Why glass kettles shatter and how to prevent it

Glass kettles have several advantages. Unlike plastic or metal, glass makes it easier to spot limescale build-up. It's free from chemicals such as BPA, an industrial chemical with possible harmful health effects, and it won't taint the smell or taste of your drink.

However, we’ve heard feedback from our members and read online comments about glass kettles shattering unexpectedly.

Some comments even express concerns about glass kettles suddenly exploding, but we haven't found any evidence of this in kettles purchased from well-known brands.

Having a glass kettle break while you're using it can be distressing, but what might cause this? Read on to find out why and how glass kettles shatter and get tips to help prevent it from happening to you.

Discover the best kettles we’ve tested and get tips on choosing the one for you or see all our kettle reviews.

What might cause a glass kettle to shatter?

Glass kettles aren’t as durable as plastic or stainless-steel ones, and can chip or crack if you accidentally knock them.

We’ve found that the issue isn't specific to any one brand, although it first came to light when a Which? member reported that it happened with their Russell Hobbs Classic glass kettle. 

In their letter, our member wrote: ‘It worked well for a few weeks, until a crack appeared in the spout. It continued to function until one day, the glass jug disintegrated in my daughter’s hand.’

We’ve tested this particular kettle in our lab and didn’t find any defects, but we have seen online that some owners have reported theirs cracking or shattering, even when they're not in use.

We’ve also received feedback from other Which? members about the same issue. One member wrote: ‘It’s remarkably fragile, and a knock on it with the tap while filling it broke the glass enough to render it unusable.’

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What the manufacturer said

When we asked Russell Hobbs about the first incident mentioned, they told us:

‘It is common for glass kettles to break around the rim when the kettle has been filled via the spout. This can happen if the spout has been in contact with the tap and can result in fractures within the glass that are not always noticeable. When the water inside the kettle heats up when it’s switched on, these cracks can spread, which could potentially cause the kettle to break.

'Within our instruction manuals, we do stress that the user should remove or lift the lid for filling, thus removing the potential for the glass to get damaged.'

Other glass-kettle manufacturers also include safety warnings in their instruction manuals about issues such as this. The Sage Tea Maker includes this safety warning in its instruction manual: 

‘The glass jug has been specially treated to make it stronger, more durable and safer than ordinary glass, however, it is not unbreakable. If dropped or struck extremely hard, it may break or weaken, and could at a later time shatter into many small pieces without apparent cause.’

Of the 300+ online Amazon reviews for the Sage Tea Maker, a few mention broken glass, but these instances occurred after using the product for more than three years or when cleaning it.

To see all our Best Buy, Eco Buy, and Great Value recommendations, head to our kettle reviews.

Aren't glass kettles made from toughened glass?

Yes, kettles made by well-known brands are more likely to be made from either tempered or borosilicate glass. These are heat-resistant and capable of handling high temperatures of up to 170°C. They are also more resistant to mechanical stress, so are less likely to shatter than ordinary glass.

• Tempered glass is soda-lime glass that has been heat-tempered. This material has a higher mechanical stress threshold than borosilicate glass, so it can take more physical damage before it breaks. It’s also much cheaper and more environmentally friendly because it is 100% recyclable.

• Borosilicate glass is a type of glass that contains silica and boron trioxide in high concentrations. It has a very low coefficient of thermal expansion, which means it won't crack under extreme temperature changes and is more resistant to thermal shock than tempered glass. Thermal shock is the sudden extreme changes in temperature, which is what happens when you pour hot water into a cold glass, causing it to crack.

As well as using tougher materials like these, manufacturers are also obliged to carry out rigorous safety tests to meet legal requirements designed to keep us safe while using their products.

Read our article on common kettle and toaster faults and how to fix them.

How to avoid damaging a glass kettle

Despite being made from tougher materials, glass kettles are still less durable than those made of plastic or stainless steel, and extra care needs to be taken when using and cleaning them.

These quick tips should help to prevent accidental damage.

• Clean gently to avoid scratches Avoid using scouring pads, scrubbing brushes or scourers – these can leave small scratches on the glass. Household acids such as white vinegar and lemon juice are gentler than all-purpose descalers. If these don’t lift all the limescale, a microfibre cloth or a soft brush will help finish the job.

• Handle with extra care A knock on the body of a glass kettle, or even putting it down with force, can be enough to cause tiny cracks to appear. This can speed up wear and tear, and, over time, may lead to a hazardous incident such as the kettle shattering while in use.

• Avoid sudden temperature changes You might empty your kettle and then realise that you need more hot water. The best practice is to refill with lukewarm water or to wait for the kettle to cool a little before adding cold water.

Read how to descale a kettle to keep your morning tea limescale-free.