156  Paints y  Colors  and  their  Analysis.     { Amx&8P78arm' 
tity  of  resinous  matter,  benzin  and  turpentine  are  required  to  be  known, 
the  modus  operandi  becomes  very  complicated,  and  a  strictly  correct 
solution  of  this  task  is  in  some  cases  impossible. 
We  will  first  consider  the  analysis  of  the  dry  colors,  or  those  freed 
from  oil  by  extraction,  and  finally  dwell  on  the  ways  and  means  by 
which  we  may  get  some  information  on  the  nature  of  the  vehicle  of  a 
paint  and  the  quantitative  relations  of  its  constituents. 
The  main  difficulty  in  these  analyses  consists  in  the  separation  of 
the  different  lead  salts  which  may  be  present,  as,  for  instance,  plumbic 
sulphate  and  chromate  from  each  other,  and  from  baric  sulphate  and 
silicates  (clay).  We  have,  however,  in  sodic  hyphosulphite  an  excel 
lent  solvent  for  plumbic  sulphate,  by  means  of  which  we  may  separate 
it  from  the  above  compounds.  The  analysis  of  a  chrome-green,  con- 
taining a  blue  and  a  yellow  color,  is  among  the  most  complicated  ones  ; 
the  analytical  method  most  suitable  in  these  cases  may  be  illustrated  by 
the  recapitulation  of  an  actual  analysis.1  Let  us  suppose  the  qualitative 
analysis  indicated  the  following  constituents  :  Plumbic  chromate,  ferric 
ferrocyanide,  plumbic  sulphate,  calcic  sulphate,  zinc  oxide,  silicates  of 
lime  alumina  and  magnesia  (clay),  baric  sulphate  and  moisture.  The 
estimation  of  these  constituents  is  best  done  in  the  following  manner  : 
1  or  2  grams  are  used  for  the  determination  of  moisture,  and  the 
dry  powder  is  then  ignited  in  a  procelain  crucible  at  a  low  heat  just 
Sufficient  to  destroy  the  ferric  ferrocyanide,  and  reweighed  to  find  the 
loss  on  ignition. 
The  calcined  residue  is  extracted  with  sodic  hyposulphite  to  dissolve 
the  plumbic  sulphate,  which  is  subsequently  precipitated  with  sulphuric 
acid,  reconverted  into  plumbic  sulphate,  and  finally  weighed.  Sodic 
hyphosulphite  may  be  advantageously  used  in  the  color  analysis  as  a 
test  for  plumbic  sulphate.  Plumbic  carbonate  is  perfectly  insoluble  in 
the  hyposulphite  solution,  while  traces  of  gypsum  and  minute  quantities 
of  lead  chromate— particularly  of  the  orange  and  red  basic  chromates 
will  dissolve — a  fact  which  must  be  borne  in  mind  when  performing  a 
quantitative  analysis  of  a  paint.  The  errors  thus  caused  are,  however, 
very  slight,  and  of  no  significance  for  practical  purposes.  It  is  advis- 
able, and  may  serve  as  confirmation  for  the  correctness  of  a  result,  to 
1  For  other  paints,  like  Paris  green,  white  lead,  zincs,  etc,  the  usual  analytical  rules 
will  give  satisfactory  results. 
