Complex sentence: The fibre swelling explains why paper filters cannot be used to filter zinc chloride solutions and explains why a number of paper plies were used to build up to the desired vulcanized fibre thickness rather treating a single paperboard thickness.
Split into simpler sentences: The fibre swelling explains why paper filters cannot be used to filter zinc chloride solutions. It is also the reason why a number of paper plies were used to build up to the desired vulcanized fibre thickness, rather treating a single paperboard thickness.