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Clear Boro Quality: Simax vs Schott Duran vs Import Clear

What separates Simax and Schott Duran clear boro from generic import clear: published specs, defects like airlines and striae, scum, and COE 33 fit.

By Joe Blanchard, sixteen years on the distribution side of the glass industry · Updated

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Short answer: Simax (Kavalier) and Schott Duran are documented borosilicate 3.3 glasses, but a COE 33 label alone does not establish that any two clears will form a sound join. The useful buying differences include published specifications, traceable stock, defect tolerances, and consistency. Kavalier and Schott publish technical datasheets and identify conformance to ISO 3585; Schott also publishes tubing quality information. For an unfamiliar clear, establish the manufacturer and glass type, then evaluate the actual batch and intended process. Time spent grading rod, chasing bubbles, or polishing haze belongs in the price comparison.

Clear is the glass you use the most of and blame the least, so it pays to be pickiest about it. This guide covers what the standards actually guarantee, the defect vocabulary that matters at the bench, and how to evaluate a clear you’ve never used. (For where boro sits against soft glass and quartz in the first place, see soft glass vs boro vs quartz.)

What COE 33 actually guarantees, ISO 3585 in plain English

“Borosilicate 3.3” is a defined material, not a marketing phrase. ISO 3585 requires a mean linear thermal expansion coefficient of (3.3 ± 0.1) × 10⁻⁶ K⁻¹ measured from 20 to 300 °C, plus minimum chemical durability: acid resistance under 100 µg Na₂O per dm² (per ISO 1776) and alkali resistance of class ISO 695-A2 or better. Any glass legitimately sold as 3.3 borosilicate sits inside that expansion band.

That’s what the standard guarantees. Here’s what it doesn’t guarantee: freedom from bubbles and airlines, absence of striae or cord, dimensional consistency of rod and tubing, or how the surface behaves after twenty minutes in your flame. Two glasses can both pass ISO 3585 and still work very differently. The expansion band also does not describe viscosity through cooling or certify every possible combination and heat history. For that distinction, see glass compatibility beyond COE.

The big names: Simax (Kavalier) and Schott Duran

Simax is made by Kavalier in Sázava, Czech Republic, a glassworks founded in 1837 by František Kavalír, focused from the beginning on pharmacy and laboratory glass. Kavalier’s historical catalog dates the development associated with Simax to 1954, under a team led by Miloš Volf, and the Simax trademark to 1958. Those are separate milestones from the glassworks’ founding; the full story is in Pyrex, Duran and Simax: from laboratory glass to art. Per Kavalier’s 2025 technical data sheet, Simax has a COE of (3.3 ± 0.1) × 10⁻⁶ K⁻¹ (20 to 300 °C, ISO 7991), density of 2.23 ± 0.02 g/cm³, and a composition of roughly 80.3% SiO₂, 13.0% B₂O₃, 2.4% Al₂O₃, and 4.3% Na₂O+K₂O, essentially the same family as Schott’s Duran 8330. It conforms to ISO 3585.

Schott Duran is borosilicate 3.3 meeting ISO 3585:1998 and ASTM E438 Type I. Schott states that Duran tubing and rod are produced to very tight geometric tolerances with high optical quality, and its datasheet defines the working behavior by standardized viscosity points, with a maximum permissible operating temperature around 500 °C, a transformation temperature near 525 °C, and a softening point near 825 °C (the first is a service limit for finished ware, and none of them is a flameworking temperature).

The point isn’t that these two glasses are exotic, it’s that both makers publish what they’re selling. You can pull the TDS, see the expansion band, the composition, and the standard it’s certified to, and use those specifications when assessing the next case.

The defect vocabulary: airlines, knots, stones, striae

Drawn glass has a standard defect taxonomy, documented in tubing-industry technical literature (VitroCom’s “Glass defects made visible” paper is a good plain-language reference):

DefectWhat it isWhat it costs you at the torch
AirlinesElongated gas bubbles drawn into fine linesVisible streaks in clear work; can open into surface lines when worked
Bubbles/seedsRound trapped gasCosmetic flaws; large ones can burst or leave pits
KnotsTransparent viscous inhomogeneities, formed mostly at the melt surface from incomplete evaporation/homogenizationOptical distortion; can lump or drag in a gather
StonesCrystalline inclusionsStress risers, a common origin point for cracks
Striae/cordLayered compositional streaksVisible “grain” in thick clear; slight viscosity differences while working

This vocabulary matters because it’s exactly what the top-tier makers quantify. Schott publishes explicit tubing defect specifications, airlines are limited by aggregate length per 10 m of tubing and by width, stones and knots are measured by core size, and for its pharmaceutical FIOLAX tubing Schott moved to 100% inspection with zero-defect specs for airlines of 0.08 mm width and up, cracks, and similar flaws. Flameworking rod isn’t held to pharma specs, and a pharma tubing spec says nothing direct about a maker’s art rod, but it does show the maker measures these defects and will put tolerances in writing. Most generic import clear ships with no published defect spec at all, which doesn’t mean it’s bad, only that you’re the inspection department.

Scum and haze: glass problem or flame problem?

The most common complaint about “cheap clear” is scum, a grey or white haze that develops as you work. Before blaming the glass, know that haze has several distinct causes, and most of them are technique:

  • Flame chemistry. A dirty or reducing flame deposits and reduces material on the surface. This is the first thing to rule out, see flame chemistry: neutral, oxidizing, reducing.
  • Devitrification. Crystallization of the glass surface, presenting as a white/grey hazy, scummy, or chalky texture. It nucleates when glass is held too hot for too long, or when surface contamination provides nucleation sites.
  • Surface contamination. Fingerprints, dust, marker ink, or shelf grime give devit somewhere to start.
  • Alkali volatilization. Extended working drives alkali out of the surface, changing its chemistry and finish.

Because technique is a confounder in every one of these, a haze problem cannot be automatically pinned on the brand. The fair test is side by side: same torch, same flame setting, same working time, two rods. If one clear scums noticeably sooner under identical treatment, that’s real information about the glass. If both haze, look at your flame and your prep first. (Color-specific scumming and boiling are their own topic, see boro color troubleshooting.)

Generic import clear: what actually varies

The honest framing on import clear is this: claims that it’s universally dirtier or scummier are community lore, not manufacturer-verified data. “Import clear” describes a supply route, not a formulation or compatibility specification. Ask for the maker, glass designation, and supporting documentation rather than treating its origin or a nominal COE label as proof of fit.

What you genuinely give up is the paper trail and the consistency it represents. Kavalier and Schott publish TDS documents, certify to ISO 3585, and control their melts batch to batch. A generic import may be excellent, and the next pallet may not match it. The costs of an inconsistent clear are quiet ones: time spent flame-polishing haze, rod culled for airlines and stones, a stone-seeded crack discovered after annealing. If your time is worth anything, “cheap clear” is only cheap when the batch is good, and you can’t know that until you’ve worked it. For hobby quantities the price gap is small; for production, the calculation depends on how much grading and rework a given batch demands.

Cross-brand compatibility: mixing the 3.3 family

Published expansion values are close, but their measurement intervals matter. Corning lists 32.5 × 10⁻⁷/°C over 0 to 300 °C for code 7740. Schott’s Duran data sheet and Kavalier’s 2026 Simax tubing catalog list 3.3 × 10⁻⁶ K⁻¹ over 20 to 300 °C. These figures identify a low expansion family; they do not measure the full behavior of a joined piece while it cools.

Kavalier’s tubing catalog permits joining Simax to other brands of the same borosilicate type. Keep that condition attached to the advice. It does not authorize unidentified clear, every glass sold under the Pyrex name, or arbitrary colored glass. Bullseye’s technical explanation of compatibility also makes the general materials point: viscosity matters alongside expansion. Its kilnforming examples are not borosilicate processing instructions.

Confirm the actual material and manufacturer guidance, then assess representative joins under the intended heat history and geometry. Our compatibility guide explains why a COE match is insufficient. If a test cracks, use why did my glass crack to investigate the process as well as the combination; a crack alone does not identify its cause.

Rod vs tubing: different jobs, different defect risks

Rod and tubing fail differently. In rod, defects live in the body of the glass: striae and cord show as grain in thick solid work, stones sit buried until they seed a crack, and airlines streak through what should be optically clean mass. In tubing, geometry joins the list, wall-thickness variation and out-of-round tubing make even heating and clean seals harder, and airlines in a thin wall can open into actual leaks or surface lines. This is why Schott’s tubing specs quantify both defects and dimensional tolerance. When you evaluate a new clear, evaluate the form you’ll actually buy: a maker’s rod and tubing can behave differently, and tubing is less forgiving of sloppy drawing.

How to evaluate a clear you haven’t used

A repeatable acceptance test costs one evening and a few rods:

  1. Inspect cold. Sight down each rod or tube against a dark background in good light. Count airlines, seeds, and visible stones or knots. Check tubing for wall uniformity.
  2. Melt test. Work a rod in a known-neutral flame for an extended period. Watch for early haze, boiling, or scum against a reference rod of your usual clear worked identically.
  3. Weld test. Fuse the new clear to your standard clear, work the joint, and anneal the test piece properly (see annealing schedules for glass).
  4. Polariscope check. A crossed-polarizer strain viewer can reveal residual stress. Compare representative, properly annealed joins with a control. A pattern does not by itself distinguish poor annealing from a material mismatch, and a clean field is not proof of compatibility for every process. Treat a simple visual check as screening, not a calibrated stress measurement or an unlimited guarantee.

These checks help screen a candidate batch. Keep its identity and test conditions recorded, and reassess if the batch, combination, wall thickness, or working process changes. A successful small sample does not certify every larger or more complicated project.

Key takeaways

  • Legitimate 3.3 borosilicate sits in the ISO 3585 expansion band, but that number alone does not establish compatibility for every combination and process.
  • Published specifications and traceability make buying decisions easier. Read the document for the actual glass and product form instead of assuming every brand or generic clear is identical.
  • Know the defect vocabulary, airlines, seeds, knots, stones, striae, because it’s what you’re screening for when no one else has.
  • Haze is not automatically the glass. Flame chemistry, devitrification, contamination, and alkali loss all cause it; test side by side before blaming a brand.
  • Cheap clear is only cheap when the batch is good; inconsistency costs cleanup time, culled rod, and post-anneal surprises.
  • Screen new clear with a cold inspection, melt test, representative weld test, and polariscope check before committing to volume. Treat a successful sample as evidence within its test conditions.

Sources

Editor’s note: compare expansion values with their measurement intervals. The Simax composition above is explicitly from the 2025 data sheet; the 2026 tubing catalog gives slightly different rounded values. A service-temperature limit, transformation temperature, and viscosity-defined softening point describe different things, and none is a torch setting. Follow the manufacturer’s current documentation for the specific material and form you buy.

About the author

Written and edited by Joe Blanchard, who spent 2007 to 2023 on the distribution side of the glass industry in Asheville, North Carolina. GlassTorches.com is his independent, manufacturer-first buyer's guide. More about the editor.

How this guide was sourced: figures come from the manufacturer's own pages and manuals where we have them and are labelled as estimates where they are ours; the catalog on this site is the source of truth, and where sources disagree the guide says so. Confirm the current spec with the maker before you buy. How we work.