The Types of Borosilicate Glass, Explained Simply

“Borosilicate glass” sounds like one material. It isn’t. The types of borosilicate glass span a lab-grade formula built to ASTM standards, a cookware-grade version sold at a fraction of the price, and an art-glass grade made for torch work.

Mixing them up is how people end up with a baking dish that shatters or a beaker that cracks over a Bunsen burner.

Manufacturers use the word loosely, and even “Pyrex” doesn’t mean what most people think anymore. This guide sorts the classification system scientists use from the grades you’ll see on a store shelf, and shows exactly which one fits your situation.

What Makes Glass “Borosilicate” in the First Place?

Borosilicate glass is glass where boron trioxide (B₂O₃) has been added to the standard silica network, and that single change is what separates it from ordinary window or bottle glass.

The Core Definition (Silica + Boron Trioxide)

Borosilicate glass is a glass built primarily from silica (roughly 80%) and boron trioxide (roughly 13–15%), with small amounts of sodium and aluminum oxide rounding out the mix.

Boron trioxide acts as a second network-former alongside silica, creating a tighter, more cross-linked structure than standard soda-lime glass. That structure is why borosilicate resists heat, chemicals, and thermal shock better than the glass in a typical drinking cup.

Why Boron Changes How Glass Behaves

Ordinary soda-lime glass relies on sodium oxide and calcium oxide as “modifiers” to lower its melting point.

Those modifiers loosen the silica network and leave behind mobile sodium ions, the same ions that let the glass expand quickly under heat and slowly leach into whatever it’s holding.

Boron does the opposite. It replaces most alkali modifiers and bonds directly into the silica network, keeping the glass dimensionally stable through temperature swings.

That’s why a borosilicate flask can go from an ice bath to an open flame without cracking, while a soda-lime jar can shatter from a splash of boiling water.

The Two Ways “Types of Borosilicate Glass” Gets Used

When people search for “types of borosilicate glass,” they’re actually asking two different questions, depending on who they are, and almost no source separates them.

A lab manager wants the ASTM or ISO classification system. A home cook or glass-art hobbyist wants to know which store-bought grade is safe and durable enough for their project.

Here’s the bridge between the two vocabularies, the technical standard on the left, what it actually shows up as on the shelf on the right:

Scientific/Regulatory Classification (for lab & industrial buyers)

If you’re buying glass for a lab, a pharmaceutical line, or an industrial process, the classification system determines whether your glass survives the job. ASTM E438 and USP Type I/II/III exist because “borosilicate” alone doesn’t tell you the expansion coefficient or the hydrolytic resistance you need.

Skip the spec sheet, and you risk a vial that leaches silica into an alkaline solution, or a flask that cracks under a heating mantle it was never rated for.

Commercial/Consumer Grades (for shoppers & hobbyists)

If you’re buying a water bottle, a casserole dish, or a glass rod for a torch, nobody expects you to read ASTM E438. What matters is the grade of lab, cookware, or art, and which brand actually delivers it.

This is where most buying confusion happens, since packaging rarely states the classification, and “borosilicate” gets printed on products that don’t consistently meet any standard.

Borosilicate Glass Classification by Standard

Three standards bodies define borosilicate glass, and they don’t always agree on the cutoffs, which is exactly why the same jar of glass can be marketed differently in the US and Europe.

ASTM E438 — Type I Class A vs. Class B

Class A is the one to reach for when thermal shock resistance is the priority. It’s the formula behind most beakers and flasks in a chemistry classroom.

Class B trades some of that expansion resistance for chemical durability, which is why it dominates the vial and ampoule market instead.

USP / Ph. Eur. Type I, II, III (Hydrolytic Classes)

Type I is the only one of the three that’s actually borosilicate. Types II and III are soda-lime glass. Type II exists because surface-treating soda-lime with ammonium sulfate improves chemical stability enough to pass a stricter test, but it’s still not the same base material as Type I.

ISO 3585 and the “3.3” Designation

ISO 3585 is the international standard that defines borosilicate 3.3 glass, named for its thermal expansion coefficient of 3.3 × 10⁻⁶ per Kelvin.

When a product is labeled “borosilicate 3.3” or certified to ISO 3585, that’s a specific, checkable claim, not marketing language.

It’s the strongest signal you’ll find on a retail label that you’re getting real lab-grade borosilicate rather than a soda-lime product wearing the name.

Borosilicate 3.3 vs. 5.0: What the Numbers Mean

The “3.3” and “5.0” numbers you’ll see on borosilicate products refer to the thermal expansion coefficient, and the lower number is the better performer for anything involving direct heat.

Thermal Expansion Coefficient Compared

3.3 wins for anything near a direct flame or extreme temperature swing. 5.0 isn’t inferior — it’s tuned for a different job, where chemical stability in a sealed container matters more than surviving a torch.

Which Applications Need Which Number

• Choose 3.3 for lab beakers, flasks, and anything headed for a Bunsen burner, autoclave, or torch.
• Choose 3.3 for kitchen bakeware that moves between freezer and oven.
• Choose 5.0 (alumino-borosilicate) for pharmaceutical vials and ampoules, where sealed chemical stability matters more than flame resistance.

Borosilicate Glass Grades by Use Case

Beyond the technical classification, borosilicate glass splits into three practical grades depending on what it’s built to survive, and picking the wrong one is the most common mistake buyers make.

Laboratory-Grade Borosilicate (Duran, PYREX®, Simax)

Lab-grade borosilicate is built and tested to a documented standard, which is why it costs more and why substituting a cheaper “borosilicate-labeled” product in a lab setting is a real risk, not just a downgrade in quality.

If you’re stocking a lab, start by selecting the right lab-grade borosilicate glassware before you compare prices.

Cookware & Food-Storage-Grade Borosilicate

• Cookware-grade borosilicate handles repeated oven, microwave, and freezer cycles, but not the sustained heat of a lab burner.
• It’s non-porous, so it won’t absorb food odors or leach flavor into acidic dishes like tomato sauce.
• Genuine cookware-grade borosilicate is certified free of lead, cadmium, and BPA, and reputable brands publish FDA or LFGB compliance.
• It costs more than soda-lime bakeware — that price gap is usually the clearest sign a product is real borosilicate.

For a full breakdown of what to look for before buying, see Choosing the right borosilicate cookware.

Art & Lampworking-Grade Borosilicate (Northstar, TAG, Glass Alchemy)

• Art-grade borosilicate resists devitrification (surface clouding) from repeated torch exposure, unlike ordinary glass.
• Northstar, TAG (Trautman Art Glass), and Glass Alchemy are the color brands most lampworkers reach for.
• Clear rod for structural work is typically Simax or Schott, chosen for consistency over color.
• Wall thickness matters as much as brand — thin-wall tubing shatters more easily under a torch than thick-wall stock.

If you’re setting up for glass art, start with borosilicate rods and tubing for lampworking rather than buying color first.

Colored Borosilicate: Opaque, Transparent, Striking, Color-Change

• Opaque colors are solid tones — whites, blacks, vivid reds, and blues — that don’t let light through.
• Transparent colors are jewel-toned and see-through, favored for layering and encasing.
• Striking glass shifts color as it’s worked, depending on whether the flame is reducing or oxidizing.
• Color-change glass shifts hue based on the light source or glass thickness.

Borosilicate vs. Other Glass Types (So You Don’t Buy the Wrong One)

Borosilicate gets compared to three other materials constantly, and the differences are specific enough to settle with numbers rather than vague reassurance, starting with how borosilicate compares to soda-lime glass in daily use.

Borosilicate vs. Soda-Lime Glass

Soda-lime wins on cost and is fine for everyday glassware that never sees direct heat. Borosilicate wins decisively anywhere thermal shock or acid/alkali exposure is a real factor, not a close call, given the nearly threefold difference in expansion rate.

Borosilicate vs. Lead Crystal (the Safety Myth, Debunked)

Borosilicate glass contains no lead. The viral “borosilicate is dangerous” claims online are, almost without exception, confusing borosilicate with lead crystal, a different material containing 24% or more lead oxide by weight.

Lead crystal’s health concerns come from that lead leaching into acidic liquids like wine over time, a mechanism that doesn’t apply to borosilicate since there’s no lead to leach.

If food safety is your actual concern, the relevant comparison is whether borosilicate glass is safe for food and drinks, not a myth built on mistaken identity.

Borosilicate vs. Tempered Soda-Lime Glass

Tempering doesn’t change a glass’s expansion rate; it changes how it fails. Tempered soda-lime survives a drop better than borosilicate, but still cracks under a temperature swing that borosilicate shrugs off.

The PYREX vs. pyrex Problem (Why Your “Borosilicate” Pyrex Might Not Be)

If your bakeware says “PYREX” in lowercase, it’s almost certainly soda-lime glass, not borosilicate, and that single capitalization detail is the most common source of buying mistakes in this entire category.

How to Tell From the Label

1. Check the wordmark case: all-capitals “PYREX” indicates borosilicate, typically found on lab glassware and pre-1998 US cookware.
2. Check the country of manufacture on the base — cookware marked “Made in France” (International Cookware) is still borosilicate.
3. Check for “USA” markings — Pyrex-branded cookware made in the US today, sold as lowercase “pyrex” by World Kitchen/Corelle Brands, is soda-lime.
4. If the packaging doesn’t specify, contact the manufacturer directly — reputable brands will confirm the base glass on request.

Why Corning Changed the Formula in 1998

Corning sold the Pyrex cookware business, and the new owner, World Kitchen, switched US-made Pyrex bakeware from borosilicate to tempered soda-lime glass.

The change lowered manufacturing cost and improved drop resistance, but it also cost US Pyrex bakeware the thermal-shock advantage that made the original formula famous.

European Pyrex, made under a different license by International Cookware, kept the borosilicate formula, so the same brand name means two different materials depending on where it was made.

How to Verify You’re Actually Buying Borosilicate Glass

Since packaging doesn’t always specify the classification, verifying a product yourself takes two steps: checking the paperwork, then checking the glass.

Label & Certification Checks

1. Look for an explicit “borosilicate 3.3” or ISO 3585 statement — vague terms like “high borosilicate” with no number are a warning sign.
2. Check for FDA food-contact compliance on cookware and food-storage items.
3. Check for LFGB certification (a German food-safety standard) on products sold into the EU.
4. For lab glassware, confirm that the ASTM E438 class and expansion coefficient are listed, not just the word “borosilicate.”

Physical Tests You Can Do at Home

1. Weigh it — borosilicate is noticeably lighter than soda-lime of the same size, since it’s a lower-density formula.
2. Check the edge color — soda-lime often shows a faint bluish-green tint from iron impurities, while borosilicate is clear.
3. Run a small thermal test on a disposable or already-damaged item — boiling water on chilled soda-lime is far more likely to crack it than the same test on borosilicate.
4. Compare the price against known soda-lime alternatives — a price matching cheap glassware is a signal to double-check the label.

For the complete step-by-step version of this process, see a full step-by-step borosilicate verification test.

Which Type of Borosilicate Glass Should You Buy?

Three questions settle this for almost every buyer, and they matter more than brand names or price.

First, how much heat exposure will the glass see? Going into a lab burner, stovetop, or direct flame means you need 3.3-class borosilicate — Class A under ASTM E438, or ISO 3585 certified. Anything less isn’t rated for that stress.

Second, how much chemical exposure is involved? For sealed pharmaceutical or lab chemical storage, alumino-borosilicate (Class B, the 5.0 expansion glass) usually makes more sense than 3.3, since durability matters more than flame resistance there.

Third, does the price match the claim? Genuine borosilicate — lab, cookware, or art grade — costs meaningfully more than soda-lime. A “borosilicate” product priced like everyday glassware is worth double-checking before you buy, not after it cracks.

Match those answers to the grades above: lab-grade for direct heat and chemistry, cookware-grade for the kitchen, art-grade for torch work, and Class B specifically for pharmaceutical storage.

Frequently Asked Questions

What are the different types of borosilicate glass?

The types are split into a classification system (ASTM E438 Class A and Class B, plus ISO 3585) and a commercial grade system (lab, cookware, and art/lampworking).

Class A and ISO 3585 are the low-expansion formulas used in labs and premium cookware; Class B (alumino-borosilicate) is used mainly in pharmaceutical vials.

What’s the difference between borosilicate 3.3 and 5.0?

The numbers are the thermal expansion coefficient — 3.3 × 10⁻⁶/K versus roughly 5.0 × 10⁻⁶/K. 3.3 handles direct heat and thermal shock better, which is why it’s used for lab glassware and torch work; 5.0 trades some of that for chemical durability suited to sealed pharmaceutical containers.

Is all Pyrex-brand glassware actually borosilicate?

No. All-caps “PYREX” lab glassware and European Pyrex cookware are borosilicate, but US Pyrex-branded cookware made since 1998 is tempered soda-lime glass — check the wordmark case and country of manufacture before assuming.

What is Type I Class A vs. Class B borosilicate glass?

Class A is low-expansion borosilicate (~33 × 10⁻⁷/°C) used for general lab glassware like beakers and flasks. Class B is alumino-borosilicate with higher expansion (48–56 × 10⁻⁷/°C), used mainly for pharmaceutical vials where chemical durability matters more than thermal shock resistance.

Is borosilicate glass safer than soda-lime glass?

For anything involving heat or acid/alkali exposure, yes, its tighter molecular structure leaches far less than soda-lime’s mobile sodium ions. For everyday glassware that never sees direct heat, the difference is negligible.

Can borosilicate glass still crack or shatter?

Yes. It resists thermal shock far better than soda-lime, but it’s actually more brittle under direct impact than tempered soda-lime glass. Dropping it on a hard floor can still break it, even though a temperature swing won’t.

What borosilicate grade do glassblowers use for pipes and art?

Most lampworkers use 3.3-class clear rod (Simax or Schott) for structural work and color brands like Northstar, TAG, or Glass Alchemy for the colors. The 3.3 rating matters because it resists devitrification, the clouding caused by repeated torch exposure.

How can I tell if a product is genuinely borosilicate before buying?

Check the label for an explicit “borosilicate 3.3” or ISO 3585 statement rather than a vague “high borosilicate” claim. If the label is unclear, weigh the item; it’s noticeably lighter than soda-lime, and check the price isn’t matching cheap glassware.

Why does borosilicate glass cost more than regular glass?

Boron trioxide is a pricier raw material than the sodium and calcium compounds in soda-lime glass, and borosilicate needs higher melting temperatures to manufacture. Both costs show up directly in the retail price.

Is borosilicate glass the same thing as tempered glass?

No. Borosilicate is a chemical formula that changes how much glass expands under heat. Tempering is a treatment applied to glass, often soda-lime that changes how it breaks on impact, not how it handles temperature swings.