Mischna zu Esther 4:18

If one makes a hollow in a water-spout to collect pebbles, its water disqualifies the mikveh; In the case of a wooden [spout] if it holds even a little, But in the case of an earthenware [spout] if it will hold a quarter-log. Rabbi Yose says: also in the case of an earthenware [spout] if it holds even a little: they have spoken of "a quarter-log" only in the case of broken sherds of an earthenware utensil. If the pieces of gravel moved about inside [the hollow], it disqualifies the mikveh. If dirt went down into it and was pressed down, [the mikveh continues to be] valid. If the spout was narrow at each end and wide in the middle, it does not disqualify [the mikveh] invalid, because it had not been made to gather anything in it.