MANUFACTURE  OF  ULTRAMARINE. 
161 
kaoline  is  introduced  into  the  solution,  and  the  whole  evapo- 
rated to  dryness,  the  charcoal  powder  being  added  from  time  to 
time.  The  dried  mixture  is  then  gently  ignited  in  a  reverbera- 
tory  furnace,  powdered,  and  mixed  as  intimately  as  possible  by 
shoveling  and  sifting. 
The  proportions  in  which  the  raw  materials  are  used,  are 
various  in  each  case. 
1.  Soda,  either  as  sulphate  or  carbonate,  must  be  present  in 
such  quantity  that  half  the  silica  of  the  clay  may  be  neutralized 
by  it. 
2.  There  must  be  a  sufficient  quantity  of  soda  and  sulphur  to 
form  a  certain  quantity  of  bi-  or  polysulphuret. 
3.  There  must  be  an  excess  of  sulphur  and  sodium  present  as 
monosulphuret,  besides  the  quantity  which,  according  to  the  best 
analyses,  is  necessary  to  form  green  ultramarine  with  the  silica 
and  clay  present  in  the  mixture. 
The  French  manufacturers  use  soda  as  soda-salt ;  the  Germans, 
on  the  other  hand,  use  only  sulphate  of  soda,  or  a  mixture  of 
sulphate  of  soda,  and  carbonate  of  soda.  By  using  sulphate, 
more  carbon  and  no  sulphur  is  required ;  by  using  carbonate  of 
soda,  less  carbon  and  much  sulphur  is  necessary ;  and  hence  the 
Germans  work  somewhat  more  cheaply  than  the  French. 
The  three  following  mixtures  may  be  taken  as  a  sample : — 
I.  II.  III. 
Kaoline  (anhydrous)  100        100  100 
Calcined  sulphate  of  soda,  83 — 100  •  •  41 
Calcined  carbonate  of  soda,  •  •  100  41 
Carbon  17         12  17 
Sulphur  .  .         60  13 
100  parts  calcined  carbonate  may  be  replaced  by  80  parts  cal- 
cined sulphate;  100  parts  of  the  latter  by  60  parts  dry  sulphide 
of  sodium. 
The  ignition  is  effected  in  crucible-formed  vessels  or  pots,  in 
furnaces  whose  construction  is  similar  to  the  small  porcelain  fur- 
naces, at  a  high  and  uniform  temperature,  and  with  as  complete 
exclusion  of  air  as  possible.  The  mixture  is  placed  in  the  pots 
by  means  of  small  shovels,  and  pressed  firmly  by  wooden  clubs. 
The  temperature  to  which  the  pots  are  gradually  raised  is  between 
a  bright  red  and  a  white  heat :  the  duration  of  a  firing  varies 
11 
