These gaseous inclusions are called seeds but also are known as blisters or bubbles. The term “blister is often used for larger seeds. Seeds have many possible causes, including but not limited to inadequate refining, problems with the glass chemistry, reboil, contaminants, or by mechanical trapping of air during bottle formation. Depending on their location and type of seed, they can be considered critical defects or non-critical yet strength-reducing. Seeds at the inside knuckle lower the resistance of the container to internal pressure and thermal shock loads. Seeds on the inside surface at the contact points can reduce the resistance of the container to impact loads.

We took artistic license in transforming this Scanning Electron Microscope image into a snowy winter scene. The green “branches” are crystals of sodium carbonate (Na2CO3), the white “snowflakes” are hexagonal crystals of calcium carbonate (CaCO3), and the blue “sky” is the inside surface of a glass bottle. These crystals are created by a process referred to as weathering, where moisture and carbon dioxide in the air react with an uncoated glass surface. If severe enough, weathering can cause bottles to have a cloudy white appearance, not unlike a frost-covered window.

Although the pictured defect is shaped like a heart, there is nothing lovely about this particular flaw. Spiticules are filaments of glass adhered to the inside surface of a pharmaceutical tubing-type vial. Caused by problems in the tubing conversion process, spiticules may be fragile and are classified as critical defects due to the risk of glass-in-product.

This colorful Twinkling Star is actually dendritic zirconia (Zr02) that is formed from melted AZS refractory material that dropped into the molten glass. The melted refractory material cools in the molten glass and subsequently the zirconia forms into dendrites (six legged crystals). The alumina and silica portion of the refractory assimilates into the glass matrix. The picture was taken at 400x magnification using a polarizing microscope with a 1st order red insert.