How kiln temperature changes colour in fused glass

Ask a glass artist what colour something will fire and the honest answer is: it depends on the schedule. The same sheet of blue can come out pale and watery, deep and saturated, or something else entirely — and the kiln decides.

What's actually happening in there

Most fusing happens between 750°C and 830°C. At the lower end — a tack fuse — pieces keep their edges and their surface texture, and colours stay close to what you cut. Push to a full fuse at the top of the range and everything flattens, layered colours mix optically, and some pigments genuinely shift.

The dramatic cases are the strikers: colours that transform with heat. Some reds and oranges load into the kiln looking like pale straw and strike to full colour during firing. The first time you see it, it feels like a trick. Copper blues and greens are touchier still — they can shift towards turquoise or muddy entirely depending on how long they sit at top temperature, and whether they're capped under clear glass or exposed.

Why time matters as much as temperature

Two schedules with the same top temperature can produce visibly different colour, because what matters is total heat work — how long the glass spends hot, not just how hot it gets. A slow ramp with a long soak does more to a copper blue than a fast spike ever will. This is why we keep a firing log for every kiln load, and why our schedules for saturated colour work run overnight.

The practical upshot for commissions

When we colour-match — to a fabric, a tile, a stretch of sea — we fire test tiles first, every time, in the same kiln position the final piece will occupy. It adds a week. It's also the only honest way to promise a colour. Great glass isn't about controlling the kiln; it's about knowing it well enough to predict what it will do.