The most desirable fuels are, as a rule, the most expensive. It may be possible to burn some of the smaller sizes of anthracite with the furnace or egg size and thus effect a saving. The price a ton of these smaller sizes is less in proportion to their calorific value because they are in less demand and they can be used to advantage in banking the fire overnight or in carrying a slow fire in moderate weather. Pea coal is probably the best size for such use, but unfortunately it and the smaller sizes are obtained with difficulty in many of the markets where the larger, or domestic, sizes are sold.

Among the various kinds and grades of bituminous coal, the differences in fuel value and in their physical characteristics are much greater than among anthracites, and for that reason should receive especial attention. In every locality a number of coals will be available and the most satisfactory one in any particular case will usually have to be determined by trial, unless the characteristics of each and the effects of these are known.

Uniformity of size is a desirable characteristic, as it permits easier control of the fire. This is particularly true of anthracite, a mixture of sizes often accounting for poor results from a certain lot of coal. Sized or screened bituminous coal may be bought for its greater convenience, even though its cost may exceed that of lump coal or run-of-mine. In some localities slack coal has been burned successfully, although it is believed that this would not ordinarily be possible because of the strong draft required.