Stir  gently  and  allow  to  stand  for  30  minutes.  When  the  liquor
is  clear  and  of  an  amber-yellow  color  it  is  ready  for  use.  Take
a  test-skein  of  cotton  yarn  which  has  been  boiled  out  and  squeezed
(but  not  dried)  and  pass  it  through  this  indigo  vat  without  heating.
Take  care  to  manipulate  the  dyeing  so  as  to  disturb  the  liquor
as  little  as  possible,  as  much  exposure  to  the  air  will  cause  undue
oxidation  and  considerable  indigo  will  be  precipitated  in  the
vat.  When  the  skein  has  become  thoroughly  and  evenly  satu-
rated with  the  liquor,  squeeze  it  out  well,  and  then  expose  the
skein  to  the  air  for  5  or  10  minutes.  When  the  skein  first  comes
from  the  vat  it  should  be  of  a  yellowish  green  color;  on  exposure
to  the  air  it  soon  turns  blue.  The  dyed  skein  is  then  washed
well  in  water  and  afterwards  in  a  warm  soap  solution  in  order  to
remove  all  alkali  and  unfixed  dyestuff  (420).  Dye  a  second
test-skein  of  cotton  yarn  in  a  similar  manner,  but  after  oxidizing
in  the  air  give  it  a  second  passage  through  the  indigo  vat  and
oxidize  again,  after  which  wash  and  soap.  This  will  represent
the  color  obtained  by  two  dips  (421).  In  the  same  manner
dye  a  third  skein,  giving  it  four  dips  (422).  Also  dye  skeins  of
woolen  yarn  in  the  same  manner,  giving  one  dip  (423),  two
dips  (424)  and  four  dips  (425).  For  the  woolen  yarn  use  a


256  DYEING   AND    TEXTILE   CHEMISTRY.
