Cement dust builds up gradually on the tank sides, forming a crust as time and precipitation both do their work. In places the weight of this buildup is too much for the grip of the gunk on the metal of the tank, and lumps fall off. On the model this effect can be shown by using a cocktail stick, or similar precision tool, to chip away at the previously created mess. The pointed end will remove small areas at a time, or it can be used at a much shallower angle to reduce the height of the lumps more quickly than an old brush.