Defect of the Month

February, 2019
Atmospheric Weathering
This piece of weathered glass was viewed in our Scanning Electron Microscope (SEM), magnified 800X and shows the crystalline structures on the inside surface of a glass container. By colorizing the elements we were able to differentiate between the Calcium and Sodium Carbonates. In this view the calcium carbonate (blue) is along the edge of the sodium crystals (purple). This atmospheric weathering is evidence of the reaction of the mobile ions of calcium and sodium in the glass with the atmosphere.
September, 2018
While this defect is similar in appearance to the little insect called a silverfish it is actually a stone (i.e. solid inclusion) in the glass composed of molybdenum oxide, sodium molybdate, or calcium molybdate. With a high melting point of 2623°C, molybdenum is commonly used as an electrode material for supplying additional heat in container glass furnaces. When the electrodes degrade or are exposed to air, they can create “silverfish” shaped stones in the glass.
August, 2018
Copper Comet
This colorful compositional map shows an unusual stone on the surface of a glass container in colors that correspond to different elements. The stone was primarily composed of copper (red) with minor amounts of nickel (yellow) and tin (green). The background glass is mostly silica (blue). The flame-like shape is not typical of copper stones, which usually appear as oval inclusions in the glass. This stone’s location on the exterior of a bottle may mean that it was created by contact with a copper object during bottle formation instead of copper contamination in the furnace.
July, 2018
Chunky Cheese
Could we have inadvertently sprinkled our glass surface with microscopic, crisp-cut blocks of gouda? Or is this rectangular irregularity yet another one of our famous glass defects? In fact, this false-color SEM micrograph shows a well-defined, rhombohedral calcite crystal (CaCO3) produced by atmospheric weathering. When a glass surface reacts with moisture and CO2 in the air, calcium carbonate (i.e. calcite) and sodium carbonate crystals are created. This particular crystal was remarkable because most calcite crystals produced by weathering form in the shape of small hexagonal plates.
June, 2018
Alchemy in the Glass Plant
Did you know that furnaces in a glass plant can also make rubies? Due to its high melting point and chemical durability, alumina (Al2O3) is a common component of refractories. When alumina comes into contact with chromium, either from colorant or chromite refractories, it can be transformed into the red-colored variety of alumina – ruby. This particular stone originated from a chromite refractory in the throat of a furnace, and a fuzzy rind of chromic oxide (Cr2O3) is still clinging to the grain of alumina/ruby. Don’t plan on making a ring out of this gem any time soon – the grain is only 1 millimeter wide!