In cleaning an oil lamp burner all gauze or perforated parts should be well brushed. These parts cannot be thoroughly cleaned by boiling, and it is often impossible to brush them in the ordinary way. In such a case a pointed piece of wood, with the end broken and made like a brush, could be used, though this process is rather tedious. The perforated parts of the burner may look clean, but if not carefully done there may be left a matting of fine hair or fibre material, which will prevent the air passing through freely. If insufficient air passes through, combustion becomes imperfect; the burner also becomes dangerously hot. Dirty burners cause lamps to smoke.