Hot objects, like stoves and steam pipes, lose much of their heat by radiation, and the blacker the object the more it will lose; hence, stoves and steam pipes should be black if they are intended to give out heat, but hot-air pipes and cooking utensils should be bright, for example tinned or nickeled, in order to lose as little heat as possible. A stove nickel plated all over will give out only about half as much heat as the same stove at the same temperature if black.
* From U. 8. Bur. Standards, Circ. 55.
A bright nickel or aluminum kettle will cool very much more slowly than a black kettle. On a coal or wood stove or directly over a coal or wood fire, a kettle is heated largely by heat radiated from the stove or fire; therefore, if the bottom is black the kettle will heat more rapidly than if bright. Over a gas, gasoline, or similar blue flame the condition of the bottom will not make so much difference, since here most of the heat is received by contact with the hot gases. The best kettle for general use is, therefore, one with the bottom black and the remainder polished, but for use on a gas stove it makes little difference whether the bottom is black or not.