Defect of the Month

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.
July, 2017
Cassini Flyby
The Cassini-Huygens space probe has been orbiting Saturn since 2004, producing many spectacular images of the planet’s moons and rings. In spite of its resemblance to the latter, this image is not one of them! Instead, the “rings” shown here are actually streaks of cord in a sample of container glass. The cord streaks have a slightly different composition than the surrounding glass, inducing stresses that can be visualized with polarized light. These stresses are permanently locked into the glass, and can combine with other stresses to cause breakage at lower loads than would otherwise be expected.
June, 2017
Klingon Warship
A Klingon Warship or a Y-wing Starfighter? When imaged with a Scanning Electron Microscope (SEM), this stone is evocative of spacecraft from popular science fiction franchises. In reality, the vaguely menacing profile was created by slicing a partially-formed hexagonal crystal at an oblique angle. Although SEM images themselves are black-and-white, colors can be assigned to different elements in a spectroscopic method called “compositional mapping.” These colors reveal that the stone was created by an AZS refractory that recrystallized into prisms of nepheline. Our new SEM makes it possible to routinely use compositional mapping to help identify stones. If you have stones in your glass, send them in to AGR via ground, air, or space for analysis.