Cerium oxide

Until the 1940's, iron oxide was generally used in glass polishing procedures, although other materials such as silica and tin oxide were also used. In the 1950's, cerium oxide was found to be a superior polishing agent, and is still used in preference today.

Cerium oxide, belonging to the group of elements known as the rare earths, occurs in nature in diverse forms. The two most commercially important are bastanite, which is a complex fluorocarbonate, and monazite, which is a phosphate.

To produce the polishing powder, about 80% of cerium oxide and 20% of other rare earths are used. When the polishing powder is applied to glass, it reacts with the surface to produce a complex cerium-oxygen-silicon compound softer than glass. This softer surface layer can then be more easily applied to produce the final polished surface. As polishing is the final step in the surfacing process, it should not be expected to remove errors made during previous steps when the shape is formed and smoothed. It is therefore necessary that previous steps, bevelling and smoothing be done correctly and accurately.