154  Paints,  Colors  and  their  Analysis,     { '^Xi^t™' 
Another  sample  analyzed  as  follows  : 
Moisture,  ....      oioo  per  cent. 
a  |  ^ 
f  Plu 
1  Chi 
Ferric  oxide, 
Calcic  carbonate, 
Baric  sulphate,  . 
Sulphuric  acid  (soluble  in  HC1), 
mbic  oxide, 
romic  acid, 
2'IOO 
3"ioo 
9*934 
72^900 
o*8o2 
8-437 
3-014 
100*387 
Direct  estimation  of  carbonic  acid  =  4*2  per  cent,  a  Prussian- 
blue  \  b  chrome-yellow.  The  practical  working  formula  for  this  color 
would  be  :  Chrome-yellow  10,  blue  5,  barytes  75  and  whitening  10 
parts— total,  100. 
This  green  contained,  besides  the  constituents  mentioned,  also  oxalic, 
tartaric  and  citric  acids  in  minute  quantities. 
Brown  and  Black  Pigments.— Quite  a  number  of  iron  ores,  par- 
ticularly the  manganiferous  oxyhydrates,  either  raw  or  in  the  calcined 
state,  are  frequently  used  as  paints  and  termed  umber  and  burnt  umber. 
A  number  of  argillaceous  iron  ores  have  found  similar  application 
and  yield  brown  paints  of  great  durability.  Light-brown  shades  of 
leather  color  are  produced  by  boiling  a  neutral  solution  of  ferrous  sul- 
phate with  potassic  bichromate.  For  darker  shades  a  small  quantity  of 
caustic  soda  is  added,  together  with  the  bichromate.  Although  these 
precipitates  are  of  a  leather  color,  their  application  is  but  very  limited- 
Cupric  ferro-cyanide,  doubtless  one  of  the  most  handsome  brown-red 
mineral  pigments,  also  belongs  to  this  class.  It  is  rather  expensive> 
which  circumstance  seems  to  have  prevented  a  more  general  applica- 
tion. 
The  coloring  principle  of  most  of  the  black  mineral  pigments  is 
carbon,  as,  for  instance,  lampblack,  boneblack  and  graphite  or  plumbago. 
The  following  is  an  analysis  of  a  plumbago  lately  found  in  this  State, 
of  which  I  happened  to  obtain  a  sample.    It  consists  of  fine  leafy 
particles,  and  is  of  sufficient  purity  to  find  useful  application  in  either 
manufacture  of  crucibles  and  paints  : 
Moisture,  .....       o  26  per  cent. 
Ash,    .  .  .  .  .  8-55 
Carbon,     .....  91-19 
I00"00 
Some  of  these  black  pigments  possess  great  colorific  intensity  and 
