Coal plies and particles have different relative densities, determined by vitrinite content, rank, ash value/mineral content and porosity. Coal is usually washed by passing it over a bath of liquid of known density. This removes high-ash value particle and increases the saleability of the coal as well as its energy content per unit volume. Thus, coals must be subjected to a float-sink test in the laboratory, which will determine the optimum particle size for washing, the density of the wash liquid required to remove the maximum ash value with the minimum work. 
Float-Sink testing is achieved on crushed and pulverised coal in a process similar to metallurgical testing on metallic ore.