158  Paints,  Colors  and  their  Analysis.  {^^l™' 
To  test  whether  red  anilin  or  corallin  (rosolic  acid)  is  contained  in  a 
scarlet  color,  it  is  best  to  extract  with  water.  If  the  resulting  filtrate 
is  colored  red  or  pink,  rose-anilin  is  very  probably  present.  Corallin, 
the  more  frequently  used  pigment  for  this  purpose,  is  insoluble  in  water, 
but  dissolves  easily  in  alcohol  and  alkaloids.  Rose-anilin  and  corallin 
(a  derivate  of  phenol)  are  very  easily  distinguished  from  each  other  by 
addition  of  ammonic  sulphide,  which  converts  rose-anilin  into  colorless 
leukanilin,  while  it  does  not  change  the  beautiful  scarlet  shade  of 
corallin. 
Sometimes  a  chemist  is  asked  to  determine  the  relation  between  the 
dry  pigment  and  the  vehicle  of  paints,  either  liquid  or  ground  in  oil. 
In  this  case  a  weighed  quantity  of  the  pasty  or  pulpy  mass  to  be 
analyzed  is  freed  from  the  fatty  and  resinous  bodies  by  extraction  with 
ether.  The  weight  of  the  dried  pulverulent  residue  collected  on  a 
tared  filter,  gives  the  necessary  data  for  the  information  desired.  In 
cases  where  the  nature  of  the  pigments  will  not  be  changed  on  ignition, 
oil  and  resinous  matter  may  be  destroyed  by  combustion,  and  the  quan- 
tity of  tlie  dry  pigment  determined  on  re-weighing  the  crucible.  A 
white  paint  in  oil  thus  left  31*412  per  cent,  dry  powder,  consisting  of 
baric  sulphate,  clay  and  calcic  carbonate. 
Far  more  difficult  than  these  investigations  is  the  analysis  of  those 
so-called  liquid  or  ready-mixed  paints,  in  case  that  vehicle  as  well  as 
the  suspended  coloring  matter  are  objects  of  analysis.  The  vehicle  for 
paint  is  combined  with  the  aim  of  obtaining  approximately  the  same 
specific  gravity  as  that  of  the  pigments,  so  as  to  keep  the  latter  sus- 
pended (most  of  them  "  settle,"  however,  nevertheless).  Among  the 
most  commonly-occurring  constituents  of  the  vehicle  may  be  men- 
tioned benzin,  oil  of  turpentine,  resins,  linseed  oil,  etc,  ;  soap,  water, 
glue,  small  quantities  of  soda,  borax  or  alum,  besides  all  kinds  of  low- 
grade  oils,  are  frequently  met  with.  The  nature  of  a  vehicle  may  be 
approximately  determined  by  fractional  distillation  ;  exact  and  reliable 
analytical  methods  are  so  far  not  known. 
In  the  following  I  add  such  an  analysis  of  a  liquid  (white)  paint  : 
Solid  incombustible  white  pigment  obtained 
on  extraction  with  benzin,  ether  and  sub- 
sequent ignition,    ....    34-405  per  cent 
Linseed-oii  and  resinous  matter,      .  .  49  095 
Oil  of  turpentine,     ....  16-500 
IOO'OOO 
