Is tempered glass cups toxic?

Glass itself is non-toxic because it is a high-temperature sintered product of silicate, and the cleaning of glass products mainly involves removing pollutants from the surface of the glass. But ordinary glass is fragile and has sharp edges and corners, which can easily harm people. Therefore, after softening at high temperatures, it suddenly cools down and becomes tempered glass. Tempered glass, even if broken, has very small fragments, no sharp edges, and is not easy to harm people. Therefore, it is widely used in car and ship glass windows, as well as floor to ceiling door and window glass for home decoration. The same applies to making baby bottles.

Why does a tempered glass cup burst?

Tempered glass has good compressive and thermal stability, so why does it also crack?

The raw material for tempered glass cups is also ordinary glass, which is first used to make thicker glass cups (wall thickness 3.4-3.8 millimeters), and then heated to 650 ° C in the furnace. At this point, the glass has softened, but has not yet deformed. Quickly pour in oil to cool it sharply. This process is called "tempering", also known as quenching. Just like steel makes fire, it makes glass hard and sturdy.

Glass is a poor conductor of heat. When the glass cup suddenly cools down, the surface layer first cools, hardens, and contracts, while the interior of the glass is still in a highly thermoplastic state with an outward expansion force. So the tempered glass cup made of it has compressive stress on the surface and tensile stress inside. In general, these two should be balanced across all parts of the cup body, and there will be no problems during use.

However, in some cases, due to improper quenching, the cup body may experience uneven stress, which may cause it to explode without reason. For example, if the internal stress in a certain part of the cup body is greater than the external stress, the force exerted from the inside is very large, while the pressure exerted from the outside is very small. Although it appears that there is no problem with this cup on the surface, it is actually tense inside. For example, the glass panel on the desk may occasionally explode, just like tempered glass cups. These thick glass products are not quenched afterwards, but during the process of transitioning from liquid to solid, there is also uneven stress due to the surface first cooling and hardening, followed by internal cooling and hardening.

The principle is actually very simple. After the boiling water enters the cup, the cup expands due to heat. Due to the relatively poor thermal conductivity of the glass, the inner layer of glass expands, but the outer layer has not yet received heat. Therefore, the expansion of the outer wall of the glass lags behind, and the pressure on the outer wall suddenly increases in an instant. The pressure exceeds the strength of the cup, leading to cracking

Why don't you fry it when you put in a small iron spoon?

There are two main reasons: firstly, water is poured onto the spoon first, which absorbs some of the heat, indirectly reducing the thermal expansion coefficient of the glass. Secondly, adding an iron spoon is equivalent to preheating the cup in advance, reducing the suddenness of heating, so the cup does not explode. So when both the inside and outside of the cup are heated at the same time, and hot water is poured in, the degree of expansion between the inside and outside of the cup is not that different, and it will not burst.