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.

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.

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.

As winter sets in over the northern hemisphere, brisk mornings reveal delicate frost crystals fanning across the windows on our houses and cars. The crystallized “porthole” seen in this polarized light micrograph is a close-up of a stone in a piece of flat glass. Telltale 60⁰ branching, along with other optical characteristics, identify the fernlike crystals as tridymite, a phase of silica (SiO2). Because silica (i.e. sand) is the primary component of most commercial glass compositions, tridymite stones can occur in food/beverage containers, tableware, and architectural glass.