Raw Materials - What is Optical Glass?

The use of cold glass techniques is not limited to optical glass. However, optical glass is the perfect material to to take full advantage of advanced cold processes.

Optical quality glass refers to the bubble free, striae* free, precision annealed** glass that is used in the optical industry. Each type of glass is formulated to have specific properties that allow it to meet criteria for its use. An example of a common use for optical glass would be professional camera lenses. Each lens may have multiple individual component lenses made of many different glasses shaped to varying curvatures and coated with thin vacuum films to create the perfect optics for a particular type of photography.

Optical glass also is produced in color. The colored filters that can be purchased to place on a cameral lens are the most common use for these materials. More exotic color filters are also created, mostly for high technology and military uses. These glasses are usually made by the optical glass manufacturers in very limited quantity and may also be quite prohibitively expensive. We have several glasses in our studio, tucked away for special use that are much more rare than gemstones.

*Striae - striae are flaws in glass that produce wavelike distortion of the light passing through it. You may see it quite commonly in "art" glass. Especially in blown or cast forms. Striae in high quality bubble free glass is the result of glass not being mixed well enough during the initial melt phase, the the chemical makeup of glass is not homogeneous through the entire piece. To mix glass at high temperatures to prevent striae from forming, one must use platinum coated ceramic stirrers. Any material placed in molten glass must withstand the high temperature and also be non reactive with the glass. The differences in the chemical makeup of the different areas of glass cause wave like distortions. Severe striae seen in the more crude forms of glass may also be the result of streams of microscopic trapped bubbles. Antique stained glass is a good example of glass that shows this second type of striation.

•• Annealed - annealing of glass refers to the slow cooling of glass after the glass has been melted. The slow cooling allows the molecules of glass to move within the glass to relieve the strain that is produced by the outside of the molten glass cooling first. If glass is not annealed properly, the glass will retain physical strain within its structure. For cold glass purposes, fine annealing is a crucial component of the process. Glass that has not been annealed well enough will continue to twist and torque with environmental changes. This can cause adhesion joints to fail. Badly annealed glass will easily break when stressed, unannealed glass large chunks are considered an explosive danger. Large pieces of glass may take weeks or months in a computer controlled kiln to become fully annealed. The opposite of annealed glass is tempered glass, as in car windshields. Tempered glass is made to shatter into many small pieces when broken.