| 1 cord hickory or hard maple | 4,500 pounds equal to | 1,800 pounds coal |
| 1 cord white oak | 3,850 pounds equal to | 1,540 pounds coal |
| 1 cord beech, red and black oak | 3,250 pounds equal to | 1,300 pounds coal |
| 1 cord poplar, chestnut and elm | 2,350 pounds equal to | 940 pounds coal |
| 1 cord average pine | 2,000 pounds equal to | 800 pounds coal |
The above values in pounds of coal may be taken to represent average bituminous coals. As given by Suplee’s “Mechanical Engineers’ Reference Book,” eight samples of coals representing bituminous coals from mines east of the Mississippi River give an average heating value of 13,755 B.t.u. per pound.
Charcoal.
—This is made from wood by driving off the volatile constituents; the residual carbon, which forms the charcoal is a fuel that burns without smoke or flame. Charcoal is made by piling wood in a heap, which is covered with earth. In the bottom of the heap a fire generates the necessary heat for distilling off the volatile matter. Charcoal holds to wood the same relation that coke bears to coal.
Coke.
—This is the residue from the distillation of coal. It comprises from 60 to 70 per cent. of the original coal and contains most of the carbon and all of the ash of the coal. Coke is gray in color and has a slightly metallic luster; it is porous, brittle and in handling gives out something of a metallic ring. It is often sold for fuel as a byproduct by gas factories. In heating value gas-coke gives about 14,000 B.t.u. per pound when dry and as a consequence is rated as an excellent fuel. Clean coke burns without flame and is capable of producing an intense heat. On account of its porous nature it occupies a relatively large volume per ton. It is most successfully burned in stoves and furnaces with large fire-boxes.
Gas-coke,
which is the residue from the gas retorts, is somewhat inferior in heating value to coke made in ovens but it is an excellent fuel where furnaces are adapted to its use. Gas-coke is often stored, by piling it in heaps, in the open and on account of its porous nature it absorbs considerable moisture. Where exposed to the weather the amount of contained moisture depends on the amount of rain or snow the coke has absorbed. This amount is easily determined by weighing a fair sample and driving off the moisture in an oven. The sample should be weighed several times until the weight remains constant.
Briquettes.
—Briquetted coal and other fuels are produced by compressing coal dust or other powdered fuel, mixed with coal tars or other bituminous binder in sufficient quantity to cause the adhesion of the particles when pressed into form under great pressure. Owing to the relative cheapness of fuel, briquettes have been used but very little in the United States. With the advance in the price of coal of the past few years, they have found a place on the market and have become a common form of fuel.