The restoration comes out of the furnace and there is a horizontal band across the incisal third. It was not visible in the design software and it was not visible before sintering. Staining will not remove it, and attempting to usually makes it more obvious.
Where the line comes from
Multilayer zirconia is manufactured by pressing layers of differently shaded and differently yttria loaded powder into one puck. How those layers meet is the whole game.
In a well made disc the composition changes continuously, so there is no plane where one layer stops and the next begins. In a cheaper disc the layers are discrete, and the boundary between them is a real physical plane. Mill through it at the wrong height and you expose it as a line.
Why nesting position decides it
Even with a continuous gradient, where you place the restoration in the height of the disc controls what the finished unit looks like. Place a crown too high and the whole thing sits in the translucent layer — it looks grey and lifeless because there is no opacity at the cervical. Place it too low and the incisal loses translucency.
Place it across a discrete boundary and you get the band. The design software will happily let you do all three.
What to check
First, whether your disc has a continuous gradient or stacked layers. The manufacturer should publish a transmittance range rather than a set of discrete layer values. A range implies a gradient.
Second, your nesting height convention. Most labs set this once and never revisit it, even after changing disc supplier — and different manufacturers put the gradient at different heights.
Third, whether the same operator nests every case. Inconsistent banding across a week usually means inconsistent nesting, not an inconsistent disc.
The disc side of the fix
A disc with a continuous 43% to 57% transmittance gradient and 800 to 1,200 MPa distributed across the layers removes the hard boundary entirely, which removes the failure mode rather than managing it. That is the specification to look for, in any brand.
How to tell gradient from stacked layers before you buy
Ask the manufacturer for the transmittance figure. A continuous gradient is described as a range across the puck. Stacked layers are described as discrete values per layer, often with a named layer count.
Then look at the cut edge of a disc you already own. A stacked product frequently shows visible bands in the green state under raking light. A graded one shows a smooth transition.
This is a 30-second check and it tells you more than the datasheet.
Sintering makes it worse or better
Shade layers in zirconia develop through sintering. A boundary that is faint in the green state can become pronounced after firing if the curve is wrong, because grain growth differs slightly between compositions.
So a lab can genuinely have a banding problem that is part disc and part furnace. If the line varies in severity between batches sintered on different programs, the furnace is contributing.
Fixing it in the design, not the finish
Once you know where the gradient sits in the puck, nesting becomes a deliberate act. Most CAM software lets you set the vertical position of the restoration within the blank, and some will show the shade zones graphically.
Set a house convention: incisal edge at a defined height for anterior units, and a different one for posterior. Write it down. The most common cause of inconsistent banding across a week is two technicians nesting to two different habits.
What not to do
Do not try to grind out a transition line. Removing material from a sintered unit to chase a shade boundary thins the restoration in exactly the area where it is already thinnest, and introduces the surface flaws that become crack initiation sites.
If the line is visible after sintering, the case is a remake with corrected nesting. That is cheaper than a fracture.
Go deeper
Milling platforms and nesting software: milling units compared for 2026.
Pricing
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