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.
August, 2022
Lava lamp
This microscopic image was taken at the edge of a larger inclusion in an amber glass bottle. If you look closely, numerous metallic spheres can be seen nestled within the trail of seeds (i.e. bubbles). Those spheres are composed of elemental silicon (Si), caused by chemical reaction of the glass melt with aluminum contaminants. The seeds are a byproduct of the reaction or due to the vaporization of other contaminants associated with the aluminum. Silicon balls create extremely high induced stresses in the glass, often leading to premature breakage of the host container.
July, 2022
Cat Scratch
Cat scratch cord appears as a series of parallel lines or curves on the outside surface of a container. They have very shallow relief, but can sometimes be felt with a fingernail as a subtle irregularity on the glass surface. Cat scratch cord is best viewed by reflecting a bright light off of the glass surface. Like other types of cord, cat scratch cords are usually vertically elongated parallel to the long axis of the container. They are most commonly observed in the sidewall, but can be found anywhere on the container’s exterior. Like other types of cord, cat scratch defects result from inhomogeneities in the glass melt due to refractory erosion, batch errors, or mixing problems. Cat scratch cord is specifically caused by a streamer of viscous glass flowing along the bottom of the forehearth. This streamer ends up as a cord on the outside of the gob as it passes through the orifice ring. Cat scratch cord is usually a cosmetic defect with a negligible effect on the glass strength. In rare circumstances, secondary decoration processes such as etching may be adversely affected by surface compositional variations in the cat scratch cord.
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.