Complex sentence: If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state, and it is therefore advised that a lime mortar should be "knocked up" prior to its use.
Split into simpler sentences: If left to stand following agitation a lime putty will slowly revert from a thick liquid to a putty state. It is advised that a lime mortar should be "knocked up" prior to its use.