In the previous installment, we discussed common issues that occur before zirconia restorations enter the sintering furnace. Those early steps are crucial, but they are not the only potential sources of failure. Because zirconia is a brittle ceramic, any improper handling during sintering, adjustment, or porcelain application may also result in cracks, delamination, or complete fractures.
This article continues from where we left off and focuses on problems that appear after sintering, with practical insights for labs and technicians.
Zirconia typically undergoes a 20% volumetric shrinkage during sintering. This means the entire firing process must be carefully controlled. In long-span bridges—especially half-arch restorations with uneven thickness, mixed pontic sizes, or telescopic crowns—differences in heat exposure can cause uneven shrinkage. This uneven behavior is a major reason for mid-sintering fractures.
Reduce the heating rate for long-span or half-arch zirconia bridges.
Allow sufficient time for heat to penetrate evenly through thick and thin sections.
Follow a dedicated sintering curve designed for large restorations (example curve shown below).
| 4D Pro Multilayer Sintering Curve | ||||
| Sintering step | Start Temperature(℃) | End Temperature(℃) | Time(Min) | Rate(℃/Min) |
| Step 1 | 20 | 300 | 70 | 4 |
| Step 2 | 300 | 1000 | 175 | 4 |
| Step 3 | 1000 | 1500 | 167 | 3 |
| Step 4 | 1500 | 1500 | 120 | 0 |
| Step 5 | 1500 | 800 | 140 | -5 |
| Step 6 | 800 | natural cooling 20 | / | / |
Uniform thermal exposure ensures the entire prosthesis shrinks consistently, minimizing the risk of cracking during the peak-temperature phase.
Once the zirconia is sintered, technicians often proceed with contouring or polishing. Improper adjustments at this stage can easily damage the restoration.
Bur diameter limitations
Milling burs cannot reach very tight interproximal spaces. As a result, excessive grinding may be needed during manual adjustment. Heavy grinding produces heat, sparks, and local stress, increasing the chance of fractures—often appearing at the connector.
Insufficient embrasure space in the CAD design
The interproximal area should be opened properly in the design stage, allowing the milling bur to shape the embrasure rather than relying on extensive manual trimming afterward.
Over-adjustment due to inaccurate design
Ideally, a restoration should follow the principle of “mill what you designed.”
The closer the digital model is to the final shape, the less adjustment is needed. The more grinding required, the higher the fracture risk.
The veneering porcelain and external stains/glazes introduce additional thermal cycles. Because zirconia has low thermal conductivity, the core heats and cools at a slower rate than the veneering layer. This mismatch can create internal stress within the composite structure.
If the furnace cools too quickly, the outer veneering porcelain cools and contracts faster than the zirconia substructure.
This temperature imbalance generates internal tension and may cause crazing, chipping, or complete veneer detachment.
Zirconia CTE: 9.5–10.5 × 10⁻⁶ /K
Veneering porcelain CTE: 9.0–10.0 × 10⁻⁶ /K
The difference should ideally remain within 0–0.5 × 10⁻⁶ /K.
When the veneering porcelain has a higher CTE than zirconia, tensile stress forms in the porcelain during cooling, leading to cracking.
When zirconia has a much higher CTE, excessive compressive stress may cause the porcelain to flake or delaminate.
Long-span zirconia restorations require modified firing schedules. A single-unit crown and a 12-unit bridge cannot be fired using the same porcelain program.
Use a lower heating rate to ensure balanced heat penetration.
Extend the holding time, compensating for zirconia’s slow heat transfer.
Prolong the cooling phase to prevent internal stress cracks caused by rapid temperature drop.
| No. | Product Type | Preheating Temp (℃) | Drying Time (h) | Heating Rate (℃/min) | Start Temp (℃) | End Temp (℃) | Holding Time (min) | Cooling Time (min) |
|---|---|---|---|---|---|---|---|---|
| 1 | 1–3 Units — Standard Sintering | 450 | 8 | 55 | 450 | 830 | 1 | Natural Cooling |
| 2 | 1–3 Units — Fast Sintering | 450 | 2 | 99 | 450 | 830 | 0.5 | Natural Cooling |
| 3 | 4–7 Units — Sintering | 450 | 8 | 40 | 450 | 830 | 1 | 5 |
| 4 | 8 Units or More — Sintering | 450 | 10 | 30 | 450 | 830 | 1 | 10 |
