Why mineral glass still matters
Mineral glass keeps a distinct place in eyewear. Corning, a long-standing producer of ophthalmic glass, puts it simply: “scratch resistance is a natural quality of ophthalmic glass”. Its range runs from refractive index 1.6 to 1.9, the latter described as “the thinnest mineral lens in the world”, alongside photochromic glasses and sunglass materials [1].
The sun lens segment shows how strategic glass remains. When Luxottica acquired Barberini in 2018 for EUR 140 million, it described the company as “the world’s top manufacturer of optical glass sun lenses”, with annual net sales of about EUR 85 million [2]. Glass sun lenses are an industrial business in their own right.
Glass lenses are polished with a cerium oxide slurry [4]. This article sets out what the public record says about that step: the process, the chemistry, the link between polishing and lens strength, and how to evaluate a compound.
1. The process: generate, fine, polish
Lens surfacing follows three stages. The curve is first generated by grinding; the surface is then fined with an abrasive pad pressed against a tool; finally, the pad and water are replaced by “a flow of a fine abrasive slurry and a soft pad” [3]. For glass, that slurry is cerium oxide: Corning’s ophthalmic glossary defines polishing as rubbing the ground surface on a tool covered with felt or a plastic sheet “while the lens is constantly sprayed with water containing finely powdered cerium oxide in suspension” [4].
Two market facts confirm that mineral lens polishing is a production discipline in its own right. Machine builders continue to cover it: OptoTech states that “all surface geometries on organic and mineral lens materials” can be processed on its polishing machines [5]. And compound suppliers sell grades designed specifically for it, rated by spindle speed — for example cerium oxides marketed for ophthalmic glass at “speeds up to 1200 RPM” and “up to 1500 RPM” [6]. Throughput is part of the specification.
2. Chemistry: why two cerium oxides do not polish alike
Glass polishing is chemical as well as mechanical. In Cook’s reference model, the polishing particle bonds with the hydrated glass surface and removes material as it moves on [7]; Hoshino and co-workers showed that CeO₂ forms Si–O–Ce bonds with silica, which is then torn away in clusters [8].
How the powder is made matters. A 2016 study in the journal Wear found that calcination temperature is the critical parameter governing the polishing efficiency of cerium oxide: only powders prepared above 700 °C polished glass well, with a maximum for samples annealed at 1050 °C, and removal rate correlated with crystallinity and Ce(III) content [9]. Commercial products also differ widely in composition: a 2018 review tabulates polishing powders from about 30 to 99.9 wt% CeO₂, with 0 to 35 wt% La₂O₃ [10]. A product name alone does not predict behaviour on your glass.
3. A scratch is not only a cosmetic defect: it is a strength defect
In the United States, eyeglass and sunglass lenses are regulated for impact resistance. Under 21 CFR 801.410, all lenses must be impact-resistant, and each finished glass lens for prescription use must be tested individually: a 5/8-inch steel ball weighing approximately 0.56 ounce is dropped from 50 inches onto the lens, which must not fracture [11].
The FDA’s guidance on this regulation states the link with surface quality explicitly [12]:
FDA GUIDANCE, 2010
“All lenses lose their impact strength in direct proportion to the breakdown of the polished and edged surfaces. The greater the number and depth of scratches or the poorer the edge finish, the weaker the lens becomes.”
For a mineral lens producer, polishing quality therefore acts twice: on cosmetic yield at inspection, and on the mechanical reliability of the lens. International requirements for uncut finished lenses are set by ISO 14889 (current edition 2025) and ISO 8980-1 [13].
4. Where scratches come from
The most rigorous public work on polishing defects comes from Lawrence Livermore National Laboratory. It shows that scratch density rises exponentially with the size of a small number of oversized “rogue” particles — dried ceria agglomerates among them [14] — and that final roughness correlates with the coarse tail of the slurry’s particle size distribution, average-sized particles having negligible influence [15].
The slurry itself changes during use. Polishing slurries typically contain 5–10 % solids, and the powder “loses its polishing ability gradually during usage” as other compounds accumulate on it and its particle size distribution shifts [10]. A slurry tank that runs across a shift does not contain the same slurry at the end as at the start.
5. The pad is part of the system
A study of polyurethane pads in optical glass polishing found that the most conspicuous change over time is the incrustation of abrasive grains in the pad microstructure. Pad wear had no significant influence on surface roughness during the first two hours of polishing, but shape accuracy was affected as the pad filled and its elasticity changed [16]. On a lens, shape is optical power. A compound should therefore be evaluated together with the pad, and over a full pad life rather than a short run.
6. Throughput: what Preston’s law implies
Removal rate is proportional to pressure and relative velocity, multiplied by a coefficient that captures the slurry, the pad and the glass [17]. On a production polisher, pressure and speed are set by the machine. When a powder lot or an ageing slurry changes that coefficient, removal rate changes at constant machine settings. A drift in cycle time can therefore originate in the slurry as well as in the machine.
7. How to evaluate a polishing compound for mineral lenses
| Criterion | What to record | Basis |
| Particle size tail | d90, d99 or maximum size on recent lots; behaviour of dried or settled slurry | LLNL [14][15] |
| Composition and process | CeO₂ and La₂O₃ content; consistency from lot to lot | [9][10] |
| Cosmetic yield | Rejects at inspection per 1,000 lenses, by defect type (scratch, haze) | ISO 14889, ISO 8980-1 [13] |
| Strength-related rejects | Drop-ball failures, tracked against polishing lot and slurry batch | 21 CFR 801.410 [11][12] |
| Cycle time stability | Polishing time per lens at the start and end of a slurry batch | Preston [17]; slurry ageing [10] |
| Pad interaction | Lens power and roughness across a full pad life | Belkhir et al. [16] |
| Documentation | Certificate of analysis per lot; safety data sheet for your country | Good practice |
The comparison that settles the question is cost per good lens: powder consumed, pad life, cycle time and rejects, measured on your own line with one variable changed at a time.
8. Supply and industry calendar
Cerium and lanthanum are not covered by China’s April 2025 export controls, which concern samarium, gadolinium, terbium, dysprosium, lutetium, scandium and yttrium. Price is the variable to watch: the US Geological Survey reports cerium oxide (99.5 % minimum) at US$1.21/kg in 2024 and US$1.71/kg in 2025 [18].
The sector’s main annual meeting is MIDO, in Milan: the next edition takes place on 6–8 February 2027 at Fiera Milano Rho, with over 1,200 exhibitors from 50 countries according to the organiser [19].
Key takeaways
- Mineral glass offers natural scratch resistance and indices up to 1.9, and glass sun lenses are an established industrial segment [1][2].
- Glass lenses are polished with cerium oxide slurry on a pad; throughput is part of the grade specification [4][6].
- Under US rules, scratches reduce impact strength: polishing quality affects both cosmetic yield and the drop-ball test [11][12].
- Scratches come from the coarse tail and from agglomerates, and slurry changes during use [10][14][15].
- Evaluate powder and pad together, over a full pad life, and compare on cost per good lens [16].
Working with Illumine Consultants
Illumine Consultants supplies cerium oxide polishing powders for mineral glass sun and prescription lenses. CERITOP is intended for operations that prioritise throughput and cost per lens; LUCERIS for the tightest defectivity requirements. We start from your glass type, your machines and your current rejection pattern, then propose a grade and an evaluation sample with a trial method.
Request an evaluation sample · Book a technical call with Olivier — illumine-consultants.com
About the author. Olivier M. is the founder of Illumine Consultants. Before founding the company, he spent eight years in rare-earth polishing solutions at Solvay. Illumine Consultants supplies cerium- and lanthanum-based polishing compounds (CERITOP and LUCERIS ranges) with an application-first approach and a documented supply chain.
This article relies exclusively on public sources, listed below. Brand names are cited for descriptive purposes only; Illumine Consultants is not affiliated with the companies mentioned.
References
[1] Corning Inc., “High Index Glass Blanks” (ophthalmic glass). corning.com (accessed 1 Oct 2026).
[2] Glass Online, “Luxottica buys sun lens maker Barberini”, June 2018. glassonline.com (accessed 1 Oct 2026).
[3] Laramy-K Optical, “All About Lens Surfacing”. laramyk.com (accessed 1 Oct 2026).
[4] Corning Inc., “Glass Glossary — Ophthalmic Glass” (definition of polishing). corning.com (accessed 1 Oct 2026).
[5] OptoTech, “Ophthalmics — Polishing”. optotech.net/en/products/ophthalmics/polishing (accessed 1 Oct 2026).
[6] Gerard Kluyskens Company, “Cerium Oxide for Mineral Glass Lenses” (product descriptions). gkci.com (accessed 1 Oct 2026).
[7] L. M. Cook, “Chemical processes in glass polishing”, Journal of Non-Crystalline Solids 120, 152–171 (1990). doi:10.1016/0022-3093(90)90200-6.
[8] T. Hoshino et al., “Mechanism of polishing of SiO₂ films by CeO₂ particles”, Journal of Non-Crystalline Solids 283, 129–136 (2001). doi:10.1016/S0022-3093(01)00364-7.
[9] P. Janoš et al., “Chemical mechanical glass polishing with cerium oxide: Effect of selected physico-chemical characteristics on polishing efficiency”, Wear 362–363, 114–120 (2016). doi:10.1016/j.wear.2016.05.020.
[10] C. R. Borra, T. J. H. Vlugt, Y. Yang, S. E. Offerman, “Recovery of Cerium from Glass Polishing Waste: A Critical Review”, Metals 8(10), 801 (2018). doi:10.3390/met8100801.
[11] US Code of Federal Regulations, 21 CFR 801.410, “Use of impact-resistant lenses in eyeglasses and sunglasses”. ecfr.gov (accessed 1 Oct 2026).
[12] US Food and Drug Administration, “Guidance for Industry and FDA Staff — Impact-Resistant Lenses: Questions and Answers”, 2 September 2010. fda.gov/media/71020/download.
[13] ISO 14889:2025, Ophthalmic optics — Spectacle lenses — Fundamental requirements for uncut finished lenses; ISO 8980-1:2017, Ophthalmic optics — Uncut finished spectacle lenses — Part 1. iso.org.
[14] T. Suratwala et al., “Effect of rogue particles on the sub-surface damage of fused silica during grinding/polishing”, Journal of Non-Crystalline Solids 354(18), 2023–2037 (2008). doi:10.1016/j.jnoncrysol.2007.11.015.
[15] T. Suratwala et al., “Microscopic Removal Function and the Relationship Between Slurry Particle Size Distribution and Workpiece Roughness During Pad Polishing”, Journal of the American Ceramic Society (2014). doi:10.1111/jace.12631.
[16] N. Belkhir, D. Bouzid, V. Herold, “Morphological behavior and wear of polyurethane pads used in glass polishing process”, Precision Engineering 36(4), 641–649 (2012). doi:10.1016/j.precisioneng.2012.05.006.
[17] F. W. Preston, “The Theory and Design of Plate Glass Polishing Machines”, Journal of the Society of Glass Technology 11, 214–256 (1927).
[18] U.S. Geological Survey, Mineral Commodity Summaries 2026 — Rare Earths. pubs.usgs.gov/periodicals/mcs2026/mcs2026-rare-earths.pdf.
[19] MIDO Eyewear Show, official website. mido.com (accessed 1 Oct 2026).
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