Can you put glass in the microwave safely? Well, turns out it depends. As a general rule, glass is microwave safe. After all, many microwaves have a rotating plate inside the heating area and this is often made from glass.
But, even if your microwave comes with a glass plate, it's important to check that your actual dishes are microwave safe before heating them. This is because it could break or shatter if it cannot go in the microwave. It could also melt or heat up so dramatically that it burns your hand.
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What is glass, really?
Glass is made from sand. The main component is silicon dioxide which melts at around 3090 degrees Fahrenheit. This changes the state of it and turns the sand into a liquid.
As it cools, it becomes a crystalline structure and is known as an amorphous solid. This means that it never fully solidifies and has instead become a kind of suspended liquid.
Glass is a relatively heat resistant and non-reactive material. It is transparent and can be shaped into whatever you like when heated until melting point.
Glass is reusable many times over, and is unlikely to release chemicals into your food like plastics and metals are prone to.
Glass is incredibly strong but can shatter easily if dropped or exposed to large temperature changes.
What are the types of glass?
Natural
This tends to be relatively thick glass and can be microwaved.
It is unlikely to be affected by temperature changes.
Synthetic
This is glass that has been manufactured in laboratories specifically for use in industry, where the demands are much higher. They can withstand hazardous chemicals and extreme temperatures.
There are 2 main types of synthetic glass: borosilicate and soda-lime. Borosilicate is made of around 15% boron trioxide. This helps to ensure your glass can hold up to temperature changes. This property is known as thermal shock resistance.
Soda-lime glass is very cheap to manufacture and easy to find, making up 90% of all synthetic glass. This is used to create everything from windows to wine glasses. It expands twice as fast as borosilicate when heated, making it much more susceptible to breakages.
Tempered
Tempered glass has been heated and rapidly cooled during the creation process. This causes the glass to have a different structure to untempered glass, which comes with benefits.
Normally, glass shatters into large and sharp pieces that can easily cause injury. Tempered glass breaks into even, smaller fragments and has much less sharp edges.
This makes it a much safer option for using around the home. It can also be referred to as safety glass. It is heat, shock, and scratch-resistant.
How do you know if something is microwave safe?
The easiest way to tell is to check the base of the container. Most containers will have an indicator on the underside as to whether or not they are suitable for use in the microwave.
If there is no engraving or marking to indicate either way there is a simple test you can perform.
Turn your microwave to high power - the default is often the highest setting anyway. Pour one cup of water into a glass measuring jug and place this in the microwave with the dish you wish to test.
Do not pour the water into your dish, place the measuring jug on or inside the dish instead. Microwave for one minute.
If your cup of water is hot and your dish cool, it is microwave safe. If the dish becomes hot, it is not suitable for microwaving. If you put the measuring jug on the dish, the section where it was sat may heat up but the rest should be cool.
Thin glass is much more likely to shatter, so where possible try to only use thicker glass in the microwave.
What cannot be microwaved?
Drinking glasses are less likely to be suitable for use in the microwave than dishes and cookware.
If your glass dish has a metal (i.e. gold or silver) rim, this should not be microwaved.
What could happen if it is not microwave safe?
If you microwave non-microwavable glass, the glass could shatter. This is because inside the glass structure there are many tiny air bubbles. These will expand as they heat up and could result in the glass shattering.
The metal-rimmed glass will spark during microwaving and again, could result in the glass shattering. In extreme cases, this could even result in fire.
Some china is made from a mixture of glass and ceramic. If these are heated and then cooled rapidly they can crack and shatter. This could be through something as simple as placing the hot dish on a cold countertop or by running it under cold water. The rapid temperature change puts the structure under a lot of pressure and this is why it is so prone to shattering. Sometimes dramatically.
Microwave glass safety tips
It is important to ensure frozen food that has been stored in a glass container is thawed completely before microwaving. This is to reduce the thermal changes the glass endures, making it less likely to shatter.
We also recommend wiping any condensation off of the glass prior to microwaving.
If you do wish to thaw from frozen in the microwave, it is vital to only use the ‘defrost’ or low power setting.
Take great care when removing cookware and dishes from the microwave. If they have been in there for an extended period, they could be very hot. This can cause you to develop burns and could lead to other health hazards.
Summary
You can microwave glass as a general rule, however, it is always best to check before you use it. This is usually clearly marked with words or a symbol on the underside.
If not, follow our tips above to check easily whether your glassware can be microwaved.
Never microwave glassware decorated with gold, silver, or other metal rims, as this could cause sparks to form.
Thicker glass holds up better to microwaving than thinner glass and is the preferable option.
Synthetic glass is often more heat-resistant than natural glass forms.
