanding why requires understanding what heat actually does to glass.

The materials

The working stock is sheet glass, supplied in transparent, opalescent and streaky forms, supplemented by frit (crushed glass, graded from powder to coarse), stringer (thin drawn rod), confetti (very thin shards) and dichroic glass. Dichroic glass carries an extremely thin multilayer coating of metal oxides applied by vacuum deposition — a technology developed for optical and aerospace use — which reflects one part of the spectrum and transmits another, so the same piece shows two different colours depending on whether light passes through it or bounces off it.

Compatibility: the rule that cannot be broken

Glasses are sold by coefficient of expansion. COE 90 and COE 96 are the two common art-glass systems, and they cannot be mixed. A piece fused from incompatible glass may look perfect leaving the kiln and crack apart days or weeks later.

Every glass expands as it heats and contracts as it cools, at a rate specific to its formulation. Fuse two glasses with different rates and the cooling contraction pulls them against each other; the resulting stress is locked into the finished piece. Sometimes it fails immediately. More often it fails later, which is what makes it so unforgiving — the mistake and the consequence are separated by enough time that a beginner may not connect them. Testing compatibility before committing to a large piece is standard practice.

The firing schedule

A kiln firing is not a temperature, it is a programme: a series of ramps and holds. The characteristic stages are broadly consistent across studios, though exact figures vary with the glass, the kiln and the thickness of the piece.

• Initial ramp — deliberately slow through the lower range, because glass is brittle and uneven heating cracks it before it is soft enough to relieve stress.
• Tack fuse, roughly 1,350–1,400°F — pieces bond where they touch but keep their individual shapes and edges. Used where relief and separate elements are wanted.
• Full fuse, roughly 1,450–1,500°F — the layers flow together into one sheet with rounded edges and a continuous surface.
• Slumping, roughly 1,200–1,250°F — a fused flat sheet is reheated over or into a mould so that gravity forms it into a bowl, plate or curved panel. Almost always a second firing.
• Annealing — a controlled hold and slow cool through the annealing range (commonly in the region of 900–960°F for typical art glass) that lets internal stress relax before the glass becomes rigid. This is the stage that determines whether the piece survives.

Thickness drives everything. A thick piece needs a much longer anneal soak and a much slower cool than a thin one, because the interior and the surface must pass through the annealing range together. Rushing the cool is the second classic cause of delayed failure, after incompatibility.

Designing for transmitted light

The most significant conceptual difference between glass and paint is that glass is usually seen by transmitted rather than reflected light, and colour behaves differently as a result. Layering transparent colours is subtractive in a way that mixing pigments only approximates: two transparent layers produce a colour more saturated and darker than either, and the effect changes completely with the light behind it. A panel designed under studio fluorescents can look entirely different in a south-facing window.

This also means a fused piece has no single correct appearance. Its colour is a function of where it is hung and what time of day it is — closer to the logic of stained glass than to that of a painting, and one reason the craft sits so naturally beside still life painting, which spends much of its effort on exactly this problem in a harder medium.

Glass in the Southern Highlands

Glass arrived in the region's craft schools comparatively late — the American studio glass movement dates from the early 1960s, decades after weaving, pottery and woodwork were established here — but it took hold quickly. Penland School of Craft in Mitchell County runs one of the country's better-known glass programmes, and glass now sits alongside the older media in the Southern Highland Craft Guild's membership. The Smithsonian American Art Museum's Renwick Gallery is the principal national collection for American studio craft, including glass.