Chapter 9 deals with the subject of glass. As in the previous chapter, it starts by describing how glass is manufactured, what different grades of glass are available on the market and how they are used. This is followed by valuable tips for the commercial washing process. Special attention is paid to the possible optical changes in glass. The various causes are described, ranging from stress cracks and scratch-like changes to the development of glass corrosion, and how these changes can be prevented in the future.