Glasfehler des Monats

June, 2022
While the air is filled with the sound of birds chirping this time of year and they are plentiful on the tree branches, a Birdswing is not a welcome sight in your glass. A Birdswing defect is a thin strand of glass stretched between two points on the inside surface of the container, sometimes referred to as a filament or in some part of the world a Monkeyswing. They generally occur in the body of the container, usually extending from side to side. They are caused when glass viscosity is too low and the opposite inside surfaces of the parison touch, causing them to fuse. During the final blow process, a filament will be drawn between the opposite inside surfaces. Birdswings or Monkeyswings are usually considered critical defects by most in the industry and pose a risk for breakage during filling or use with a potential for glass fragments in product.
May, 2022
Stained Glass
Alumina (Al2O3) is an extremely useful material in the glass industry. When incorporated into refractory brick, alumina’s high melting temperature and chemical resistance make it ideal for furnace components. In a different chemical form, alumina-containing minerals are intentionally added as a raw material to improve the durability of the glass. But grains of pure alumina – the mineral corundum – can occur as impurities in the mined materials, and are part of a family of contaminants called “refractory heavy minerals.” The crystalline grain shown here in polarized light, creating an effect reminiscent of a stained glass window, is an example of a corundum stone caused by naturally occurring contamination of feldspar, one common source of alumina for glass manufacturers.
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.