IMPRESSION    MATERIALS.  5^1

IMPRESSION    MATERIALS

Must  possess  the  following  properties:  (1.)  Plasticity  in  sufficient
degree  to  copy  mucous  tissues,  avoiding  the  extremes  oi'  softness,  which
permits  them  to  flow  from  the  cup,  and  of  haidiiess,  which  requires
excessive  pressure.  (2.)  The])roperty  of  hardening  within  a  siiort  time,
and  under  conditions  not  incompatible  with  tlu'  ujouth.  (8.)  Absence
of  expansion  or  contraction,  except  in  very  nioiK'rate  degree.  It  may
also  be  added  that  the  materials  sliould  not  be  such  as  in  taste,  smell,
or  appearance  are  calcuUxted  to  disgust  a  patient.
There  are  three  materials  answering  to  these  requirements,  and  pos-
sessing properties  as  distinctive  as  the  sources  whence  they  are  derived.
From  the  Animal  kingdom,  Beeswax  ;  from  the  Vegetable  kingdom,
Gutta-Percha  ;  from  the  Mineral  kingdom,  Plaster.  After  their
separate  description,  a  brief  review  of  their  distinctive  properties  will
be  given.  No  one  of  the  three  can  be  dispensed  with  ;  no  one  should
be  exclusively  used.
Beeswax.  —  Formerly  the  only  material  used.  It  is  still  the  only
one  fit  for  certain  cases,  and  is  absolutely  indispensable.  The  best
wax  is  from  virgin  combs,  and  has  a  rich  golden  color.  Commercial
adulterations  with  tallow,  etc.,  injure  it,  and  mixture  with  resin  makes
it  harsh  and  difficult  to  manage.  Gutta-percha  is  sometimes  incor-
porated with  it  to  give  hardness  in  warm  weather ;  bleached  or  white
wax  is  also  used  for  the  same  purpose.
