The  iron  and  copper  sulphates  are  decomposed,  liberating
sulphur  trioxide,  which  in  turn  converts  the  silver  sulphide
Ag,S  into  silver  sulphate  AgjSO4.  In  many  places  where
this  method  is  employed  the  ores  to  be  treated  contain  very
little  copper,  but  several  ounces  of  silver  and  varying
amounts  of  gold  to  the  ton.  Such  ores  are  treated  as  copper
ores  by  alternate  roastings  and  meltings  so  as  to  produce
a  coarse  regulus,  or  they  are  smelted  direct  with  copper
regulus,  in  order  to  concentrate  the  precious  metals.  The
regulus  containing  the  gold  and  silver  is  smelted  with
siliceous  substances  to  slag  off  iron  and  other  impurities,  and
the  enriched  regulus,  corresponding  to  "  blue  metal,"  is  then
subjected  to  the  Ziervogel  process.  The  roughly  crushed
material  is  first  roasted  to  remove  excess  of  sulphur,  then
removed,  allowed  to  cool,  crushed  to  a  very  fine  powder,  and
roasted  at  a  low  temperature  for  about  one  and  a  half  hours,
when  any  iron  is  converted  to  sulphate ;  the  temperature  is
then  raised  and  the  roasting  continued  for  a  similar  period
to  convert  the  copper  into  sulphate ;  the  iron  sulphate  is  also
decomposed.  The  temperature  is  again  raised,  when  the
copper  sulphate  is  largely  converted  into  oxide  and  the  silver
into  sulphate.  If  much  lead  or  antimony  sulphide  is  present
a  caking  of  the  mass  occurs,  and  the  operation  is  very  im-
perfect. The  presence  of  zinc,  arsenic,  and  antimony  in  the
ore  is  said  to  cause  loss  of  silver  by  volatilisation  ;  also  in
the  case  of  arsenic,  arseniates  are  formed,  which  are  very
difficult  to  decompose.  The  roasting  process  may  be  per-
formed in  a  single  or  in  a  double-bedded  reverberatory
furnace.  In  the  latter  case  the  ore  is  placed  on  the  top  bed
for  a  preliminary  roasting,  then  raked  on  to  the  lower  and
hotter  one  to  finish,  with  frequent  stirring  during  the  roast-
ing. The  operation  requires  about  five  and  a  half  hours.
