Here at AGR, we never get tired of seeing zirconia dendrite stones in glass containers. The little snowflake crystals swarm in the middle of a glassy knot enriched in alumina, a bit like dandelion tufts floating in a puddle or a star cluster observed through a telescope. These stones are created when AZS refractories in the melter erode and drip down into the molten glass. Upon cooling, the zirconia recrystallizes (devitrifies) to form the familiar snowflake shapes. The resultant glassy knots and crystals usually have a low amount of stress, but are nevertheless undesirable in glass articles.

While golf balls and glass windshields have teamed up for some notoriously, awful outcomes,  metallic stones in glass containers are essentially their equal in terms of consequences. The metallic stones often form a sphere, presumably due to surface tension of the molten metal, but the dimples formed in this specimen evoked the cover of a golf ball. Compositional analysis revealed that it primarily was constituted by iron with a small amount of nickel. Thus, it was likely caused by the unplanned introduction of carbon steel or a cast iron impurity into the internal or external cullet. Although iron stones are not highly stressed, this particular stone was the origin and primary cause of breakage for this beverage container that failed during a retort operation under the influence of a thermal shock load.

Although glass is valued for its purity and chemical inertness, certain conditions can foster interactions between a container and the product. Utilizing untreated bottles for certain alcoholic products with an approximately neutral pH might cause it to “snow” in July. Alcoholic beverages, such as vodka or gin, are generally not buffered against changes in pH.  Over time, alkali leaching from the glass can cause the pH of the product to increase, which becomes more alkaline. If the pH rises high enough, the solution will attack the glass surface, resulting in siliceous glass flakes or particles in the product. 

Like a dandelion blooming in your yard, this stone in glass is an unwelcome addition to a glass container. The smooth grey surface belies extremely high tensile stresses in the surrounding glass, created by the stone’s lower coefficient of thermal expansion. Silicon balls such as this stone are created when aluminum contamination in the cullet reacts with the glass melt, creating elemental silicon and aluminum oxide. When the aluminum oxide dissolves, only the silicon sphere remains. The high induced stresses sometimes cause spontaneous cracking, such as in this stone, or spontaneous fracture sometime during processing. For that reason, silicon balls are among the very worst of stones!