You picked the right shade disc — then the furnace shifted it. Sintering temperature is a colour variable, not just a strength one.
The mechanism
Above ~1500 °C, translucency rises but chroma can drop as the structure densifies; a long hold also lets trace colourants diffuse. A 20 °C overshoot can lighten a shade perceptibly.
Keep the shade stable
- Use the disc-maker's exact peak (usually 1450–1530 °C by grade).
- Avoid mixing shades in one batch at different peak temps.
- Calibrate the furnace thermocouple every 6 months; drift of 15 °C is common.
The AI Furnace Series stores per-shade programs so the temperature tracks the disc, not the operator's memory.
The phase-colour link
Translucency rises as the tetragonal phase fully densifies; under-sintered zirconia stays chalky because pores scatter light. Over-sintering pushes grains past 0.5 µm and the disc yellows as it absorbs more.
| Defect | Cause | Fix |
|---|---|---|
| Chalky | Under 1450 °C | Re-fire 20 min longer |
| Yellow | Over 1550 °C | Use a fresher puck |
| Patchy | Uneven load | Centre the tray |
HIP and shade
Hot isostatic pressing closes residual pores and deepens translucency by a visible step — many premium shades are HIP'd by design.