Defect of the Month

July, 2021
Space Jam
This group of crystals resembles a cosmic traffic jam, but don’t bother looking for basketballs or NBA Superstars. Devitrification (i.e. recrystallization) of the glass melt typically occurs in areas of the furnace that are too cool. Usually, either wollastonite (CaSiO3) or diopside (CaMgSi2O6) crystals are formed depending on whether or not the glass composition contains magnesium; however, both types of stones were present in this piece of float glass. The diopside can be identified by concentric rings of pastel green and pink birefringence colors, while most of the wollastonite crystals display only a single color. Other optical properties, such as extinction angle, can provide more conclusive identification.
February, 2021
Evergreen Forest
While this images evokes an evergreen tree line along a cliff’s edge, the photograph is actually an example of the oxide of zirconium (zirconia), which has melted and re-crystallized to form a stone in a glass container. Zirconia is a key component in a variety of high temperature refractory materials. Typically, these crystals are created through viscous flow of the surface layer of an AZS (alumina, zirconia, silica) refractory located above the flux line in the glass tank. When the melted AZS flows into the molten glass bath, it is actually cooled and recrystallizes into the structures depicted in the photograph. The image was captured with a polarizing microscope at 100 X magnification and with a first order red tint plate in place.
January, 2021
Sea Creatures
The objects in this image resemble a cross between waffle fries and red blood cells, but are actually the silica skeletons of microscopic sea creatures called diatoms. You may be familiar with diatoms as an ingredient in toothpaste or for killing garden pests. But these useful remnants of marine life are also found in glass plants as a furnace insulation material and an oil absorbent. This particular SEM micrograph was taken from a stone composed of diatoms that was pressed into the outside surface of a glass container.
December, 2020
Bubbly Way
When seeds (i.e. bubbles) are created via normal melting processes, they are usually found randomly distributed throughout a container. In contrast, this bubbly streak was formed by reactions between the hot glass and a contaminant, visible as a faint grey band. Investigation into this stone outbreak suggested that the problem was contamination of the internal cullet supply, highlighting the importance of keeping non-glass waste material out of the cullet bins.
November, 2020
Percussion Cone (Bruise)
Percussion Cones also known as Bruises are usually caused by mechanical damage with the source mechanism being a blunt, hard object pressed into the surface under perpendicular directed force. They are strength reducing flaws. As the radius of contact area decreases both the stress level and potential for damage increases. A percussion cone is usually identified by the form of circular ring cracks and crushed glass often on the outside surface near the center of the circle.