Defect of the Month

March, 2018
A Bottle’s Worst Nightmare
Silicon balls can be a bottle’s worst nightmare. This type of stone is caused by aluminum contamination in the cullet. When the aluminum reacts with molten glass (mostly silicon dioxide), it exchanges places to create aluminum oxide and elemental silicon. Because silicon has a lower coefficient of thermal expansion than glass, these stones exert a very high stress on the surrounding matrix after the glass cools. They severely weaken the container and can cause it to break due to relatively mild loads later in the filling process. This SEM micrograph shows a damaged silicon ball (often they are found in a perfect spherical condition) that was found at a fracture origin.
February, 2018
Temple Ruins
Looming like the weathered ruins of an Egyptian Temple, this crystal of calcite (CaCO3) rises out of a roughened desert of corroded glass. It is well known that some liquors can attack the inside surfaces of glass containers by leaching sodium and calcium out of the glass. The leached glass is vulnerable to corrosion or delamination, leading to the roughened surface seen in this SEM image. The calcite crystal possibly grew on the roughened surface due to the increased concentration of calcium in the product after leaching.
January, 2018
Damascus Dagger
Damascus steel blades are known for their extreme resilience and swirling two-toned bands. The method by which historical Damascus steel was produced is hotly debated, but high-quality knives made via a similar process are still available for the discerning aficionado. The banded inclusion draped over the finish of this container is also made of iron, but unlike Damascus steel, it would not make a very good sword. Most of the metal has reacted with sulfur in the glass melt to create iron sulfide. The iron sulfide blister was then elongated during the forming process into the blade-like inclusion shown here.
December, 2017
December Snowflake
We hope you enjoy these pristine snowflake-shaped crystals as much as we do. Unlike their meteorological doppelgangers, though, these snowflakes don’t melt – and that’s just the problem. In a glass furnace, erosion from refractory sidewalls creates a viscous zirconia-rich drip that doesn’t dissolve in the glass melt. As the glass moves into cooler parts of the furnace, the zirconia crystallizes into the dendrites that you see here.
November, 2017
Swimming Seahorses
These colorful “sea-horses” are actually crystals of tin oxide embedded on the surface of a piece of flat glass. In float glass production, molten tin on the top of the tin bath can oxidize to form crystals of tin oxide (SnO2), also referred to as cassiterite. The tin oxide can then become embedded in the hot glass ribbon as it floats over the bath. Usually, the tin oxide grows in white clumps of very small nodules, but occasionally larger lath-shaped crystals can form. The crystals’ characteristic “knee-bend” twinning, as well as high birefringence and refractive index, is indicative of tin oxide.