162  MANUFACTURE  OF  ULTRAMARINE. 
from  7  to  10  hours  ;  the  furnace  is  then  allowed  to  cool,  and  the 
contents  of  the  crucible  appear  then  as  a  caked  mass  of  a  gray  * 
or  often  yellowish-green  appearance ;  it  is  then  frequently 
watered.  In  this  state  the  ultramarine  appears  as  alight  spongy 
mass,  consisting  of  small  and  large  porous  pieces,  which  are 
ground  in  mills  to  the  finest  powder  ;  the  powder  is  then  washed, 
and  afterwards  dried.  After  the  product  has  been  again  pow- 
dered, and  passed  through  sieves,  it  may  be  either  sold  as 
green  ultramarine,  or  used  for  conversion  into  blue  ultramarine. 
II.  The  preparation  of  blue  Ultramarine  from  green  offers  no 
difficulties.  At  present  it  is  generally  effected  by  roasting  green 
ultramarine  with  sulphur  at  a  low  temperature,  and  under  access 
of  air,  so  that  the  sulphur  may  burn  to  a  sulphuric  acid.  At  the 
same  time  a  part  of  the  sodium  oxidizes,  and  is  then  extracted 
from  the  ultramarine  as  a  sulphate  of  soda.  The  sulphur  con- 
tained in  the  green  ultramarine  remains  completely  behind,  but 
combined  with  less  sodium. 
In  the  German  roasting-process  small  iron  cylinders  are  used 
which  are  built  over  a  furnace.  The  posterior  end  of  the  cylin- 
der is  fixed,  and  is  provided  with  a  hole  in  which  the  axis  of  a 
stirrer  may  be  placed.  The  anterior  part  may  be  easily  taken 
away  ;  it  has  an  opening  for  the  projecting  axis  of  the  stirrer,  a 
small  opening  below,  and  a  larger  one  above,  which  serve  for 
adding  sulphur,  and  which  may  be  closed  by  bolts.  At  the  upper 
side  of  the  cylinder  there  is  another  small  opening  for  the  exit 
of  sulphurous  acid.  The  cylinder  provided  with  the  stirrer  is 
filled  with  20  to  30  pounds  of  green  ultramarine,  closed  and 
heated  in  the  furnace.  From  time  to  time  the  axis  is  turned  in 
order  to  heat  the  ultramarine  uniformly.  When  it  has  been 
heated  so  far  that  a  piece  of  sulphur  thrown  into  the  opening  of 
the  cylinder  spontaneously  takes  fire  and  burns,  the  fire  is  moder- 
ated, about  a  pound  of  sulphur  is  added,  the  stirrer  is  turned, 
and  the  filling-hole  left  open  in  order  to  allow  the  sulphur  to 
burn  off.  The  mass  is  turned  slowly  until  no  more  sulphurous 
vapors  escape.  This  treatment  with  sulphur  is  repeated  until  a 
sample  exhibits  the  highest  purity  and  intensity  of  the  blue  color. 
In  some  manufactories  the  roasting  is  not  finished  at  once  ;  but 
the  product,  before  it  has  become  quite  blue,  is  once  more  washed, 
ground,  mixed,  and  sieved.    By  this  means  a  more  uniform  blue 
