Not solid and not liquid: a researcher from PNIPU shared fascinating facts about glass.

Fact 1: Glass is neither solid nor liquid

Unlike solid materials such as salt or quartz, glass lacks an ordered crystalline structure. Its atoms are arranged randomly, similar to a liquid. However, the material has an extremely high viscosity, which means its molecules move very slowly in relation to one another.

“Yes, the material possesses some liquid-like properties, but its low fluidity classifies it as a solid. Referring to glass as a liquid is incorrect. A more accurate term would be ‘amorphous solid’ or ‘glass-like state,’” explains Regina Larionova, a senior lecturer at the Department of Civil Engineering and Materials Science at PNIUP.

Fact 2: Glass does not conduct electricity

Solid glass is considered a dielectric – a substance that lacks free charges, as all are bound to neutral molecules.

“For electrical conductivity, mobile charge carriers, or ions, are needed. However, such carriers are absent in solidified materials. Nonetheless, the material can accumulate an electric charge. In contrast, molten glass conducts electricity easily, as its molecules can move freely,” the expert clarifies.

Fact 3: Volcanic lava is the precursor to glass

Both the well-known porous pumice and semi-precious obsidian are forms of solidified lava, or volcanic glass. Like liquids, they lack the ordered atomic arrangements characteristic of crystalline substances. This structural characteristic allows natural glasses, particularly obsidian, to form thin, sharp edges upon impact, making them easy to work with.

“Obsidian and the dragon glass from ‘Game of Thrones’ are among the first inorganic materials mastered by humans. The ancestor of silicate glass is rapidly solidified volcanic lava in an amorphous form. The main components of obsidian are quartz (25–35 percent) and feldspar (60–65). Additionally, it contains mica (5–10), hornblende, and biotite (5–10). The hardness of obsidian on the Mohs scale is 5 out of 10. In comparison, the hardness of platinum and steel ranges from 4 to 4.5,” Regina Larionova shares.

Fact 4: Ancient people used volcanic glass for crafting

According to the expert, if we consider the use of obsidian historically, for instance, during the Paleolithic period, it was crafted into ritual blades, piercing and cutting tools: scrapers, knives, arrowheads, and spear tips. There are also records of its use in jewelry and decorative sculpture. During that time, obsidian was extracted using abrasives (wet sand, wet corundum) and bow drills.

Today, due to its high silica content and low viscosity, this material is utilized in the production of fiber optic cables. In the industry, it is used as an additive in Portland cement – a mixture for various types of concrete, construction mortars, and dry building mixes for erecting concrete and reinforced concrete structures. In medicine, obsidian has found application in the production of scalpels.

Furthermore, the stone is used to create jewelry and decorative elements.

Fact 5: Glass cullet is used to create modern glass

Glass consists of a molecular network of silicon dioxide, alternating silicon and oxygen.

“Currently, glass is made from natural raw materials, including silicon dioxide, limestone, and calcined soda. These are melted at a temperature of 1728 degrees Celsius. To achieve good light transmission properties, the surface of the material is meticulously polished, and impurities are minimized, while also reducing the number of cavities during the melting process,” explains Regina Larionova, a senior lecturer at the Department of Civil Engineering and Materials Science at PNIUP.

Depending on the intended purpose of the glass, dolomite, limestone, cullet, and other chemicals may be added during its production. For example, dolomite powder enhances transparency, reduces the expansion coefficient, lowers the working temperature during molding, and ensures resistance to chemical reagents.

The role of limestone is to introduce calcium oxide, which contributes to the chemical durability and longevity of the material. The use of cullet reduces energy consumption and carbon dioxide emissions during production. Soda is added to lower the heating degree of the material and achieve its unique fluidity.

Fact 6: Not all glass can be recycled

According to the expert, there are several types of glass that cannot be recycled or reused due to their specific composition and structure.

Glass with a high lead oxide content

This type of glass is used to create crystal and is known for its transparency and brilliance. However, due to the release of hazardous gases when attempting to remelt it, it cannot be recycled and is disposed of as household waste.

Photo-sensitive or photoresistive glass

This type is used in electronics for creating integrated circuits and as a layer on films or CDs/DVDs/Blu-rays. However, it contains silver elements, making it unsuitable for recycling.

Ceramic glass

This material contains zirconium oxide and other metals. It is used to produce kitchenware such as heat-resistant pans, baking trays, or plates. This glass cannot be remelted due to the high number of additives.