PREPARATION OF CHARCOAL BY STEAM. 279 
by smothered combustion in a pile, or by destructive distil- 
lation in an iron retort, a large proportion is on this account 
always unfit for making gun-powder, from the impossibility 
of so regulating the temperature as to avoid passing the 
proper limit of carbonization, and producing black thorough- 
ly-burned charcoal, fit only for fuel. MM. Thomas and 
Laurens first conceived the idea of applying highly heated 
and rarified steam to the restoration of animal charcoal 
which had lost its decolorizing properties, and the success 
of this process suggested to the author the propriety of 
applying the same agent to the direct carbonization of 
wood. The result was successful in every respect. 
M. Violette first endeavored to determine the exact limits 
of temperature proper for producing charcoal of the best 
quality, by exposing small pieces of wood to the heat of a 
fusible metal bath, furnished with a thermometer. Below 
570° Fahr., the heat was insufficient; between that tempe- 
rature and 660° Fahr. (the highest range of the mercurial 
thermometer) excellent charcoal was produced, and beyond 
this, overburned or black charcoal only. 
The first experiments with heated steam, made by means 
of a small apparatus capable of containing about two pounds 
of wood, were so encouraging, that permission was obtained 
to make arrangements for carrying out the process on a large 
scale. The apparatus now employed consists of two con- 
centric horizontal cylinders of sheet iron, enclosed in a sub- 
stantial brickwork erection, and having below a furnace 
whose principle office is to heat to the necessary extent the 
spiral coil of wrought iron pipe through which the steam is 
made to circulate before entering the cylinders. The outer 
cylinder is closed by an iron plate, which can be made 
tight by luting, in the manner of a coal-gas retort; and de- 
fended from the cooling influence of the air by an external 
cast-iron door. The charge consists of about sixty pounds 
of wood, which is placed in an envelope of perforated sheet- 
iron, capable of freely entering the inner cylinder of the 
