Glass is a rigid, non-crystalline solid used widely in everything from windows to cookware. While robust, the material is notoriously susceptible to sudden temperature changes, which often results in shattering. Understanding why glass fails when rapidly heated requires examining its unique atomic structure and the physical laws that govern how materials respond to thermal energy.
The Amorphous Structure of Glass
Glass is classified as an amorphous solid, meaning its internal structure lacks the uniform, ordered arrangement of atoms found in crystalline solids like quartz. Instead, the atoms are randomly networked together, giving the material a disordered structure similar to a frozen liquid. This disordered structure influences how heat energy moves through the material.
The amorphous nature of glass makes it a comparatively poor conductor of heat, possessing a thermal conductivity orders of magnitude lower than metals. For common soda-lime glass, this value is typically around $0.8$ to $1.0\ \text{W}/(\text{m}\cdot \text{K})$. This low conductivity impedes the rapid transfer of thermal energy, allowing uneven heating to occur. When heat is applied to one surface, it is absorbed locally, and the slow movement of energy through the bulk creates a sharp temperature gradient.
Thermal Expansion: A Universal Principle
Materials change size when heated due to thermal expansion. As temperature increases, the kinetic energy of the constituent atoms rises, causing them to vibrate more intensely. This increased vibration forces the atoms further apart, which manifests as an increase in the material’s overall volume.
This change in size is proportional to the temperature change, a relationship quantified by the material’s coefficient of thermal expansion. Conversely, when a material cools, the atoms vibrate less vigorously and move closer together, causing the object to contract.
Differential Stress and Thermal Shock
The combination of glass’s poor heat conductivity and thermal expansion causes it to shatter. When cold glass encounters rapid heating, the surface layer heats up instantly and attempts to expand. The inner layers remain cold because the material conducts heat very slowly.
This uneven temperature distribution creates internal resistance known as differential stress. The hot outer layer, trying to expand, is rigidly held in place by the cold, unexpanded interior. This generates tensile stress on the hot surface, stretching it beyond its limit. Glass is strong under compression but weak when subjected to tensile forces.
The stress generated can be around $0.63\ \text{MPa}$ for every $1\ ^{\circ}\text{C}$ of temperature difference between the surfaces. When this tensile force exceeds the material’s inherent strength, the glass fails abruptly in an event called thermal shock, characterized by a rapid fracture originating from the surface.
Engineering Solutions for Thermal Resistance
Manufacturers employ engineering methods to reduce a glass object’s susceptibility to failure under thermal stress.
Thermal Tempering
Thermal tempering intentionally creates balanced internal stresses. The glass is heated to a high temperature and then rapidly cooled with forced air drafts, causing the outer surfaces to solidify and contract before the interior. This process locks the outer surface into a state of permanent compression. This compression must be overcome by an external tensile force before the glass can break.
Chemical Composition Alteration
Another approach alters the chemical composition of the glass to minimize thermal expansion. Standard soda-lime glass has a high coefficient of thermal expansion, causing it to change size significantly under temperature changes. Borosilicate glass, often sold under trade names like Pyrex, incorporates boron trioxide into its structure. This drastically lowers the coefficient of thermal expansion to values as low as $3.3 \times 10^{-6}\ \text{K}^{-1}$. This minimal size change reduces the differential stress created during rapid heating, allowing borosilicate cookware to withstand large temperature differentials without fracturing.