They are mostly drinking green tea in those jars. Green tea brews at a much lower temperature (150-180F); indeed, since they are leaving the tea leaves in there

They are mostly drinking green tea in those jars. Green tea brews at a much lower temperature (150-180F); indeed, since they are leaving the tea leaves in there all day and refilling it, they can use even cooler water and let it steep slowly. Heavy glass is also a decent insulator, keeping it warm all day.