32 BULLETIN 753, U. S. DEPARTMENT OF AGRICULTURE. 
Another point worth bearing in mind in connection with the burn- 
ing of wood in place of coal is the difference in the amount of ash 
produced. A cord of hardwood will make only about 60 pounds of 
ashes, while a ton of hard coal will make from 200 to 300 pounds.2 
A pound of wood briquet, irrespective of species, should have 
about the same heat value as dry wood, probably a little higher, on 
account of the heat value of the organic binder (if one is used), 
which may have a greater unit heat value than wood and thus raise 
the average slightly. If the resins in the wood are used as binders 
the same result may be expected. In comparing briquets with cord- 
wood or stove wood it must be remembered that the briquet is usually 
drier and will therefore generate more heat per pound of material 
than will wood. 
In actual use wood fuel does not always show up as favorably in 
comparison with coal as the above heat values would indicate. This is 
probably due to the fact that it is not the actual heat-producing’ 
power of the fuels that is compared but the efficiency of the appara- 
tus for utilizing the heat. Wood requires about one-third more grate 
surface and two-thirds more cubical space than coal for generating 
an equal amount of steam. 
In logging engines a ton of good grade bituminous coal is consid- 
ered equivalent to a cord and a half of air-dry oak or two cords of 
softwood. Two and a half cords of pine knots (about 125 cubic 
feet) are thought to furnish about the same amount of steam as 1 
ton of southern soft coal. For general calculations for stationary 
engines 1 ton of coal is considered equivalent to 2 cords of wood, or 
1 pound of coal to 24 pounds of wood. During the winter of 1917-18 
1Since potash is now greatly in demand, the quantity which may be obtained from 
wood ash is worth consideration. 
The quantity of ashes obtained from a cord of wood varies with the conditions under 
which it is burned. About 30 cords of hardwood produce a ton of ashes equal in quan- 
tity to the Canadian wood ashes of commerce; but the same quantity of wood consumed 
as fuel in a cook stove or other small, closed burner would be far more completely reduced 
and would produce only about one-third to two-thirds of a ton of ash. On the other 
hand, commercial hardwood ashes contain only 5 per cent of the valuable fertilizer 
potash, whereas stove ashes contain from 10 to 15 per cent, so that the amount of potash 
to be had from a cord of wood is about the same however the wood is burned and re- 
gardless of the bulk of the resulting ash. Softwood ashes contain on an average about 
one-third less potash than hardwood ashes, and the quantity of ash obtained from 
softwoods is less than from the same bulk of hardwood. The present price of potash, 
about 25 cents a pound, or $500 a ton, almost prohibits its use in fertilizers. 
It is important always to keep wood ashes under cover, as they leach rapidly if allowed 
to become damp. New ashes should be allowed to cool before they are dumped on the 
ash heap. 
It is estimated that the ashes from a cord of northern hardwoods will furnish about 
20 pounds of lime, more than 3 pounds of potash, and a half pound of phosphoric acid, 
and that they have a value at present prices of about $1. 
H. J. Wilder, agriculturist of U. S. Department of Agriculture (letter to Mr. A. F. 
Hawes, July 18, 1918) : Hardwood ashes which have not been wet analyze about 5 per 
cent potash, 30 to 35 per cent lime, both in desirable forms. Potash contents of soft- 
woods is rarely below 3 per cent. Hardwood ashes have 600 to 700 pounds lime per 
ton of ashes. Mixture of coal ashes from factories would do no harm. 
