Defect of the Month

September, 2022
Old Coin
Metallic stones in glass are difficult, if not impossible, to identify by visual examination alone. Although metals such as iron and copper are the most common, reaction with the glass can create different sheens that alter the apparent color. On top of that, less common metals occasionally crop up, such as this nickel sulfide stone. Nickel sulfide is well-known in the flat glass industry as a cause of breakage for thermally tempered windows. This stone, however, was found in a glass bottle, and was caused by contamination of the internal cullet supply.
April, 2022
Double Vision?
Almost all fractures originate from a single origin where the tension stress exceeds the glass strength. In rare instances, bottles might exhibit two origins that are usually separated by some distance on the bottle surface; for example, when bottles break under an unusually high internal pressure load. The picture is a situation that is almost never observed where two origins are side-by-side, as shown by the double fracture mirrors. Each origin initiated from the ends of a collapsed blister that can be seen as the thin dark gray line just below the original glass surface (remnants of the collapsed blister are seen by the irregularly shaped semi-circle of irregular features just above the origins). What is so unique about this example is that two fractures started at exactly the same time but from two discrete points of origin. If either of the two origins had initiated first, only one fracture mirror would have been observed.
March, 2022
Swimming Sea Turtle
While this may appear as swimming sea turtle in the East Australian Current, in reality polarized light can reveal a hidden world of strain in ordinarily colorless, transparent glass. The bright blue streak in this image is cord (i.e. glass with a different composition than the bulk) caused by erosion of an AZS refractory containing alumina, zirconia, and silica. Because of the orientation of the sample relative to the polarized light source, the blue color indicates that the cord is under compression. In this case, the turtle is actually, a small dendrite crystal of ZrO2 that formed due to local enrichment of that element within the cord.
February, 2022
Quenched Sword
As a blacksmith submerges a red-hot blade into a barrel of water, the hiss of steam evokes a sense of high adventure as we wait for the hero to claim his newly forged sword! This micrograph of a copper inclusion in glass certainly resembles a quenched sword. Copper stones are caused by contaminants in the internal or external cullet supply, usually consisting of coins or scraps of wire. The unusual striped pattern on this copper ribbon is likely due to crystallization of the metal. Numerous seeds (i.e. bubbles) surround the metal where it has reacted with the molten glass.
January, 2022
View from the Top
This top-down image of a finish shows a thin line that extends radially across the sealing surface. These flaws are referred to as lineovers or “line over the finish.” Lineovers differ from split finishes in that they are only a surface flaw and do not penetrate into the glass; however, like split finishes, lineovers can cause sealing problems such as leakage, loss of carbonation, or spoiling.