Complex sentence: At this stage a clay plug at the level of the scum would be smashed out and the scum run off, similarly a plug at the bottom would then be removed and the iron run off into a prepared sand bed.
Split into simpler sentences: At this stage a clay plug at the level of the scum would be smashed out and the scum run off. Similarly a plug at the bottom would then be removed and the iron run off into a prepared sand bed.