318 
ON  THE  PREPARATION  OF  DAMMARA  VARNISH. 
minute  water-bubbles  produce  a  dull,  rough  appearance  on  the 
surface  of  the  varnish,  so  that  the  latter  can  never  produce  a 
truly  glass-like  coating.  At  every  change  of  temperature,  these 
watery  particles  either  expand  or  contract,  until  at  last,  from 
frequent  repetition  of  this  process,  or  in  consequence  of  a  greater 
elevation  of  temperature  than  usual,  the  coat  of  varnish  bursts 
or  cracks,  and  falls  off  as  a  scaly  powder.  In  order  to  get  rid 
of  this  defect  entirely,  the  water  adherent  to  the  resin  must  be 
completely  removed.  This  is  best  done  by  boiling  the  resin  with 
the  oil  of  turpentine  in  an  open  vessel,  as  in  this  case  the  water 
enclosed  in  the  resin  is  dissipated  below  the  boiling  point  of  the 
oil.  The  object  is  equally  attained  -when  the  resin  is  well  dried 
in  a  drying  oven  before  solution,  and  then  dissolved  in  cold  oil 
of  turpentine ;  if  the  resin  were  sufficiently  dried,  a  perfectly 
clear  and  transparent  varnish  is  obtained,  possessing  all  the  pro- 
perties of  a  good  varnish  ;  this  mode  of  preparation,  however, 
from  its  complicated  nature,  is  not  to  be  recommended  for  adop- 
tion on  the  large  scale.  If  a  very  small  quantity  of  water  be 
added  designedly  to  a  perfectly  clear  and  well-boiled  varnish, 
and  the  whole  is  shaken,  the  latter  immediately  acquires  the  tur- 
bid appearance,  and  all  the  properties  of  a  bad  varnish. 
In  the  preparation  of  dammara  varnish,  the  author  employs 
enamelled  cast-iron  pots,  capable  of  containing  about  50  lbs. ;  in 
these,  25  to  30  lbs.  of  varnish  may  be  conveniently  prepared. 
The  dammara  resin  is  put  into  the  pots  in  a  solid  state  (the 
powdering  of  the  resin  is  disadvantageous,  as  when  in  this  state 
it  forms  a  mass  during  the  fusion,  and  the  varnish  thus  generally 
acquires  a  color),  the  proper  quantity  of  turpentine  (5  parts  to  4 
parts  of  resin)  is  then  poured  to  it,  and  the  whole  put  upon  the 
fire.  As  soon  as  the  boiling  of  the  oil  begins,  the  water  origi- 
nally included  in  the  resin  is  dissipated  in  the  form  of  vapor,  and 
the  resin  acquires  a  softer  consistence.  When  all  the  water  is 
expelled  and  the  oil  (or  varnish)  boils  quietly,  the  solution  is 
completed,  and  the  vessel  may  be  removed  from  the  fire.  As 
long  as  even  traces  of  water  exist  in  the  varnish,  its  boiling  is 
attended  with  a  bubbling  movement;  but  as  soon  as  all  the  water 
is  got  rid  of,  the  varnish  boils  quite  quietly.  That  even  a  very 
small  quantity  of  water  is  sufficient  to  produce  this  bubbling, 
may  be  shown  by  blowing  with  the  mouth  into  some  quietly-boil- 
