To be able to use external cullet in production, any contaminants should be removed as much as possible. Typical contaminations are:
* Organics: Paper, plastics, caps, rings, PVB foils for flat glass
* Inorganics: Stones, ceramics, porcelains
* Metals: Ferrous and non-ferrous metals
* Heat resistant and lead glass
Manpower or machinery can be used in different stages of purification. Since they melt at higher temperatures than glass, separation of inorganics, the removal of heat resistant glass and lead glass is critical. In the modern recycling facilities, dryer systems and optical sorting machines are used. The input material should be sized and cleaned for the highest efficiency in automatic sorting. More than one free fall or conveyor belt sorter can be used, depending on the requirements of the process. Different colors can be sorted by optical sorting machines.