PCB are mainly composed of plastics with contents of flame-retardants and paper. The type of plastics is predominantly C-H-O and halogenated polymers. Nylon and polyurethane are also used sometime in smaller amounts. Metals in PCBs consist of a large amount of base metals such as copper, iron, aluminum and tin; rare metals like tantalum, gallium (and other rare platinum groups metals); noble metals such as gold, silver, and palladium. Hazardous metals such as chromium, lead, beryllium, mercury, cadmium, zinc, nickel are also present. Ceramics present in the PCB are primarily silica and alumina. Other ceramic materials include alkaline Earth oxides, mica and barium titanate.