Am.  Tour.  Pharm. 
May,  1919. 
Modem  Paint  Vehicles. 
293 
There  are  several  other  chemical,  as  well  as  physical  character- 
istics of  drying  oils,  which  are  very  important  for  their  proper 
valuation.  Among  these  we  may  mention  acid  number  or  value, 
saponification  value,  specific  gravity  and  index  of  refraction. 
The  acid  number  is  a  measure  of  the  free  fatty  acids  present  in 
an  oil.  It  is  always  small  in  quantity,  but  very  injurious  when  ex- 
cessive, tending,  as  experiments  indicate,  to  cause  livering  and 
hardening  of  paints  in  the  can. 
The  maximum  acid  number  permitted  for  raw  linseed  oil  in  pre- 
pared paints  when  pure  is  six. 
Acid  numbers  of  samples  of  various  oils  will  vary  roughly  as 
follows : 
Perilla    8-12 
Linseed    2-  4 
Menhadden    . !   6.8 
Soya  bean    2-  4 
Lumbang    1-  9 
One  sample  of  perilla  oil  which  came  to  my  attention  showed 
an  acid  number  of  11.2.  The  very  high  acid  value  of  perilla  oil, 
contrary  to  expectations,  does  not  seem  to  cause  any  harm.  Perhaps 
research  would  show  the  free  acids  in  perilla  oil  to  be  of  different 
composition  than  those  of  linseed.  However  that  may  be,  paints 
made  from  perilla  oil  remain  soft  in  the  can,  while  paints  made 
from  linseed  oil  of  equally  high  acid  number  would  harden  and 
"  liver  "  in  less  than  six  months. 
The  soya  bean  oil  samples  analyzed  varied  from  2.05  to  3.70  in 
acid  number.  The  impure  sample  of  lumbang  oil  showed  the 
ridiculous  acid  number  of  52.0.  Of  course,  no  one  would  attempt 
to  use  oil  of  this  sort.  Whenever  a  very  high  acid  number  is  found 
in  an  oil,  the  presence  of  rosin  is  at  once  suspected;  and  there  is  a 
test,  the  well-known  Liebermann-Storch  reaction,  by  which  its  pres- 
ence or  absence  may  be  fairly  well  confirmed. 
The  specific  gravity  and  the  other  so-called  "  constants  "  of  oils 
are  very  helpful  in  identifying  them  when  pure,  but  when  mixtures 
are  obtained,  the  task  becomes  exceedingly  difficult. 
Some  of  the  vehicles  used  in  paints  made  on  government  speci- 
fications are  exceedingly  complex,  and  contain  various  types  of  China 
wood  oil,  rosin  and  other  varnishes.  The  results  obtained  in  the 
application  of  literally  thousands  of  gallons  of  paints  containing  all 
