3.  Archil.  —  This  coloring-matter  is  obtained  from  certain
species  of  lichens,  the  principal  varieties  of  which  are  Roccella
tinctoria,  Roccella  fuciformia,  and  Variolaria  orcina.  The  dye-
stuff  occurs  in  the  form  of  a  paste,  and  is  prepared  by  treating  the
lichens  to  a  process  of  oxidation  in  the  presence  of  ammonia.
The  principal  color-producing  compounds  existing  in  the  lichens
are  erythrin,  lecanoric  acid,  and  evernic  acid.  The  lichens  are
torn  up  into  small  fragments,  placed  in  iron  drums  provided  with
stirrers,  and  mixed  with  'a  dilute  solution  of  ammonia.  The
temperature  is  kept  at  about  100°  F.  for  several  days,  during  which
time  the  mass  undergoes  a  fermentation  which  causes  the  develop-


THE  MINOR   NATURAL  DYES.  22/

ment  of  the  coloring-matter.  When  the  latter  ceases  to  increase
(which  is  determined  by  making  tests  from  time  to  time)  the
fermentation  is  stopped.  The  product  so  obtained  is  archil
paste;  archil  liquor  is  prepared  by  removing  the  fibrous  matter  of
the  plant.  French  purple  is  a  preparation  of  archil  which  is  said
to  give  faster  shades  than  the  ordinary  product;  it  is  made  by
treating  the  lichens  with  a  dilute  solution  of  ammonia,  acidulating
the  resulting  liquid  with  hydrochloric  acid,  which  precipitates
the  coloring-matters.  This  precipitate  is  washed,  dissolved  in
strong  ammonia,  and  kept  for  about  three  weeks  at  a  temperature
of  1 60°  F.,  during  which  time  a  fine  purple  color  is  developed;
calcium  chloride  is  added  and  a  purple  lake  is  precipitated.
When  used  for  dyeing  this  lake  is  mixed  with  an  equal  weight  of
oxalic  acid  and  dissolved  in  water.
A  solution  of  archil  gives  the  following  reactions:

Acids:    Solution  yellower.
Alkalies:    Solution  bluer.
Lead  acetate:    Crimson  precipitate.
Calcium  chloride:    Red  precipitate.
Stannous  chloride:    First  redder,  then  yellower.
Alum:    Solution  redder.
Basic  alum:    Crimson-red  precipitate.
