How Does a Bottle Keep Coffee Hot for Twelve Hours? Remove the Air

Heat leaves a hot liquid by three mechanisms and a vessel that keeps it hot must obstruct all three. Conduction carries heat through material in contact, which is why a metal mug burns the hand. Convection carries it away in moving air or liquid, which is why blowing across a surface cools it. Radiation carries it as infrared light, which needs no medium at all and crosses empty space perfectly well. An ordinary insulated container slows the first two by putting a poor conductor in the way, and does almost nothing about the third. Blocking all three at once requires a different approach, and that is exactly what this design provides.

Each feature of the vessel addresses a specific mechanism:

• •Two walls with a gap, so the inner vessel touches the outer only at the neck
• •Air pumped out of that gap, removing the material conduction needs
• •The same vacuum removing the air that convection would have to move
• •A reflective coating on the facing surfaces, turning radiated heat back
• •A narrow neck of poor-conducting material, limiting the one remaining contact
• •A stopper, since an open top loses heat by evaporation regardless of the walls

James Dewar built the first such vessel in 1892 at the Royal Institution for an entirely different purpose, namely holding liquefied gases at temperatures far below anything encountered otherwise, where any heat leaking in boils the contents away. The design worked and became fundamental equipment for low temperature physics, and it is still called a Dewar in laboratories. He did not patent it, taking the view that scientific apparatus should be freely available, and two German glassblowers who made such vessels for him recognised the domestic possibility, patented a commercial version in 1903 and founded a company on it, naming the product from the Greek for heat. Dewar sued and lost.

The same vessel keeps cold things cold, which is the same physics running the other way and is worth stating because people frequently assume two different products are needed. Heat flows from warm to cold whatever the direction relative to the container, so a barrier that stops heat leaving also stops it entering, and iced water stays cold in exactly the vessel that keeps tea hot. The one practical difference is condensation, since a cold vessel collects moisture on its outer surface from the surrounding air, which is why cold flasks sometimes appear to sweat. Laboratory versions holding liquid nitrogen are left deliberately unsealed, since the contents boil continuously and a sealed vessel would burst.

These containers stop working long before they look damaged, and the reason is that the vacuum is not permanent. Gas molecules gradually escape from the material of the walls into the gap over years, and any small leak admits air directly, both of which restore the conduction and convection paths and cause performance to fall off progressively rather than suddenly. A vessel that used to hold heat all day and now manages a few hours has lost its vacuum, and no repair is practical. Getters, which are materials placed inside the gap that absorb stray gas, extend the working life considerably and are why quality varies so much between products. Dropping one cracks the inner vessel or the seal, which ends it immediately.

Conduction, convection and radiation all carry heat away, and this design blocks all three by separating two walls, removing the air between them and coating the facing surfaces to reflect infrared. Dewar built the first in 1892 for holding liquefied gases and did not patent it, and the commercial version came from glassblowers who had made them for him. They fail as the vacuum degrades, which happens gradually.