Defect of the Month

September, 2021
Lightning in a bottle
While this might look like lightning in a bottle, it is actually cord stress. Cord is generally invisible to the naked eye but can be viewed in polarized light. Cord is “an attenuated glassy inclusion possessing optical and other properties differing from those of the surrounding glass” (ASTM C162). Or stated differently, these colorful swirls are subtle differences in glass composition. It is can be caused by batching errors, problems with melting, furnace upsets, refractory erosion or insufficient mixing. High stress, tensile cord located on the exterior surface has greatest chance of causing a performance problem.
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.
October, 2020
Flying Bug
This summer in Western Pennsylvania we have had to contend with mosquitos, flies, bees, hornets, and now these flying bugs. The bug-like inclusion shown here is a common crystalline devitrification stone called Beta Wollastonite. Wollastonite is composed of calcium silicate and will form in cooler areas of the furnace and forehearth. It typically crystallizes into large prisms that produce vivid retardation colors in polarized light.
September, 2020
This sea green stone actually started as a grass-green colorant. Chromic oxide, is a batch additive used to make green glass. It can form inclusions due to insufficient melting or mixing. Excessive moisture or humidity may have caused it to agglomerate. Chromic oxide stones are difficult to melt and therefore should not be returned to the cullet supply.