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Defect of the Month

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Stone in glass looks like a flower
Spring Flower
June, 2024

Like a dandelion blooming in your yard, this stone in glass is an unwelcome addition to a glass container. The smooth grey surface belies extremely high tensile stresses in the surrounding glass, created by the stone’s lower coefficient of thermal expansion. Silicon balls such as this stone are created when aluminum contamination in the cullet reacts with the glass melt, creating elemental silicon and aluminum oxide. When the aluminum oxide dissolves, only the silicon sphere remains. The high induced stresses sometimes cause spontaneous cracking, such as in this stone, or spontaneous fracture sometime during processing. For that reason, silicon balls are among the very worst of stones!  

Gliding Frog in glass
Gliding Frog
May, 2024

No, that isn’t a new yoga pose!  With a little bit of whimsy and imagination, this photomicrograph of a stone in glass looks like a frog swimming through green water. The cracked surface texture and tight solution sac are clues as to its origin, since both traits are characteristic of aluminosilicate refractory material. When this stone was analyzed via SEM-EDX, the interior consisted of recrystallized alumina and nepheline – both are commonly created by the reaction of an alumina-rich refractory with the glass melt.

Grandma's Yarn Crystals in glass
Grandma's Yarn
April, 2024

Perhaps nothing says “cozy” like an image of grandma in a rocking chair, knitting a blanket with a big basket of yarn at her feet. The bunches of crystals in this Scanning Electron Microscope image appear remarkably similar to jumbled skeins of yarn, but have a very different source. They were found inside a molybdenum blister created in the glass melt when boosting electrodes were exposed to air in the furnace. The crystals themselves are composed primarily of sodium molybdate with a smaller amount of calcium molybdate.

Road to the Mountain Defect of the Month
Road to the Mountain
March, 2024

While this might appear to be a road to a mountain, it is a magnified view of the horizontal crack revealed that the origin was related to a vertical scuff mark with a few associated crescent cracks.  Further investigation led to the conclusion that the scuff marks had been created by the downward sliding contact of the heel of one bottle against the fractured bottle.  This action most likely had occurred during case packing.