Defect of the Month

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.
October, 2017
Galileo’s Saturn
In 1610, Galileo became the first person to observe the rings of Saturn. His small, home-made telescope made it difficult to fully resolve the rings and consequently some of his drawings showed ambiguous lobes extending from a central sphere. Today, we have high resolution optics that allow us to magnify small nearby objects, such as inclusions in glass. This copper stone, with its brownish extensions, resembles Galileo’s early Saturn drawings. Copper stones in glass are caused by cullet contaminated with copper wire or coins.
September, 2017
A River runs through it
The element cobalt is intentionally added to a glass melt in order to produce deep blue-colored glass. Occasionally, cobalt can slip into the glass melt when it is not desired, causing intensely blue streaks like in this wine bottle. One source of cobalt is leftover colorant in the furnace when there is a color change from blue to a lighter color. A second source comes from metallic thermocouple housings that contain a significant percentage of cobalt. As they degrade, cobalt is released into the molten glass to create a blue streak.
August, 2017
Tricky Trilobite
Is this a fossil of a prehistoric trilobite or another glass defect? You’d be correct if you guessed the latter! This microscope image is of a defect called a “bump check.” Bump checks are caused by momentary, glancing contact between containers, producing a characteristic pattern of opposing crescent cracks. Bump checks that occur soon after container formation have tell-tale signs of hot-end contact, such as small pieces of missing or adhered glass.