Can I use my Pyrex® jug to make lye?

Lye, as we know, is extremely alkaline, with a pH of 14. Most materials cannot withstand such caustic liquids, causing them to leach or erode. In addition, lye can also get very hot. So, in short, the container used for making lye must be heat resistant, able to withstand high alkaline solutions, and preferably be made of sturdy material that doesn’t break easily.

Let’s look at glass.

Glass comes in many different compositions. The most common form is soda lime glass, dating back to at least 3600 BC, which makes up about 90% of all glass used in the world, including window panes, kitchenware, and lighting products. It is the so-called “original” or “true” glass, and is made by mixing sodium oxide (soda) with calcium oxide (lime).

Soda lime glass is the most inexpensive and easiest glass type to mass produce, but it is a very soft glass with a low melting point. It also has a very low thermal shock resistance, which means that it cracks and breaks easily, and chemicals will leach the glass over time.

There is, however, a way to make soda lime glass stronger by tempering. This is when glass is heated up to a high temperature and then rapidly cooled down. This produces a hard glass that is three times as strong as non-tempered glass, and instead of breaking into jagged shards, it crumbles into small unsharp pieces when it breaks.

This can be a desired feature, for example, in car windscreens or glass tabletops, but at the same time it can also pose a risk in other situations. To break tempered glass it does require a hard enough impact, but if the glass is damaged, it is only a matter of time until the glass will shatter completely. This is why law requires car windscreens to be replaced when there’s even only a small crack.

What about borosilicate?

Borosilicate, a very strong, hard glass, on the other hand, doesn’t shatter. It is an engineered glass, specifically developed for use in laboratories. The main components of borosilicate are is SiO2 (silicone dioxide) and boron. It is chemically resistant and thermally stable up to at least 450C.

Due to its high thermal shock resistance and stability, borosilicate glass wasn’t just used in laboratories, it also became popular in households, because you could cook with it.

Casserole dishes, pudding forms, baking moulds, and even pans were made of borosilicate. And this is where we come to Pyrex.

Pyrex®

Introduced as a brand in 1915, Pyrex specialised in glass kitchenware and bakeware, in particular heatproof glassware. It is probably the most popular and best-known brand of heat-proof glass kitchenware worldwide.

Although some people believe the name is made up from the Greek “pyr”, meaning fire, and Latin “rex”, meaning king, it is more likely the name is just arbitrary.

Pyrex jugs were known for their solid, durable and long-lasting properties. A reputation that was well deserved, as they rarely broke and made the purchase worthwhile, despite the high price tag.

The reason for Pyrex’s durability? Borosilicate.

Things change in 1998

Pyrex jugs were made of borosilicate glass. Well, they were until 1998, at least. Then the company split up and Pyrex, the brand, was licensed to various other companies.

Corelle Brands, which acquired the US production, changed the composition to a tempered soda lime glass. It was a quiet move, and initially no one suspected anything. But suddenly more and more reports about Pyrex jugs exploding popped up.

There is quite a debate around this. Corelle Brands stated that they have been producing both borosilicate glass and soda lime glass cookware long before 1998, and contended that most of the reported incidents are due to unsafe practices and not following their usage guidelines.

A study reviewing the incidents warned that soda lime silicate cookware was not adequate for household cooking, and a lawsuit followed in which the company sued the publisher of the article (Corelle lost).

In 2018 another class action was filed against Corelle Brands, which is still ongoing. All this concerns the US manufactured Pyrex products, which supplies North and South America and Asia.

In Europe, the Middle East and Africa, Arc International acquired the rights to the brand, and they are still committed to using borosilicate in their products.

What about New Zealand?

Here in New Zealand, our Pyrex products are manufactured in the US. So this means, unfortunately, that our beloved Pyrex jugs are likely made of tempered soda lime glass, especially if purchased after 1998.

However, even if you have a jug that you found on the flea market or inherited from your great aunt, there’s no guarantee that it is made of borosilicate.

The answer in PP5 plastic

Therefore, as much as I love Pyrex jugs, put them back into your kitchen cupboard and invest in a cheaper PP (polypropylene) plastic container or jug to mix up your lye solution. PP plastic, with the recycling code 5, is the only plastic that is both heat proof and chemical resistant, and with the bonus that you don’t have to worry about shattering glass.

In short:

Use a plastic container, not glass, to mix your lye and check that your container has PP5 stamped on it. The 5 is usually in a recycling triangle with PP just below the triangle.

This post was first published in the New Zealand Soap Makers’ Association Newsletter May 2020.

Update 21/08/20:
I still use my Pyrex jugs, because I’m an oldie soap maker, and this is how I’ve done it all these years. But I am very aware of the possible risks associated with it. However, if you are starting out and looking for a container to mix your lye in, I would strongly suggest to buy a PP plastic jug, available from Kmart for $2 in NZ, or use an empty 1 kg yoghurt pot, because they’re usually made from PP plastic too.

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