How is borosilicate glass made? Not one way, three different ways, depending on what’s coming out the other end.
A lab beaker, a Pyrex casserole dish, and a hand-blown glass pipe all start from the same molten batch of silica and boron trioxide, but from there, the paths split completely.
Tubing gets drawn continuously through a machine. Cookware gets pressed or blown into a mold. Artisan pieces get shaped by hand over a torch.
Same chemistry, three different factory floors. Here’s what actually happens at each stage, from the raw batch through melting, shaping, and the cooling step that decides whether the glass holds up under heat at all.
What Is Borosilicate Glass Made Of?
Borosilicate glass is roughly 70–80% silica sand combined with 7–13% boron trioxide, plus smaller amounts of soda ash and alumina.
Silica is the actual glass-former melt; it’s hot enough, and it turns into the transparent, rigid material we recognize as glass.
Boron trioxide is what separates borosilicate from ordinary soda-lime glass, and it does one specific job: it lowers the glass’s thermal expansion so it can handle rapid temperature swings without cracking.
The Core Raw Materials
| Material | Typical Share | Function |
|---|---|---|
| Silica sand (SiO₂) | 70–80% | Forms the glass structure itself |
| Boron trioxide (B₂O₃) | 7–13% | Lowers thermal expansion, adds shock resistance |
| Soda ash (Na₂O) | 4–8% | Lowers the melting point of silica so it’s workable |
| Alumina (Al₂O₃) | 2–3% | Adds chemical durability and mechanical strength |
What Boron Trioxide Actually Does
Thermal expansion is how much a material physically grows when heated. Ordinary soda-lime glass expands at roughly 9 × 10⁻⁶ per Kelvin.
Add boron trioxide, and that number drops to around 3 × 10⁻⁶ per Kelvin — about a third as much. Less expansion means less internal stress when one part of the glass heats faster than another, which is the actual mechanism behind borosilicate glass surviving a hot pan going straight under cold tap water.
It’s a direct chemistry-to-physics relationship, and it’s the entire reason boron gets added in the first place.
Step 1: Batching and Melting the Raw Materials
Making borosilicate glass starts with weighing out the exact ratio of silica, boron trioxide, soda ash, and alumina, then melting that mixture in a furnace between 1,400°C and 1,600°C until it becomes a homogeneous molten mass.
Get the ratio wrong, and the finished glass won’t hit its thermal resistance targets, so this step is measured with real precision, not eyeballed.
Measuring and Mixing the Batch
Manufacturers combine the raw materials in a mixer before they ever see a furnace, and the mix has to be uniform — pockets of uneven composition turn into weak spots or visible streaks in the finished glass.
Once mixed, the batch goes into a continuous or day-tank furnace, where it’s heated until every grain of sand and boron compound has fully dissolved into the melt.
This can take hours, depending on furnace size and batch volume, and the melt has to sit at a temperature long enough for trapped gas bubbles to rise out before moving to the next stage.
Melting Temperature vs. Working Temperature
The melting temperature (1,400–1,600°C) is where raw solids become fully molten glass. That’s not the temperature at which the glass gets shaped.
Before shaping, the melt is cooled to a working temperature — typically 1,050°C to 1,200°C for tube drawing — where the glass is still soft and flowable but viscous enough to hold a shape as it’s pulled, pressed, or blown.
Confusing these two numbers is common, but they’re doing different jobs: one turns sand into glass, the other turns glass into a product.
Step 2: Shaping the Glass — Three Different Methods
Borosilicate glass gets shaped in one of three ways, depending on the end product: continuous tube drawing for tubing and vials, press-and-blow molding for cookware, or manual lampworking for custom and artisan pieces.
These aren’t stylistic choices they’re matched to what the final shape needs to be and how many units need to come off the line.
Tube Drawing — The Danner Process
- Molten glass flows from the furnace as a continuous ribbon onto a rotating, slightly tilted ceramic mandrel called the Danner pipe.
- Air is blown through the hollow shaft of the mandrel to keep a bore open as the glass wraps around it.
- The glass is drawn off the end of the mandrel and pulled away by a drawing machine, forming a continuous tube.
- Draw speed, air pressure, and glass temperature are adjusted together to control the tube’s final diameter, which typically ranges from 5mm to 50mm.
- The tube is cut to length once it reaches the end of the line and moves on to annealing.
Tube Drawing — The Vello Process
- Molten glass flows from the furnace into a bowl containing a hollow vertical mandrel, sometimes called the Vello needle.
- Glass flows through the narrow annular gap between the mandrel and a surrounding ring, which shapes it into tube form as it exits.
- Air pressure inside the mandrel keeps the internal diameter consistent as the tube is drawn vertically downward.
- The tube is redirected from vertical to horizontal travel and carried by rollers toward the cutting station.
- This method handles a wider size range (3mm to 60mm diameter, 0.5mm to 5mm wall thickness) and runs at higher throughput than the Danner process, which is why it’s the more common choice for high-volume pharmaceutical tubing.
Press-and-Blow Molding for Cookware
Cookware doesn’t come from a continuous tube it starts as a molten gob of glass dropped into a mold, then shaped using pressure or air.
A measured amount of molten borosilicate is cut from the furnace stream and dropped into a metal mold matching the shape of the final dish, bowl, or measuring cup.
A plunger presses the glass into the mold’s contours, or in some designs, compressed air blows it against the mold walls, the way a bottle gets formed.
The piece is then removed from the mold while still hot and moved directly to annealing. This is a batch process, not a continuous one each piece is essentially made individually, which is part of why cookware production runs slower per unit than tube drawing.
Manual Lampworking for Artisan and Custom Glass
Lampworking is the odd one out no furnace stream, no automated line, just a glassworker and a torch.
A solid rod or pre-drawn tube of borosilicate glass is heated locally over a high-temperature torch flame until that section turns soft and workable, while the rest of the piece stays rigid.
The glassworker then shapes the softened section by hand — rotating it, blowing into it, pulling it, or fusing it to another piece — building up the final form section by section rather than casting or drawing it all at once.
This method is how custom scientific apparatus, decorative glass, and one-off pieces get made, and it’s the slowest of the three methods by a wide margin, but it’s also the only one that allows shape changes mid-process.
Which Manufacturing Method Should You Expect for Your Product?
The method used to make a piece of borosilicate glass depends entirely on what that piece is for, and matching product type to process explains a lot of the confusion around “how borosilicate is made.”
Product-to-Process Comparison
| Product Type | Manufacturing Method | Typical Specs | Production Speed |
|---|---|---|---|
| Lab tubing, thermometers | Danner process | 5–50mm diameter | Moderate, continuous |
| Pharmaceutical vials, high-volume tubing | Vello process | 3–60mm diameter, 0.5–5mm wall | Fast, continuous, up to 55 tonnes/day |
| Bakeware, casserole dishes, measuring cups | Press-and-blow molding | Varies by mold | Slower, one piece at a time |
| Custom scientific glass, decorative pieces | Manual lampworking | Fully custom | Slowest, one-off |
Why the Method Changes the Result
A tube-drawn piece and a molded piece can carry identical glass chemistry and still behave differently under heat, because the manufacturing process shapes wall thickness and uniformity not just the glass formula.
Tube drawing produces consistent, thin, uniform walls by design, which is why lab tubing tolerates a hot plate without issue.
Molded cookware walls vary more in thickness across a single piece, and that variation, not the borosilicate chemistry itself, is a big part of why some cookware is rated for oven use only rather than direct stovetop flame.
For more on how that plays out in practice, see this breakdown of why some borosilicate products aren’t stovetop-safe.
Step 3: Annealing — Why This Step Determines Durability
Annealing is a slow, controlled cooling process that removes internal stress from freshly shaped glass, and skipping or rushing it is what turns technically correct borosilicate chemistry into glass that cracks anyway.
This step happens right after shaping, while the glass is still hot enough to relax internally but has already taken its final shape.
What Happens Inside the Annealing Lehr
Freshly shaped glass goes straight into an annealing lehr a long, temperature-controlled tunnel oven where it’s held at a specific temperature and then cooled gradually rather than left to cool at room temperature on its own.
Cooling glass too fast locks in uneven stress between the outer surface (which cools first) and the interior (which cools more slowly), the same physical mechanism that causes thermal shock cracking later during use.
A properly annealed piece has released that internal stress before it ever reaches a customer, which is the entire point of building a dedicated cooling tunnel into the production line instead of just letting the glass sit.
What Happens If Glass Is Under-Annealed
- The glass can look completely normal and pass a visual inspection while still carrying significant internal stress.
- Under-annealed pieces often survive normal handling — being washed, stacked, or carried — because that stress doesn’t show up under everyday mechanical load.
- The failure shows up specifically under thermal stress, meaning the glass can crack or shatter the first time it’s exposed to a temperature swing that properly annealed glass of the same composition would survive without issue.
- Manufacturers test for this using a polarized light strain viewer, since residual stress patterns are invisible to the naked eye but visible under polarized light as color bands or streaks.
- This is why two pieces of borosilicate glass with identical chemistry can perform completely differently in the same kitchen — one was annealed correctly, the other wasn’t.
Step 4: Quality Control and Final Processing
Every batch of borosilicate glass goes through inspection before it ships, checking for the kind of defects that don’t show up until the glass is already under stress in someone’s kitchen or lab.
This isn’t a final glance it’s a structured check against defined tolerances at multiple points in the line.
What Inspectors Check For
- Bubbles or seeds trapped in the glass during melting, which weaken the structure at that specific point.
- Uneven wall thickness, particularly in molded cookware, creates the same uneven-heating risk that causes thermal shock cracking.
- Residual stress from incomplete annealing was checked using polarized light strain viewers rather than visual inspection alone.
- Dimensional accuracy against target specs — pharmaceutical tubing, for instance, is held to tolerances as tight as ±0.05mm on outer diameter.
- Surface defects like scratches, chips, or inclusions that occurred during handling after shaping.
Additional Finishing Steps
- Cutting tubing or molded pieces to the final length or trimming excess glass from the mold seam.
- Polishing edges and rims, especially on drinkware and cookware, where a rough edge would be a handling hazard.
- Printing or etching graduation marks on measuring cups and lab glassware is done after annealing, so the marking doesn’t distort under heat.
- Applying any coatings, such as non-stick surfaces on bakeware or protective sleeves on lab tubing for shipping.
Is Borosilicate Glass Manufacturing Environmentally Friendly?
Borosilicate glass production is energy-intensive because of the high melting temperatures involved, but the material’s durability and recyclability offset some of that cost over the product’s life.
Neither side of that trade-off cancels the other out completely, so it’s worth looking at both separately.
Energy Use in Melting
Running a furnace at 1,400–1,600°C for hours at a time takes substantial energy, and that energy draw is the single biggest environmental cost in borosilicate production more so than raw material sourcing or transportation.
Manufacturers running continuous tube-drawing lines get some efficiency advantage here, since a Danner or Vello line stays at operating temperature around the clock rather than repeatedly heating and cooling a furnace for smaller molded batches.
Can Borosilicate Glass Be Recycled?
Yes, but it has to be processed separately from ordinary soda-lime glass, because the two types melt at different temperatures, and mixing them in a recycling stream produces defective glass.
Facilities that accept borosilicate specifically will remelt it back into new borosilicate products, effectively restarting the process from the batching and melting stage described above.
Curbside recycling programs generally aren’t set up for this separation, so borosilicate cookware and lab glassware often need a specialized recycler rather than a standard glass bin.
For the specifics on how that sorting and reprocessing actually works, see this guide on how borosilicate glass recycling actually works.
Who Manufactures Borosilicate Glass?
Schott (Germany), Corning (United States), and Borosil (India) are among the largest global manufacturers of borosilicate glass, each running a mix of tube-drawing and molding operations depending on their product lines.
If you’re sourcing borosilicate glass for a specific application lab tubing, pharmaceutical packaging, or bulk cookware production matching your supplier to the manufacturing method your product actually needs matters more than picking a brand name.
This directory of major borosilicate glass manufacturers breaks down which companies specialize in tube drawing versus molded product lines, so you’re not guessing which one fits your order.
Frequently Asked Questions
How is borosilicate glass different from regular glass?
Regular soda-lime glass expands at roughly 9 × 10⁻⁶ per Kelvin when heated, while borosilicate expands at around 3 × 10⁻⁶ per Kelvin due to its boron trioxide content. That difference is why borosilicate survives sudden temperature changes that would crack ordinary glass.
What temperature is used to melt borosilicate glass?
Furnaces melt the raw batch at 1,400°C to 1,600°C. The glass is then cooled to a lower working temperature, typically 1,050°C to 1,200°C, before it’s actually shaped.
Why is annealing important in glass manufacturing?
Annealing slowly cools freshly shaped glass to release internal stress that builds up from uneven cooling. Skipping this step leaves stress inside the glass that’s invisible on inspection but causes unpredictable cracking under thermal shock later.
Is borosilicate glass made by hand or by machine?
Both, depending on the product. Lab tubing and pharmaceutical vials are machine-drawn continuously via the Danner or Vello process, cookware is machine-molded in batches, and custom or artisan pieces are shaped by hand over a torch.
Can borosilicate glass be recycled?
Yes, but it needs separate processing from regular soda-lime glass because the two melt at different temperatures. Standard curbside recycling bins typically aren’t equipped for that separation, so specialized recyclers are usually required.
What is the Danner process?
It’s a continuous tube-drawing method where molten glass flows onto a rotating, tilted mandrel and is drawn off the end into tube form. It typically produces tubes between 5mm and 50mm in diameter.
What is the difference between the Danner and Vello processes?
The Danner process uses a rotating mandrel and horizontal drawing, while the Vello process draws glass vertically through an annular gap between a mandrel and ring.
Vello generally handles a wider size range and higher throughput, which is why it’s more common for high-volume pharmaceutical tubing.