67｜Glass Manufacturing Methods: Forming and Processing (Part 5)

Hello everyone🎵Last weekend, I took some time off and went with my wife to Shima Onsen in Gunma Prefecture. We relaxed and refreshed ourselves at a hot spring inn that has been in business since the Edo period, the setting for "Spirited Away."

Now, glasses can be categorized by their material, such as soda-lime glass or crystal glass. This time, I'd like to talk about borosilicate glass, a heat-resistant material used for beakers and flasks in science experiments, as well as in microwave ovens and regular ovens.

Borosilicate glass (in Japanese kanji, 硼珪酸ガラス, or in English, borosilicate glass) is a type of glass that contains 5% or more boric acid mixed into the glass material and melted, increasing its softening temperature and hardness. It is usually referred to as heat-resistant glass or hard glass. Because it has a low coefficient of thermal expansion, it is more resistant to thermal shock than ordinary glass. It also has excellent heat resistance and chemical resistance, so it is used in laboratory equipment and kitchenware.

Chemically speaking, borosilicate glass has a small coefficient of thermal expansion, so it has a high thermal shock temperature and is resistant to thermal shock and breakage, making it durable. It has heat resistance that allows it to be used without problems even at high temperatures. It also contains a lot of boron oxide (B2O3), making it very chemically durable. It does not react with acids or alkalis and has chemical stability, which maintains the purity of its contents. It does not contain lead or BPA, and chemicals do not leach out, ensuring health safety. Furthermore, it is also called "neutral glass" because it is inherently resistant to water and should be neutral.

Compared to soda-lime glass, borosilicate glass has a higher melting point, which requires more manufacturing equipment and cost. It also has less transparency than soda-lime glass. On the other hand, borosilicate glass begins to soften at 525℃ or higher and begins to flow at 860℃ or higher, allowing for processing like lampworking that is not possible with regular glass.

Thermal expansion, as the name suggests, is the coefficient of how much something expands when its temperature rises. Borosilicate glass is softened by adding boron atoms. As a result, there is less sodium, and the expansion is about one-third that of soda-lime glass. Because glass does not conduct heat well, if there is a temperature difference between the inside and outside of the glass, the glass will distort and break. When you pour hot water into a cold glass and it breaks, it's because the glass itself is still cold, but the inside of the glass expands with the temperature of the hot water. However, since the glass itself is still cold and solidified, distortion occurs between the expanding and contracting forces, causing it to break.

Generally, soda-lime glass is limited to a temperature difference of 40 degrees Celsius by JIS (Japanese Industrial Standards). Glass is strong against heat and temperature because it is manufactured at high temperatures of 1400 degrees Celsius.
However, it is weak against temperature differences. Please avoid pouring hot liquids into cold glasses or putting ice in hot glasses, as there is a risk of breakage.