The sound of breaking glass is unmistakable. Once you hear it for the first time, you instinctively store that noise in your arsenal of dangerous things, like a tornado siren, a fire alarm, or gunfire. That reaction helps to keep you safe. Fortunately, it’s not always life or death when you hear this particular warning noise. Sometimes it means that a dish got dropped in the sink. Yet even when you’re very careful, accidentally heating glass can cause it to shatter.
What causes this problem? After all, you can melt glass to blow and shape it. Shatterproof glass made for baking can break too when heated unevenly. It doesn’t make sense at first glance. Luckily, I interned with a glassblower some years back and had the opportunity to learn all about how glass works. I’ll explain the thermal properties and demystify the issue completely, so you never need to wonder again.
Why does glass break when heated? Glass breaks when heated because of thermal shock. When you heat or cool glass, the whole piece changes temperature. However, by simply heating the outside layer, or a single spot rapidly, you are causing uneven expansion. As a result of this thermal problem, the smaller glass around it cracks to relieve the pressure built up from some of the glass trying to expand.
How Glass Reacts to Heat
Glass is heat resistant, at least by human standards. It takes a lot to break evenly heated glass, and you’re more likely to melt it if the temps get high enough. Even special heat-resistant glass can suffer a dynamic failure if part of it is cool.
Unfortunately, most of the time, when the heat is applied to glass, it’s anything but even. When you place a baking dish in the oven, the contained warmth gives the glass an all-over warmth. However, putting the same dish on a hot surface, especially when it’s already cool creates a problem as part of the dish gets much hotter much faster, causing uneven stress.
At What Temperature Does Glass Break
Unfortunately, there is no single answer to the question of how hot glass has to be to break. Glass comes in different thicknesses and types. Heat tempered glass like your pyrex baking dish is a lot more heat resistant than a float glass window pane in a house. Moreover, bulletproof glass and other similar variations can all affect the breaking point.
In general, float glass begins to crack around 150 – 200ºC. That’s about three hundred and two degrees Fahrenheit at a minimum. However, if that glass is cooled on the outside, as it would be in the winter, then those temps might be much lower. Likewise, a single crack is not the same as a full shattering effect.
What Temperature Melts Glass
Just as the question of breaking glass, the temperature that will melt glass varies. A good campfire with reasonable airflow can get hot enough to melt a beer bottle. A lot of glass is SiO2 mixed with carbonates and oxides to help lower the melting point, and affect various unrelated aspects of the final outcome. Resultantly, some glass melts at much lower temperatures.
However, in a controlled environment, such as a kiln, working with professional glass like that used by glassblowers, you need temperatures of thirteen to fifteen hundred degrees to make a puddle of glass. That’s hotter than most campfires ever get. Plus, a kiln has a heating element in the lid to help with even heating and prevent cracking and breaking of the pieces inside.
Notably, it is difficult to melt ‘just part’ of the glass without heating the whole piece. Although it is doable, usually this happens either by chance or in the hands of a trained professional. I don’t recommend trying to ‘figure it out’ by using a torch or similar heat source since this can cause the glass to crack or shatter, leaving you with extremely sharp glass fragments.
How to Heat Glass Without Breaking It
Heating glass without breaking it is fairly simple. It would help if you made sure the entire glass structure warms evenly. Similarly, cooling needs to be approached holistically. Never place cold glass on a hot surface or a glass s container, which is hot into a cold area like the refrigerator.
By simply paying attention and making a few adjustments, you can save a lot of wasted glass dishes. For example, let’s say you’re in a survival situation where you have no bowls or other dishes, but you’ve got a set of glass beer steins to serve soup. You could eat from the container you cooked in if you’re alone, but if you need to serve a family or group, you have a problem.
Especially if it’s wintertime and those glasses are cold, you could end up wasting food. Instead, heat the glasses slowly and evenly. You can even put them inside a soup pot if you do so before you begin cooking. By heating the glasses, you can easily use them to contain boiling soup. Plus, it’s easy enough to wipe them off and wrap some fabric like a mitten around the handles.
Cooling that glass down properly is also important. Setting the hot cups in snow, or even on cold rocks and ground can cause cracking and breaking. Instead, wrap the glass dishes in some fabric and allow them to cool more slowly.
Can You Pour Hot Things Into Glass
We’ve all seen someone pour boiling water into a glass container. Often, they don’t break. Some people swear by the metal spoon method, where you place a metal spoon inside to absorb some of the heat. However, that’s not an effective way to save the glass.
What happens when it doesn’t break has more to do with thin glass heating more evenly, or the liquid being below the necessary temperature to cause a failure in that particular container. However, it could also be causing micro-fractures, which will lead to an easier break down the line. In short, do not pour hot things into cold or even room temperature glass. Take the time to heat up your container or vessel, and you will have that dish a lot longer.
Why Does Glass Break When Cooled
Hot glass is more likely to break when exposed to cool temperatures. This is why glass is always cooled inside a kiln to reduce the temperature slowly. Because glass isn’t typically thin enough to cool safely and quickly, you need to be careful with hot glass.
Sure, your glass baking dishes do fine on the counter (most of the time), but non-heat resistant glass or thicker glass is much more likely to break. When the glass is hotter on the inside than it is on the outside, the contraction on the surface causes stress fractures. Essentially, the glass is forced to relieve pressure. Hence, it cracks or shatters.
Never take hot glass and place it on a cold surface. Unless your goal is to intentionally cause cracking, then you’ll probably get an undesirable result. More importantly, you can get glass explosions, which occur when you rapidly chill the heated glass.
In a survival situation, this knowledge can be applied to your benefit. Either by heating a small section of glass when you need to break it for removal or by using the thermal properties of glass as a distraction when you are in danger.
For example, placing a road flare against a pane of glass and running like your heels are on fire can cause pursuers to look elsewhere for you. Like you, they know what that sound indicates, and it’s usually trouble. Use that information to your advantage when SHTF.
Thermal Fracturing
Glass breaks from heat or cold because of the thermal differential. Heat causes the glass to expand, and cold causes it to contract. When these two forces are in opposition, it causes thermal fracturing.
Intriguingly sloped surfaces are more likely to break because they heat less every than their flat companions. This is why a cup or baking dish in the kitchen is more often the victim of thermal breakage than a window in your home.
Final Thoughts
No one wants to waste time and money on broken glass because they heated it up unevenly. Undoubtedly, you can hurt yourself or someone else, but you also have to spend your precious resources to handle the breakage. From sweeping time to replacement costs, it’s better to simply take the extra time to warm and cool glass correctly.
There are many situations where the ability to quickly and easily break glass can be a serious boon. No one wants to be trapped inside a building, but you can escape if you have a window and a fire starter. Similarly, you can break glass to make a quick cutting surface in an emergency.
The sound of breaking glass isn’t always a bad thing. As long as you’re the one breaking it, you can use heat to crack glass for survival as well.
