Notes on a Proper Thermos

The proper vacuum flask performs one transformation and otherwise keeps quiet. Coffee poured before dawn should remain recognisably hot at noon; cold water should emerge without tasting of lid or lining. The cap must turn with gloved hands, the mouth admit a brush and the seal be ordinary enough to replace. These details are less photogenic than colour, but they determine whether the object joins a household or merely passes through it.

We will test examples before naming a winner. The eventual choice should withstand dents without surrendering its vacuum and pour without sending a brown crescent down the outside. Longevity depends as much on the availability of a gasket as on the strength of the steel. The best thermos is therefore not sealed as a product. It remains open to maintenance and to another decade of mornings.