Defect of the Month

November, 2023
Bright Stripe
Stones and cord in glass can sometimes go hand-in-hand. This container has a large refractory stone in the heel that also created a highly stressed cord in the sidewall. Although a polarimeter cannot reliably detect cord when the bottle is viewed through the sidewall, high magnitude cord stress can sometimes be seen in this manner. The small knots of stress within the cord are additional particles of the stone created as it dissolved within the melt.
June, 2023
Weather Radar Map
This image bears a striking resemblance to a weather radar map. In reality it is an EDX spectral map conveys a huge amount of information by portraying each element in a particular color. The different areas are portions of a stone found in a glass container that have partially melted or recrystallized. This particular stone resulted in glassy areas (blue and yellow) that separated into calcium-rich and calcium-depleted phases. The red area is composed primarily of zirconia (ZrO2) and the green area is composed of alumina (Al2O3) with magnesia (MgO). The magnesium and zirconia are not found in the bulk glass composition, indicating that the stone likely originated from a refractory source.
May, 2023
Smoky Bones
It’s May, the official start of the summer grilling season when backyard grills will be fired up across America. This particular image of a stone in glass resembles a grilled chicken breast slathered in barbeque sauce – although maybe we’re just a little hungry. In any case, the colorful fringe around the stone consists of feathery nepheline crystals (Na2O·Al2O3·2SiO2), which were created by an aluminosilicate refractory stone dissolving in the surrounding glass. Aluminosilicate, or mullite, type refractories are most commonly used in the forehearths of container glass furnaces. The surface cracks around the stone are caused by high stresses due to differences in the coefficient of thermal expansion between glass and refractory materials.
April, 2023
Monster Teeth
In this magnified view of a cobalt-colored bottle, a series of individual knurls are partially illuminated as large blue crescents. The smaller, vertical cracks running through two of the knurls are flaws referred to as “bearing surface checks.” Bearing surface checks are most often created when rough handling causes damage to the bottom soon after bottle formation. This damage is then extended into checks due to thermal stresses caused by temperature differences between the glass and the transfer or conveyer surfaces. Bearing surface checks may lower the performance of a container subjected to loads such as impact, internal pressure, and thermal shock.
March, 2023
Four-Leaf Clover
What do leprechauns and annealing stresses have in common? They’re both invisible! Or at least, usually invisible. This photograph shows the bottom portion of a green glass bottle under polarized light (crossed polars), which is necessary to view annealing stress. The bright “four-leaf clover” pattern is due to a combination of retardation caused by annealing stress and the optical configuration of the polarimeter. Following procedures described in ASTM C148, the amount of retardation can be measured and assigned an integer value called a “temper grade,” which is a good metric for annealing quality.