Tankers are classed by size and type of cargo. In general, smaller tankers carry "clean" cargoes (refined products, such as gasoline, diesel fuel, or jet fuel). Large tankers generally carry "dirty" (black oil or crude oil) cargoes. Tankers of less than 100,000 dwt are referred to as either "clean" or "dirty". Clean tankers carry refined petroleum products such as gasoline, kerosene or jet fuels, or chemicals. The so-called dirty vessels transport products such as heavy fuel oils or crude oil. Larger tankers usually only carry crude oil. A tanker carrying dirty cargoes will require about 2 weeks of manual labor to clean its tanks and piping before carrying clean cargo.