CHEMICAL  AND  PHARMACEUTICAL  MANIPULATIONS.  23 
Moulds. — Many  substances  are  used  for  making  moulds  ;  among  the  best 
are  beeswax,  plaster  of  Paris,  fusible  metal,  and  gutta  percha. 
Wax  moulds  are  prepared  by  melting  the  wax  over  a  water-bath,  and 
stirring  in  one  ounce  of  white  lead  to  each  pound  of  wax.  The  wax  should 
be  clear  and  free  from  impurities. 
If  the  object  to  be  copied  is  a  medal,  it  should  be  brushed  over  with  sweet 
oil,  and  the  excess  of  oil  removed  with  a  cloth.  A  slip  of  metal  or  card  is 
bound  round  the  edges  of  the  medal,  so  as  to  form  a  rim.  The  wax  being 
melted,  the  medal,  to  prevent  air-bubbles,  is  held  in  an  inclined  position, 
and  the  wax;  which  should  not  be  too  hot,  poured  gently  on  the  lowest  part, 
and  allowed  gradually  to  spread  over  the  surface  of  the  medal  by  bringing 
it  to  a  level  when  it  is  filled  to  the  top  of  the  rim  with  wax.  As  soon  as  the 
wax  begins  to  set,  the  band  should  be  removed  to  prevent  cracking.  Let 
the  medal  and  wax  remain  together  until  entirely  cold;  so  that  they  may  be 
easily  separated. 
If  it  is  desired  to  take  a  wax  mould  from  a  plaster-cast  or  medallion,  a 
similar  course  is  followed,  the  medallion  being  first  prepared  as  follows : 
the  medallion  is  warmed  a  little,  brushed  over  with  boiled  linseed  oil,  and 
allowed  to  dry  perfectly.  It  then  presents  a  polished  appearance  and  is 
ready  for  the  wax. 
Instead  of  oil,  water  is  often  used  ;  the  plaster  being  saturated  with  it  by 
placing  the  back  of  the  medallion  in  the  water,  care  being  taken  not  to  allow 
the  water  to- flow  over  the  face  of  the  medallion. 
Plaster  of  Paris  moulds  are  made  by  mixing  the  finest  calcined  plaster 
with  water,  to  form  a  thin  paste  about  the  consistence  of  cream.  A  little 
of  this  paste  is  poured  upon  the  object  and  well  brushed  into  every  part 
with  a  camel's  hair  brush,  and  then  more  of  the  paste  is  added  to  produce 
the  requisite  thickness.  It  is  allowed  to  set  and  dry;  the  drying  can  be 
facilitated  by  heating  in  an  oven  or  otherwise. 
The  fusible  metal  of  which  moulds  are  frequently  made  is  an  alloy  of 
live  parts  of  lead,  three  of  tin,  and  eight  of  bismuth,  and  melts  below  212°  F. 
Care  and  practice  are  requisite  for  producing  a  good  and  sharp  casting ; 
and  the  metal  must  not  be  poured  too  hot.  Commence  by  pouring  sufficient 
of  the  melted  alloy  into  a  suitable  vessel, — taking  the  precaution  to  skim 
the  dross  from  the  surface  of  it  with  a  card, — and  when  it  is  nearly  con- 
gealed, bring  the  matrix  down  upon  it  quickly  and  with  considerable  force, 
and  let  it  remain  until  the  mass  has  perfectly  cooled.  When  done  with 
skill,  a  reverse  will  be  obtained  with  all  the  sharpness  and  perfection  of 
the  original. 
Gutta  percha  is  probably  the  substance  best  adapted  for  taking  moulds 
for  electrotyping.  It  is  applicable  to  metal,  wood,  glass,  stone,  &c.  It  needs 
only  to  be  softened  by  heat  either  in  warm  water  or  by  a  steam-bath,  spread 
into  suitable  form,  laid  and  pressed  upon  the  object  to  be  copied,  and  allow- 
ed to  cool  under  the  pressure,  when  the  mould  will  be  fit  for  use. 
Sulphur  is  sometimes  used  for  moulds  ;  and  very  beautiful  impressions 
