Filters can be constructed with different layers, called mixed bed filters. Sand is a common filter material, but anthracite, granular activated carbon (GAC), garnet and ilmenite are also common filter materials. Anthracite is a harder material and has less volatile compared to other coals. Ilmenite and garnet are heavy compared to sand. Garnet consists several minerals, causing a shifting red colour. Ilmenite is an oxide of iron and titanium. GAC can be used in the process of adsorption and filtration at the same time. These materials can be used both alone, or combined with other media. Different combinations give different filter classification. Monomedia is a one layered filter, commonly consisting of sand and is today replaced by newer technology. Deep-bed monomedia is also a one layered filter which consist of either anthracite or GAC. The deep-bed monomedia filter is used when there is a consistent water quality and this gives a longer run time. Dual media (two layered) often contain a sand layer in the bottom with an anthracite or GAC layer on top. Trimedia or mixed media is a filter with three layers. Trimedia often have garnet or ilmenite in the bottom layer, sand in the middle and anthracite at the top.