Asphalt is produced by mixing stones of various sizes with bitumen or modified bitumen in a mixer. Sand is often added and sometimes special additives are included to achieve desirable properties in the asphalt mix. Used asphalt is one of the most recycled materials in the world and can be used at many production facilities. Some production facilities also incorporate recycled glass, plastic, printer toner, steel slag and rubber to produce sustainable mixes designed to meet client performance requirements. When required, oxides to colour asphalt, cellulose fibre and several other additives can be fed into the production process to achieve structural, functional or architectural attributes.