130 



MANUFACTURE OF CHAECOAL. 



The quantity of carbon in charcoal depends upon the temperature, 

 the duration of the process, and the kind of wood. According to the 

 experiments of Violette, 100 i^arts of buckthorn wood gave: At 250° 

 (C), 50 parts of charcoal; at 300°, 33; at 400°, 20; and at a white 

 heat (about 1,8000C.), but- 15 parts, the times in each case being alike. 

 Then the duration varies ; in working at the same temperature the 

 amount of charcoal is proportioned to the time employed, more volatile 

 parts escaping in some cases than in others. The composition of char- 

 coal varies with the temperature at which made : thus, at 250°, the 

 carbon forms 05 per cent. ; at 300° it is 73 per cent.; at 400° it is 80 

 per cent. ; and at about 1,500° it is 96 per cent. 



Woods of compact grain carbonize more slowly, and yield a coal of 

 greater density and greater quantity. The following results were ob- 

 tained in woods dried at loO^ and carbonized at 300° : 



Elm, 46.99; guaiacum, 41.80; box, 40.44; ash, 38.28; oak, 34.60; 

 hornbeam, 34.44 ; birch, 34.17; maple, 33.75; buckthorn, 33.61 ; cornel, 

 33.36. 



The variable comj^osition of different woods, charred at the same de- 

 gree of heat, is given by percentages in the following table from Yin- 

 cent: 



Charcoal exposed to the air, absorbs moisture in variable quantity 

 according to the temperature at which it was burned. Thus, at 150° 

 it absorbs 21 per cent, of its weight ; at 250°, 7 per cent. ; at 350°, 6 per 

 cent.; at 450°, 4 per cent. ; and at about 1,500°, about 2 per cent. The 

 rule, then, is, that the higher the temperature the less the absorption. 



The charcoal from high degrees of heat is a better conductor of elec- 

 tricity. This property is very feeble in coals obtained at 150° to 300°, 

 but a'^t 1,500° it is two-thirds as great as iron. The carbon from gas- 

 retorts is used for the positive poles of circuits, and for the pencils used 

 in generating electrical lights. 



The kindling temperature in the open air is higher where the heat 

 has been greater. Thus, coals burned at 260° to 280° take fire at 340° 

 to 300° ; those burned at 290° to 350° take fire at 360° to 370° ; those 

 at 400° kindle at 400° ; and those at 1,000° to 1,500°, at 600° to 800° ; 

 and these latter burn with difficulty. 



Charcoals once kindled burn for a time that decreases according as 

 the temperature of carbonization has been greater— those of lower de- 

 gree lasting longer. When freshly drawn from the kiln or meiler^ and 

 after complete cooling, charcoal is liable to spontaneous combustion at 

 common temperatures, from the property it has of condensing the gases 

 in its pores. It is obviated by lightly sprinkling, so as to supply the 

 moisture it would take in from the air. 



