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Does hot glass look the same as cold glass?

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This article will explain the phenomena in which hot glass looks the same as cold glass.

Hot glass looks the same as cold glass, but it reacts differently to changes in temperature and pressure. An object that appears to be solid at room temperature will become a very viscous liquid if heated enough.

Anyway, the cold glass is always more beautiful than a hot one.

Does hot glass look the same as cold glass?

The optical properties of glass depend on several factors. The variation of these properties with temperature is an important one, but not the only one that affects the appearance of glass objects when they are held in a hand under natural lighting conditions. These variations must be taken into account by designers of glass objects, especially when they are to be viewed under quite different conditions.

The index of refraction is the property that determines how much light will bend as it passes from one medium to another. This property depends on both the wavelength and angle of incidence at which this bending takes place.

Normally, a variation of temperature will not affect these physical properties since the molecular bonds are not affected by temperature changes. However, there is a case when it does: when the glass is supercooled.

Supercooling occurs in all materials when they are cooled below their freezing points, however only in some materials will this affect the index of refraction and cause strange optical phenomena.

In normal conditions (room temperature), glass has a very high index of refraction and causes total internal reflection, which means that light cannot pass through the material because it is reflected back into its original medium.

During supercooling, or the “superfluid” phase of glass, this index of refraction varies with temperature and pressure changes. Therefore when an object made of supercooled glass is held in a hand at certain angles to the light, a change in temperature or pressure can cause the light to refract and pass through the glass.

You may also see this optical phenomenon if you place a transparent plastic rod into hot water. The rod will become milky due to variations in cooling rates inside the rod, which is another case of supercooling. In fact, most materials that do not normally display liquid-like properties will “supercool” if the temperature and the physical conditions change enough.

This effect is also seen in materials like aerogel, low-density silica foam used as a thermal insulator. Aerogel, however, has a higher density than glass and therefore doesn’t show this visible supercooling effect.

Learn the 5 reasons why is glass transparent

Why Does Hot Glass Break in Cold Water?

Unfortunately, this is not an easy question to answer. The answer lies in the high surface tension of supercooled glass and the ability (or lack thereof) for a crack to propagate through it at certain temperatures.

When you place a piece of supercooled glass into cold water, you are decreasing the temperature of the bulk glass and increasing the temperature on the surface. Since heating a piece of supercooled glass will cause it to become normal again, we can infer that there is a point where heating and cooling will cancel each other out and leave you with a state where all molecules are not moving as fast as they normally would at this “crossover temperature”.

Above this crossover temperature, the surface of the glass will become supercooled again. Below it, the molecules in the bulk glass are moving too fast for a crack to propagate through it. This is similar to how water can be easily pulled apart by currents at room temperature but harder to do so when frozen into ice.

How Does Glass React To Heat?

The glass will not melt at room temperature, but it can deform or decompose. The exact composition of the glass determines how much heat energy is required to change the state of the material. For example, ordinary window glass requires about 80 times more heat than cold water alone to melt.

Drinking glasses are slightly different because they contain other chemicals that affect the temperature at which they can be pulled apart by water currents.

Hot glass looks the same as cold glass, but it reacts differently to changes in temperature and pressure. Most importantly, an object that appears to be solid at room temperature will become a very viscous liquid if heated enough.

In which part of the flame should glass tubing be heated?

In order to best work with glass, you should always heat it in the middle of the flame. The outer regions of a gas-burning flame are hotter than the inner parts because that is where carbon dioxide and water vapor are being burned off. This creates an oxygen-rich environment for heating the inside of your glass tubing without burning it. Also, if your glass tubing is still at room temperature or colder, it will need less heat for shaping.

How Does the Heat Affect Glass?

The heat affects the glass by changing its physical state from a solid to a liquid (and back again). This transition occurs below the “glass transition temperature,” which varies with different types of glass and is also affected by pressure and impurities. Unlike most materials, the glass transition temperature is always lower than the melting point.


In conclusion, the hot glass will not look the same as room temperature glass, but it won’t necessarily be visibly different. The surface of the hot tube may remain cooled by the flame, although given enough time in a very hot environment you would see thermal gradients affect the outside of your tubing.


Rayhan Sarwar

I am a glass and ceramic engineering student at the Rajshahi University of Engineering & Technology (RUET). I enjoy exploring science and technology, which is why I chose to study engineering!

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