Defect of the Month

November, 2021
Blue Worm
This wormlike flaw is actually classified as a type of stone resulting from AZS refractory erosion that drips into the melt. Typically AZS drip stones are composed of very small particles of zirconia that are embedded in nepheline and/or carnegeite. They are produced by decomposition of the AZS refractories in the superstructure or run-down of molten silica over AZS refractories. AZS stones should not be returned to the cullet stream as they are unlikely to melt in subsequent trips through the furnace.
October, 2021
Choked Neck
Choking is to be taken seriously, whether in people or glass bottles. Choked Necks are constrictions or obstructions in the bore of the neck. They usually appear below the neck parting line on the inside of the bore. There are different varieties of a choked neck or bore that in the majority of cases are the result of temperature issues in the finish region, the glass either staying too hot or too cold due the plunger’s inability to extract enough heat from the glass (glass too hot) or the plunger extracting too much heat from the glass (glass too cold). A choked or restricted bore can become a critical defect when the restriction is sufficient to cause contact with a fill pipe in the filling line, increasing the risk of glass contamination in product.
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.
August, 2021
Unlucky Number 7
Checks are thermomechanical fissures created during the manufacturing process while the glass is still very hot. Letter checks, such as the one shown here, are typically caused by blow molds that run too hot, causing the glass to “hang-up” when the blow mold is opened. Checks are often found on the sharper (less rounded) areas of a container: decorations, knurling, or the finish. Most checks do not go completely through the glass thickness, but still lower the glass strength and can reduce container performance.
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.