ON  DECOLORIZING  CHARCOALS. 
153 
alumina.  In  preparing  sulphate  of  alumina,  the  chief  point  to 
be  attended  to  is  to  employ  a  clay  as  free  from  iron  and  lime  as 
possible,  though  the  presence  of  a  trace  of  iron  is  quite  immate- 
rial. A  clay  containing  a  considerable  quantity  of  lime  may 
likewise  be  made  available,  by  previously  removing  the  lime  by 
digestion  with  hydrochloric  acid.  The  charcoal,  which  is  now 
produced  in  large  quantity  by  the  destructive  distillation  of 
of  sawdust  for  pyroligneous  acid  by  Halliday's  patent  process, 
being  almost  a  waste  product,  is  exceedingly  well  adapted  for 
aluminizing. 
The  aluminized  charcoal  may  be  employed  to  decolorize  all 
acid  solutions  except  those  containing  much  free  sulphuric  acid  ; 
for,  after  being  heated  to  redness,  the  alumina  becomes  so  com- 
pact, that  it  is  not  soluble  in  any  but  pretty  concentrated  sulphuric 
acid.  Aluminized  charcoal  decolorizes  tartaric  and  citric  acids 
quite  as  efficiently  as  either  bone-black  or  washed  animal  char- 
coal, i.  bone-black  digested  for  some  time  with  hydrochloric 
acid,  while  it  is  much  cheaper  than  washed  animal  charcoal,  and 
does  not  introduce  nearly  so  much  inorganic  matter  into  the 
tartaric  acid  mother  liquors.  The  price  at  which  aluminized 
charcoal  can  be  produced  does  not  exceed  that  of  bone-black. 
The  only  two  points  required  to  be  attended  to,  with  regard  to 
alumnized  charcoal,  are  to  employ  it  in  very  fine  powder,  and 
to  boil  it  for  a  few  minutes  with  the  solution  it  is  wished  to  de- 
colorize. 
I  have  repeatedly  decolorized  solutions  of  crude  tartaric  and 
citric  acids  with  aluminized  charcoal,  and  have  invariably  found 
that  the  aluminized  charcoal  possessed  an  equal  decolorizing 
power  with  washed  animal  charcoal — the  material  usually  employed 
by  tartaric  acid  manufacturers.  I  likewise  found  that  the  alumi- 
nized charcoal  introduced  a  very  small  quantity  indeed  of 
inorganic  matter  into  the  mother  liquors,  compared  with  that 
which  always  results  from  the  employment  of  either  washed 
animal  charcoal  or  bone-black,  as  may  be  seen  by  the  subjoined 
experiments.  Equal  quantities  of  crude  tartaric  acid  were  de- 
colorized with  the  same  amounts  of  bone-black,  washed  animal 
charcoal,  and  aluminized  charcoal.  When  the  resulting  solutions 
were  evaporated  to  dryness,  they  yielded,  on  incineration,  the 
following  quantities  of  fixed  residue  : — 
