As their service life is said to be more than 100 years and they are strong enough to keep its shape without breaking apart even if they are thrown, they have a feature to protect the things kept inside easily even when it is roughly handled in the event of an emergency such as a fire.
耐用年数は100年以上とも言われ、その頑丈さは放り投げてもばらばらになりにくいため、火事などの緊急時に粗雑に扱われても中に保管していたものを守りやすいという特徴を持つ。