Made from Butyl Rubber, Schwalbe tubes are well known for its extremely high air retention and elasticity. Schwalbe tubes undergo vulcanization process which ensures constant wall thickness and high air pressure retention.
All tubes are inflated and stored for 24 hours to test the air pressure retention. Afterwards, each tube undergoes a careful visual inspection.
Only a good quality tube can cover very many tire sizes which isn't possible if the tube isn't highly elastic in nature. For example, tube no 17 can fit tire widths between 28mm and 47mm.