On the occlusal surfaces and proximal contact points of a zirconia crown or bridge, mechanical polishing wears down the opposing tooth enamel less than a ceramic glaze layer, because the glaze degrades over time and becomes abrasive again as it flakes off. On anterior buccal surfaces with high aesthetic demands, by contrast, staining followed by glazing gives better long-term colour stability, according to a study published in late 2025. The right protocol therefore depends on the zone of the restoration, not on a single rule applied to the whole piece.
Why does the surface treatment change depending on the zone of the restoration?
A zirconia crown or bridge, whether milled from a monolithic disc or a multilayer block, leaves the sintering furnace with a surface finish that still needs to be refined before placement. Two routes exist: mechanical polishing, which shapes and brings out gloss in the ceramic through progressive abrasion, and glazing, which applies a thin fired glass layer, sometimes preceded by staining. These two treatments do not answer the same clinical constraints: one protects the opposing tooth and ages better on a surface that works in occlusion, the other gives finer control over shade on a surface that does not experience direct friction.
Does mechanical polishing really wear opposing enamel less than glazing?
Yes, and this runs against a common assumption about zirconia's hardness. According to L'Information Dentaire, in an article dated 21 April 2020, "zirconia causes less attritional wear" than metal-ceramic restorations when its surface is properly polished, because a durable polish limits abrasion of the enamel it sits against. The drawback of glazing is that it degrades over time and retains more bacterial plaque once its glassy layer has worn, making it a less reliable choice in the long run on a functional surface. The same source notes that chairside occlusal adjustments remain possible with suitable instrumentation, but they require careful repolishing afterward; for a major adjustment, sending the piece back to the laboratory is still preferable to leaving the surface unrefined.
What mechanical polishing protocol should a laboratory follow?
Manufacturers of polishers for sintered zirconia publish precise speeds and steps that vary from one system to another: following the actual kit's instructions remains the rule, but two examples give an order of magnitude. Komet France's 94018C polisher for sintered zirconia is used at an optimal speed of 6,000 rpm, 10,000 rpm at most, in two colour-coded steps: blue for polishing, grey for high-shine finishing, after preliminary shaping with diamond burs. EVE Ernst Vetter's Diapol system recommends a speed of 7,000 to 10,000 rpm, never exceeding 15,000 rpm, in three steps: a green polisher with roughly 35-micron grit for shaping, a grey polisher with 4-to-8-micron grit for smoothing, then a pink polisher with 1-to-2-micron grit for the final high gloss. The manufacturer also notes that diamond polishers should be dressed after each use to keep their efficiency over time.
Both protocols share the same logic: never skip a step from the coarsest grit to the finest, because a final gloss step applied directly over a poorly shaped surface gives a dull, uneven result, even with the right finishing polisher.
Should you polish before glazing, after, or instead of glazing?
A complete mechanical polish can replace glazing on functional surfaces, in particular after an occlusal adjustment or a contact retouch, without a new firing cycle. On aesthetic surfaces, many laboratories keep a classic protocol of staining followed by firing the glaze, reserving pure polishing for zones that work in occlusion. A porcelain furnace able to ramp up temperature quickly makes this glaze and stain firing step easier on zirconia, alongside work on lithium disilicate.
Do staining and glazing change colour stability over time?
A study published in December 2025 in Dentistry Journal by a team from the University of Zagreb provides figures on this exact question. The authors made 60 samples of super-translucent zirconia (Vita YZ ST, shade A1), split into three groups of 20: polished, glazed, and stained then glazed. Each group received its own surface treatment: specialised paste polishing for two minutes for the first, a glaze layer fired at 750°C for the second, a reddish-brown shade followed by the same glaze for the third. Each sample's colour was measured before and after 10,000 simulated brushing cycles.
The stained-then-glazed group showed the smallest colour variation, with a ΔE00 of about 0.33, below the generally accepted threshold of clinical perceptibility, while the simply polished samples showed the largest variations and reduced optical stability. The authors conclude that, for an anterior restoration with high aesthetic demands, staining followed by glazing gives better long-term shade predictability than polishing alone, a result that does not contradict the opposing-wear argument: these are simply two different criteria, one on a surface that rubs against a tooth, the other on a surface that rubs against nothing.
Polishing or glazing: how to decide based on the zone of the restoration?
- Occlusal surfaces and proximal contact points, including on a multi-unit bridge: full mechanical polishing, every step of the kit down to the finest grit, no glazing, to limit opposing wear and plaque retention over time
- Anterior buccal surfaces outside direct occlusion, with high aesthetic demands: staining followed by glazing fired in a porcelain furnace, accepting the wear trade-off since the zone is not under friction
- Retouching after a clinical try-in or an intraoral occlusal adjustment: systematic repolishing with a dedicated zirconia kit before handing the piece back to the patient, never a surface left straight from grinding
- A piece fully reworked in both the anterior and posterior zones: treat each zone according to its function rather than applying one choice to the whole restoration
A kit of zirconia stains and glaze, available as a single bottle or a complete kit, and a porcelain furnace able to ramp up to 200°C per minute for firing the glaze and stains, cover this whole protocol in the laboratory, on zirconia as well as on lithium disilicate.
- Zirconia polishing speed (Komet, 94018C polisher): 6,000 rpm optimal, 10,000 rpm maximum (Komet France, product sheet, accessed 20 October 2026)
- Zirconia polishing speed (EVE Diapol): 7,000 to 10,000 rpm, never exceeding 15,000 rpm (EVE Ernst Vetter, Diapol product sheet, accessed 20 October 2026)
- EVE Diapol system steps: 3 colour-coded polishers, from roughly 35-micron grit for shaping to 1-to-2-micron grit for the final gloss (EVE Ernst Vetter, Diapol product sheet, accessed 20 October 2026)
- Samples tested for colour stability: 60 samples of super-translucent Vita YZ ST zirconia, in 3 groups of 20 (Žagar et al., Dentistry Journal, December 2025)
- Simulated brushing cycles before colour measurement: 10,000 cycles (Žagar et al., Dentistry Journal, December 2025)
- Lowest colour variation observed: ΔE00 ≈ 0.33 for the stained-then-glazed group, below the clinical perceptibility threshold (Žagar et al., Dentistry Journal, December 2025)
- Firing temperature of the tested glaze: 750°C (Žagar et al., Dentistry Journal, December 2025)
Sources
L'Information Dentaire, "Zircone et polissage," 21 April 2020. https://www.information-dentaire.fr/formations/zircone-et-polissage/
Komet France, product sheet for the 94018C sintered zirconia polisher, accessed 20 October 2026. https://komet.fr/fr-FR/Produits/Laboratoire/Materiaux/Zircone-frittee/Polissoirs/94018C
EVE Ernst Vetter, product sheet for Diapol diamond polishers, accessed 20 October 2026. https://www.eternaltools.com/eve-diapol-diamond-polishers
Žagar M., Kovačić I., Pongrac R., Ćelić R., study on the colour stability of super-translucent zirconia polished, glazed, or stained then glazed after simulated brushing, Dentistry Journal, December 2025, volume 14, issue 1. https://pmc.ncbi.nlm.nih.gov/articles/PMC12840539/
