CONVERSION  OF  TANNIC   INTO  GALLIC  ACID. 
455 
ON  THE  CONVERSION  OF  TANNIC  INTO  GALLIC  ACID  IN  THE 
EXTRACTS  OF  TANNING  SUBSTANCES. 
By  Prof.  Calvert,  of  Manchester,  Eng. 
During  a  recent  visit  of  M.  Calvert  to  Paris,  he  read  several 
papers  at  the  session  of  the  Societe  de  Pharmacie  of  June  the 
4th,  1856,  and  among  them  one  with  the  above  caption.  The 
first  experiments  were  to  determine  the  special  action  of  tannic 
and  gallic  acids  in  the  dyer's  vat  by  dipping  pieces  of  mordanted 
stuff  in  solutions  of  tannin  and  gallic  acid  of  definite  strength 
and  at  different  temperatures.  It  resulted  that  the  gallic  acid 
liquid  quickly  dyed  the  tissue,  but  was  very  evanescent,  whilst  the 
tannin  solution  acted  more  slowly,  giving  a  permarent  dye. 
These  facts  induced  the  belief  that  gallic  acid  acted  on  the  mor- 
dant by  reducing  the  iron  to  protoxide,  and  on  testing  the  liquor 
of  the  bath  he  found  a  large  quantity  of  protoxide  of  iron  ; 
whilst,  in  the  other  tannic  liquor,  no  reduction  had  occurred. 
This  property  of  gallic  acid  increases  in  the  presence  of  hydro- 
chloric, sulphuric  and  oxalic  acids,  whence  it  follows  that  gallic  acid 
cannot  be  used  for  dyeing  when  in  excess  or  in  presence  of  other 
acids,  whilst  in  the  case  of  tannic  acid,  acids,  except  in  decided 
excess,  do  not  favor  reduction.  He  believes,  in  consequence,  that 
under  the  influence  of  a  great  excess  of  mineral  acid,  tannic 
acid  is  separated  into  sugar  and  gallic  acid,  and  the  last  sub- 
stance acts  as  a  reducer.  These  facts  explain  why  calcareous 
water  is  found  to  yield  the  best  blacks. 
M.  Calvert  then  tried  the  relative  behaviour  of  these  acids  with 
alumina  mordant.  Pieces  of  calico  previously  mordanted  were 
put  in  baths  of  gallic  and  tannic  acids  at  212°  F.,  for  2J  hours. 
The  specimens  were  then  withdrawn,  washed  with  distilled  water, 
and  tinted  with  madder.  The  specimen  with  gallic  acid  was 
nearly  colorless,  whilst  the  other  was  of  a  beautiful  deep  red. 
Other  experiments  were  tried  with  the  same  result;  but  to  remove 
all  doubt  the  author  added  pure  hydrate  of  alumina  to  a  solution 
of  each  of  these  acids,  when  he  found  that  the  gallic  acid  dis- 
solved the  alumina,  whilst  the  tannic  acid  did  not  effect  it. 
The  author  tried  to  obtain  reds  and  blacks  with  an  extract  of 
sumach  long  made,  and  had  not  succeeded,  probably  because  its 
tannin  was  converted  into  gallic  acid.    He  found  that  whilst  a 
