The phrase trims off any waste clay to the passage, means to

In 1951, the Government decided to start a Pottery Training Centre where new and more advantaged technical methods, especially glazing, could be taught. The centre was intended to serve the whole of the defunct Northern Region, and there were several reasons for choosing Abuja . The first was the excellence of the traditional pottery made in the Emirate. Secondly, firewood is plentiful; this is a most important consideration, because in the making of glazed pottery, more firewood than clay is required. Thirdly, there are good clays, and good local sources for the raw materials needed for the glazes. Fourthly, water, which is another important raw material, is plentiful. Finally, Abuja is in a central position for the whole region and is a town where learners from many different parts can find a congenial temporary home, and where the Emir and his Council are actively interested in the project.

Nearly all the making is done by a process called throwing;, so called because the lumps of clay are thrown the potter onto a wheel-head. They are weigh out so that each pot will be roughly the same size; for example, for making pint-sized jugs, the lumps of clay will be one and a half kilogrammes. The potter sits on the saddle of the wheel and spins it by pushing a pedal with his left foot. He has a bowl of water, a loofah, a bamboo knife, a pointed stick or porcupine quill, a wooden-smoothing tool which potters call a rib, and a piece of wire-like object that is used for wedging. He makes the wheel-heal slightly damp, and throws the lumps into the middle. The first work is to force the lump to the centre, then he presses his thumb into the middle of the lump, using water to keep it slippery. When the bottom is of the right thickness, he begins to draw up the walls until they are of the right height. Then he shapes the belly and shoulder of the pot. He trims off any waste clay. In this way, a small and medium-sized pot can be made quickly and accurately.

Adapted from Robert, J.M.E and Smith, L.E.M (1978) Testing English language, AUP