Let us consider some of the methods of saving heat, and study different kinds of apparatus with this knowledge in mind.
We recall the fact, first, that some substances are good conductors of heat, and others poor.[7] If you hold a metal poker in your hand, and place the other end in red hot coals, you will realize that metal is a rapid conductor of heat. If the poker has a wooden handle, the heat of the coals does not readily reach your hand, for the wood is a poor conductor. Moreover, this good conductor is a poor holder of heat, the heat radiating rapidly from it into the surrounding air, but the poor conductor, once thoroughly heated, cools off slowly.
You can think of many illustrations from your daily life. Why do you prefer a woolen blanket on a cool night, rather than a linen sheet, merely? Why do you use a cloth holder in ironing? What is the principle of a hot water bottle? Air is a poor conductor. Can you think of an illustration of this? What is the principle of a thermos bottle?
It is not difficult to see how these facts apply in our cooking apparatus. From an oven with metal sides heat is lost by radiation. In a double oven, with an air space between the inner and outer part, some heat is saved. If the outer cover is of some non-conducting material, even less radiation takes place. This is the principle of the oven devised by Mr. Edward Atkinson. Here the inner oven is of sheet iron, and the outer covering of a non-conducting material, some composition with wood pulp or paper as the basis. If in this way heat can be trapped, as it were, in an oven, it will follow that less heat will have to be supplied, and we can use a smaller amount of fuel. This is the case in the Atkinson oven (Fig. 16), where the source of heat is either a kerosene lamp, or a small Bunsen burner of the rose type, which uses only a small amount of gas.
Fig. 16.—The Atkinson cooker.
Another illustration of the conserving of heat by the prevention of radiation is in the fireless cooker. This is a method used in Sweden in simple form, and adapted and improved to suit modern needs. Heat is supplied in the first place by gas or kerosene, and the water in the vessel containing the food is raised to the boiling point, and held there in some cases for a few minutes. The vessel is then placed in the “cooker,” which is a box with thick walls of some non-conducting material, and the heat already present is sufficient to finish the cooking process, since the radiation is very slow. In some cookers a heated stone is introduced to raise the temperature slightly. Both of these devices are excellent for the long, slow cooking that seems to mellow the food material and develop the flavors that do not result from rapid cooking.
At the same time, we need rapid processes, such as broiling and toasting, which give characteristic flavors. To meet this double need, a new type of gas stove has been made. See Fig. 17.