Abstract: A wash box for separating materials produced in, for example a mining operation, into fractions of different densities. The machine is divided vertically into compartments, including at least one stratification compartment and at least one reject compartment, the raw material being delivered onto the grid, the level of water in the compartments being higher than the grid. Vertical pulsations are applied to the water in the compartments, causing the water to travel through the perforations of the grid. The pulsations applied to the stratification compartment are effective to stratify the material on the grid, movement of the water through the wash-box tending to carry the lighter fraction from the wash-box while the pulsations applied to the reject compartment determine the rate at which the heavier fraction reject material falls from the grid plate into a reject extraction chamber adjacent to the reject compartment.