This presentation shows the advantages of re-melting post-consumer glass, but also the potential risks of using contaminated cullet in the raw material batch of glass furnaces (e.g. container glass furnaces). As an example of potential advantages: increasing the cullet % in the batch of an efficient end-port fired regenerative container glass furnace from 65 up to 75% decreased the specific energy consumption from 3.95 MJ/kg molten glass to 3.8 MJ/kg and reduced the direct CO2 emissions with 31 grams per kg glass. Additional, lower indirect CO2 emissions can be taken into account, since less primary raw materials have to be applied, saving fossil energy in raw material synthesis (e.g. synthetic soda production). Waste glass has to be sorted and prepared in dedicated cullet recycling (treatment) plants (CTP) to meet the strict quality standards often expressed in maximum mass fraction of ceramics, stones, china, metal (ferro & non-ferro) and color mismatches that can still be accepted. But, also the presence of organic components (fats, oils, sugar, food residues,..) has to be controlled to avoid production or glass color problems. Contamination of cullet may lead to: 1. Glass quality problems: inclusions or color changes; 2. Glass melting disturbances by foaming or limited heat transfer into melt; 3. Glass furnace lifetime, by downward drilling of melts of metals, present in the cullet. The most important problems today are related to the presence of glass ceramics in the post-consumer waste glass and the variable amount of different colors in the glass cullet or fluctuating contamination by organics. Even small pieces of china or glass ceramics in the cullet, with sizes less than 5 mm, may end as glass defects in glass products. Larger pieces of glass ceramics may even lead to severe interruptions in the gob formation process, due to problems with cutting of the gobs with high viscous inclusions. In modern cullet treatment plants most ferro- and non-ferro metals are rather effectively removed. Glass-ceramics are very difficult to distinguish from normal soda-lime-silica glass, because color and transparency can be almost the same. Specific techniques have to be applied to detect glass-ceramic pieces in the cullet based on X-Ray absorption, X-Ray fluorescence, Hyper-spectral Imaging or UV techniques. Such systems are recently applied in modern waste glass treatment plants in Europe, delivering recycling cullet to the glass industry. This paper will show the glass defects related to the presence of glass-ceramics and the typical compositions of these inclusions (often present as cord or big knots). Organic materials can pyrolize within the batch blanket to form carbon rich residues. Carbon or cokes can react with sulfates in the batch and will cause formation of sulfides. A high level of sulfides in the batch (instead of sulfate) will jeopardize the fining process, may cause changes in fining onset temperature, may cause foaming around the batch blanket, or may lead to chemically reduced glass or even amber cords. The process of radiant heat transmission in the melt will change with a variation of the oxidation state of the melt (redox state of batch & cullet). Examples of glass defects caused by metal contamination in the batch will be shown. The composition of the metal inclusions (glass defects) in the glass product may be very different from the composition of the original contamination, causing the defect. An example is metallic aluminum that leads to silicon inclusions in the glass. Nickel sulfide can be formed by pollution of the glass melt by stainless steel flakes. Liquid metals are very aggressive towards the refractory bottom materials of the tank. A droplet of molten lead for instance will drill a hole in the refractory layers: "downward drilling". The presentation will show the relation between glass defects and their origin in contaminated cullet and the mechanism of defect formation or defect conversion.