128 MANUFACTURE OF CHARCOAL. 



the same as of a large one, but, as in case of accident tbe loss might be 

 greater, it is thought safer to have them of moderate size. One collier 

 can watch two standing meilers (where the wood is placed upright) or 

 three lying meilers (the wood laid horizontally) at the same time. 



The yield of charcoal varies, but is usually 30 to 40 per cent, of vol- 

 ume of the wood,^ and from 18 to 22 per cent, of the weight — seldom over 

 25 per cent, by the ordinary methods, where the wood has been exposed 

 two or three months in the open air after cutting. Well-made charcoal 

 retains the form and structure of the wood, is brittle, and somewhat 

 cracked, and very sonorous. When buried in the earth it is nearly in- 

 destructible, and for this reason is sometimes used in fixing the corners 

 in land-surveys. They say that charcoal has been "too much cooked" 

 if it loses its sonorous quality, and is too much cracked. If not done 

 enough it is not black, nor is the fracture bright. 



As a rule, the heating power of charcoal from different kinds of 

 wood bears about the same proportion to one another as did the woods 

 themselves, and with like degree of dryness is in proportion to density. 

 In a given kind of timber it may vary greatly, according to climate, 

 location, exposure, soil, age, and the season when cut, as well as the part 

 of the tree from which taken. The branches have less heating power 

 than the trunks in deciduous trees, but the reverse of this in the con- 

 ifers. Woods that dry slowly generally burn slowly, because, being com- 

 pact, the pores do not admit the air necessary for combustion, and their 

 charcoal lasts longer, but gives less heat. Wood that has laid some 

 months, and contains only 20 to 25 per cent, of hygrometrical water, 

 makes better coal than that burnt green or very dry. Light porous 

 woods and their charcoal burn briskly, and often contain gases which 

 cause a crackling when burning. Charcoal used in making gunpowder 

 is from light soft woods, such as willow, alder, mountain ash, birch, 

 hazel, &c., and great care must be taken to prevent tbe introduction of 

 sand. The woods are peeled, and are burned in iron retorts, the heat 

 being externally applied. 



Kilns or ovens for reducing wood to charcoal are generally made of 

 brick, and are of two kinds, the conical and the rectangular. The usual 

 size of the conical kiln is 24 feet in diameter, with a little greater height, 

 holding about forty cords of wood. They should be lined with fire-brick 

 to a height of 10 or 12 feet from the bottom, and are often plastered 

 both outside and inside. A band of heavy strap-iron should be placed 

 around the middle, and they are furnished with doors of boiler-plate 

 iron, and drawn together with screw-bolts. The top is closed by a cir- 

 cular plate of iron that may be raised when necessary. Air-holes are 

 made around the bottom, which may be closed by loose bricks. The 

 ioints around the doors and any cracks that may appear in the wall are 

 to be closed with clay or plaster. 



The rectangular form is usually 40 feet long, 16 feet wide, and 15 feet 

 high on the outside, with a timber framework. The walls should be a 

 foot thick ; the top is arched, and air-holes are provided as in the other 

 form. Such a kiln will hold about eighty cords of wood, and will burn 

 over 3,000 bushels at a time. The yield by either of these kilns with 

 hard wood will range from 42 to 45 bushels to the cord. The large kind 

 can be filled, burned, and emptied in about four weeks, and the smaller 



' In Sweden the yield is about 63.2 per cent, of volunie, with the Finns picca and 

 Abies excelsa. Where the wood is set vertically, it is about 5 to 8 per cent. less than 

 where it is horizontal. In Upper Silesia it is 52.6 per cent, for the stem, 42.7 for the 

 branches, and 39.5 for the roota.— Percy. 



