Following on from the previous blog on thermal shock in glass containers, this article provides a more technical examination of the stresses that develop within glass during rapid temperature changes. In particular, it focuses on the tensile and compressive stress mechanisms that lead to fracture, the characteristic failure patterns observed in bottles and jars, and the material differences that explain why some types of glass are more resistant to thermal shock than others.
This deeper technical context is intended to support engineers, quality professionals, and packaging specialists in understanding not only how thermal shock failures occur, but why they occur, and how glass composition and thermal properties influence performance in real-world filling, processing, and consumer-use conditions.
Glassworks International has been supplying our many UK customers with the highest quality glass packaging, providing outstanding value and dedicated service, nationwide for 25 years.
Stuart Alexander heads up our technical and quality department, providing a wealth of glass production experience and expertise gained over many years. Stuart is always on hand to offer support and advice and to answer any questions you may have.
Glassworks International has been supplying our many UK customers with the highest quality glass packaging, providing outstanding value and dedicated service, nationwide for 25 years.
Stuart Alexander heads up our technical and quality department, providing a wealth of glass production experience and expertise gained over many years. Stuart is always on hand to offer support and advice and to answer any questions you may have.
Thermal shock in glass containers occurs when rapid temperature changes create uneven expansion or contraction within the glass structure. Because glass is a poor thermal conductor, temperature gradients develop through the thickness of the container wall, leading to internal stresses that may exceed the strength of the material.
When hot glass is suddenly exposed to a colder environment—such as in the cooling zones of a pasteurisation process—the outer surface cools rapidly and attempts to contract. This contraction is restrained by the hotter inner glass, which remains expanded. As a result, the outer surface is placed into tensile stress, while compressive stress develops within the interior of the container. Because glass is inherently weak in tension, these surface tensile stresses represent the primary mechanism by which thermal shock failure occurs.
In a hot-fill process, a relatively cool glass container is rapidly exposed to hot product, establishing steep thermal gradients through the glass wall. The inner surface heats first and attempts to expand, but this expansion is constrained by the cooler outer surface and the bulk of the glass. As a result, the inner surface is placed into compressive stress, while the outer surface is driven into tensile stress during the initial heating phase.
At the container base, the thicker and more thermally sluggish glass further restricts axial and radial expansion of the sidewall. This constraint generates bending stresses, particularly in the heel region where the sidewall transitions into the base. The combined thermal and bending effects place the outer surface of the heel region into elevated tensile stress, making it the most susceptible location for hot-fill thermal shock failure.
Because glass exhibits very low tolerance to tensile stress, hot-fill failures most commonly initiate at the outside base corners, where tensile stresses coincide with surface flaws, residual stresses, and geometric discontinuities. Consequently, control of fill temperature, optimisation of base and heel design, and preservation of surface condition are critical factors in mitigating hot-fill thermal shock failures.
Thermal shock failures are typically brittle in nature and occur instantaneously, often without prior visible warning. Common failure modes include vertical cracks along the bottle body, circumferential cracking around the base or shoulder are common fracture patterns. The resulting crack patterns frequently mirror the stress distribution present at the moment of failure, providing valuable diagnostic insight.
To accurately identify the root cause of a failure—whether attributable to thermal shock, excessive internal pressure, or mechanical impact—it is essential to retain the affected containers wherever possible. These samples should be forwarded to the glass supplier for detailed forensic examination by their technical department, enabling a definitive assessment and supporting effective corrective action.
Standard glass used for bottles and jars is typically soda-lime-silica glass. This type of glass has a relatively high coefficient of thermal expansion, meaning it expands and contracts significantly with temperature changes. As a result, it is more susceptible to thermal shock.
Ovenware glass, such as Pyrex, is traditionally made from borosilicate glass. Borosilicate glass has a much lower coefficient of thermal expansion, allowing it to tolerate larger temperature gradients with reduced stress generation. This makes it significantly more resistant to thermal shock compared to standard soda-lime glass.
Although modern consumer Pyrex products in some regions may use tempered soda-lime glass, the thermal shock resistance of borosilicate glass remains superior due to its intrinsic material properties rather than surface strengthening alone.
Glassworks International has been supplying our many UK customers with the highest quality glass packaging, providing outstanding value and dedicated service, nationwide for 25 years.
Stuart Alexander heads up our technical and quality department, providing a wealth of glass production experience and expertise gained over many years. Stuart is always on hand to offer support and advice and to answer any questions you may have.
Glassworks International takes pride in keeping up to date with the latest technologies and trends within glass manufacturing and production.