The present set up is designed to conduct the separation test. It consists of a flat table which is inclined at an angle of about 3° to the horizontal. Running approximately parallel to the top edge is a series of slats, or riffles as they are turned, about 6 mm in height. The material to be separated is fed on to one of the top corners and a reciprocating motion consisting of a forward direction and return stroke, causes it to move across the table. The particles also tend to move downwards under the combined action of gravity and of a stream of the water which is introduced along the top edge, but are opposed by the riffles behind which the smaller particles as the cleaner material tend to be held. Thus, the large particles and the less dense material are carried downwards, and the remainder is carried parallel to the riffles