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Guide
TL;DR
Pyrex is a brand name, not a type of glass. Over the years it has been used for both borosilicate and tempered soda-lime glass. Borosilicate expands roughly one third as much as soda-lime when it heats up, so it handles quick temperature changes better. When you buy a glass toy, look for a seller that names the material plainly.
"Glass" covers many different materials. They look alike on a shelf. They behave differently when they get warm, get cold, or get knocked.
For a toy, that difference matters. A glass toy gets washed again and again. Many people warm or cool it under the tap before use. A material that copes well with temperature change is a better match for that kind of routine.
The three names you will see most often are borosilicate, soda-lime and Pyrex. Two of them are types of glass. One is a brand.
Pyrex is a brand, not a glass type. Corning introduced it in the early 1900s as a low-expansion borosilicate glass. The brand has been used for kitchenware since 1915.
Over its long history, many different glass formulas have been sold under the Pyrex name. That includes borosilicate and heat-strengthened, or tempered, soda-lime glass.
Here is how it breaks down today:
US kitchen Pyrex: consumer bakeware sold in the United States is made of heat-strengthened soda-lime glass, by the brand's US licensee.
Kitchen Pyrex in Europe, Africa and the Middle East: made from borosilicate glass under a separate license.
Laboratory PYREX: lab glassware from Corning is borosilicate. Corning itself stopped making consumer cookware in 1998.
You may have heard that capital letters on the logo mean one glass and lowercase means another. That is not a reliable way to tell.
So the word "Pyrex" does not, on its own, tell you the glass is borosilicate. A baking dish from a US store is not the same material as lab glass. Neither kitchen glass nor lab glass is made to be used as a toy. Please do not use them that way.
Borosilicate is made by adding boron to the sand-based glass mix. That change gives it low thermal expansion. Standard lab borosilicate expands about 3.3 x 10^-6 per kelvin.
Soda-lime is the everyday glass of bottles, jars and windows. Ordinary soda-lime expands roughly 8.6 to 9 x 10^-6 per kelvin. That is about 2.5 to 3 times as much as borosilicate.
Why expansion matters: when glass changes temperature quickly, some parts expand or shrink faster than others. That creates stress inside the glass. The less a glass expands, the less stress builds up. This is why borosilicate resists thermal shock better than soda-lime.
Tempered soda-lime is soda-lime that has been heat-strengthened. It is tougher against impact than borosilicate. But it handles sudden temperature change worse.
None of this makes soda-lime bad glass. It is cheaper to make and works well for everyday things. It is simply a poorer match for something that gets warmed, cooled and washed over and over.
| Property | Borosilicate | Tempered soda-lime (typical US Pyrex bakeware) | Plain soda-lime |
|---|---|---|---|
| Thermal expansion | About 3.3 x 10^-6 per kelvin | Soda-lime base glass, roughly 8.6 to 9 x 10^-6 per kelvin | About 9 x 10^-6 per kelvin |
| Thermal shock | Resists it better than soda-lime | Handles sudden change worse than borosilicate | Handles sudden change worse than borosilicate |
| Impact | Less tough than tempered soda-lime | Tougher than borosilicate | Not heat-strengthened |
| Typical uses | Lab glassware, heat-resistant cookware | US consumer bakeware | Bottles, jars, window glass |
You cannot tell borosilicate from soda-lime just by looking. Clear glass looks like clear glass. The seller's description is what you have to go on.
What to look for:
The material is named plainly for every product. "Borosilicate" is a clear answer.
Vague words such as "glass" on its own or "Pyrex-style" do not tell you the material.
If a listing is unclear, ask the seller directly before you buy.
Every piece on buttglass.com is borosilicate glass, made as a single solid piece, and non-porous. It is the glass professional labs and heat-resistant cookware are made from, because it resists thermal shock. Our FAQ covers materials and ordering.
Care is simple. Each piece washes with soap and water and works with any lube. You can use tap water, the fridge or ice water. Even so, it makes sense to change the temperature gently rather than going from very hot to very cold. Our guide on how to clean glass toys walks through the steps. If you are new to glass, start with our beginner pieces.
If you have any health concern about using a toy, talk to a doctor.
Q: Is Pyrex the same as borosilicate?
A: Not always. Pyrex is a brand that has been used for both borosilicate and tempered soda-lime glass. US kitchen Pyrex today is soda-lime.
Q: Can I tell if a glass toy is borosilicate by looking at it?
A: No. The two glasses look the same. Rely on a seller that states the material clearly, and ask if it does not.
Q: Can I test glass at home with heat?
A: Please do not heat glass to test it. Ask the seller about the material instead.
Q: Why not use a glass item from the kitchen?
A: Kitchen and lab glass are not made to be used as toys. Do not use them that way.
