One of the natural impurities found in Silica is iron. The higher the iron content the more pronounced greenish colour glass will display when manufactured, using such Silica. There is however a solution to this, adding an oxidizing agent (such as manganese oxide) that converts the iron from its reduced state (strong greenish colour) to an oxidized state (much less intense yellowish colour). The manganese oxide (oxidizing agent) in contrast goes into a chemically reduced state and becomes virtually colourless.
One of the oldest used oxidizing agents (decolourizer), manganese dioxide, is believed to have been used as early as the second century B.C. During Roman times glass contained an average ratio of between 0.5% to 1%. Due to the chemical reaction of manganese dioxide in regards to altering the colour of glass it later became known as “glassmakers soap”.
What is interesting to note is that when glass with the decolorized, reduced manganese is exposed to ultraviolet light over long periods of time another chemical reaction takes place naturally. Photo-oxidization converts the manganese back into an oxidized form, which even in lower concentrations, will impart either a pink or purplish colour to the glass. This glass is called solarized or desert glass and the process can take between a few years up to decades.
There are also other chemical elements that can be as decolourizers such as selenium and cerium. These are also subject to photo-oxidization and can produce solarised colours ranging between yellow and amber.
All this once again goes to proof the changeable characteristics of glass, making it a truly organic building material.