How Pyrex Glass Cookware Is Made in Charleroi, Pennsylvania

updated May 30, 2019
We independently select these products—if you buy from one of our links, we may earn a commission. All prices were accurate at the time of publishing.
Post Image
(Image credit: Casey Barber)

Who: Pyrex
What: Iconic glass kitchenware
Where: Charleroi, PA

From the moment you step through the gates of the sprawling Pyrex factory complex, on the banks of the Monongahela River, glass surrounds you. You feel it crunching under your feet and see mountains of sparkling glass shards in the distance — building-high piles that seem as if they’re guarding the old lines where Pyrex’s distinctively colored Opalware was produced from 1936 to 1986.

Piles of broken glass wait to be reborn into brand new Pyrex dishes. (Image credit: Casey Barber)

The Role of Recycled Glass in Pyrex Bakeware

These glass shards are actually called “cullet,” or broken pieces of Pyrex and other glass (like old windowpane glass) that will be crushed and melted to become a new piece of Pyrex. Incorporating this pre-tempered glass into the mix is good for a few reasons: it makes the molten glass more durable and easier to form, and it also melts more easily than the raw materials, making it more energy-efficient to produce each batch.

Funneling glass into the furnace. (Image credit: Casey Barber)

What Else Goes Into Pyrex?

Why isn’t every single piece of Pyrex melted down from old Pyrex scrap? For one, there’s just not enough broken glass; if there was, it would signal some serious quality issues for the factory! And besides, if it were all cullet, the glass wouldn’t be strong enough to do what it needs to do — namely, stay intact in high-heat conditions.

The cullet and all the other raw materials that make up the Pyrex glass blend, like lime, soda ash, and sand, are weighed precisely on a series of electronic scales in a small room, then blended together in two-ton batches.

The signature blue-green tint of Pyrex glass comes from the particular chemistry of the materials that make up the glass, as well as a bit of cobalt color mixed in to make sure the tint stays the same from batch to batch. The same blend of glass is used for every single piece Pyrex produces, from the small measuring cups to the big casserole dishes and mixing bowls.

(Image credit: Casey Barber)

It All Starts with Melting the Gobs (Yes, Gobs) of Glass

The two-ton containers are dumped into a big, shimmying funnel that continuously feeds the single furnace that melts everything down and sends molten glass to each of the four production lines that press and shape the glass into the familiar vessels we know and love. Every day they make enough mix to fire all the pieces they’re making that day on all four lines, and keep enough mix for an eight-hour emergency reserve, just in case.

It’s 1400°C — about 2500°F — inside the tank, where a sea of shimmering orange liquid glass makes you feel like you’re experiencing the final scene from Terminator 2 firsthand or staring at the surface of the sun. (No, really! A special filter is used when peeking into the small porthole opening, and even through this small hole, you can feel the emanating waves of heat so strongly I feared they might melt my camera lens.)

The temperature drops to 1130°C to 1170°C (2000°F to 2100°F) by the time the glass flows onto the production line. Each gob — yes, that is the technical term for the blob of glass that is squished and pressed into shape — is precisely measured to make sure it’s the right amount for the piece being made on that particular line that day. The gob’s weight varies, since it takes more glass to form a four-quart casserole than a one-cup measuring cup.

Brand new glass casserole dishes going through the hardening process. (Image credit: Casey Barber)

A Fast 7 Minutes from Pressing to Shipping

Once that glowing gob flows down the chute and onto the line, things happen quickly. Only seven minutes elapse from the time a piece is pressed until the time it’s in the box and ready for shipping.

The gob falls into a cast iron and stainless steel mold and is pressed into shape via a rotating piece of machinery. Metal pins sticking out of each mold make for an ingenious piece of design — they help cool air flow up and around the mold to harden the glass, preventing bubbles and fissures from forming in the glass as it cools. Just one more thing that makes Pyrex stronger!

Iconic glass measuring cups on the production line. (Image credit: Casey Barber)

The piece is picked up by a robotic arm and moved to another rotating station for fire polishing, where the rough edges of glass are, well, polished to smoothness by jets of white-hot flame. Because the Pyrex glass hasn’t yet been heat tempered at this point, it could crack. These jets shoot out at regular intervals throughout the pressing and polishing process to regulate temperatures of the glass and the molds as well as to polish it (to make another dated media reference, it’s like the fire in the Super Mario Brothers castles).

After the Pyrex piece is fire-polished, it chugs down a conveyor belt into the kiln, where the glass is tempered for five minutes. The only exception to this process is for pieces that are screen-printed, like the measuring cups. These take a quick side trip into the screen-printing room to be swiped with their colorful decorations and measurements, then to the kiln for tempering.

Casserole dishes headed to inspection. (Image credit: Casey Barber)
1 / 16
Piles of broken Pyrex glass—which will be melted and re-used to make new Pyrex pieces—shimmer in the sun at the plant in Charleroi, PA. (Image credit: Casey Barber)

The process of thermal tempering is what makes Pyrex so amazingly durable: it compresses the surface and eliminates stress points in the glass to strengthen it, putting a bow on the glass’s ability to withstand heat in the oven, which allows it to go from cold to hot without cracking or exploding, and to bounce back from being dropped on your kitchen floor (most of the time, that is).

The final step is inspection, where each piece is examined by hand to make sure it’s up to Pyrex standards. If any piece has a defect — a chipped edge, an air bubble — it’s sent back to the beginning, turned into cullet, and repurposed as another Pyrex product. Pyrex saves every piece of glass, even if broken, to repurpose it.

After watching the Pyrex production line from start to finish, I am no longer worried in the least about what my Pyrex pieces — both modern and vintage — can withstand.