﻿550 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  shrinkage 
  of 
  the 
  clay 
  is 
  not 
  only 
  arrested, 
  but 
  the 
  alkalis 
  are 
  also 
  

   volatilized^ 
  whereby 
  the 
  hard 
  burned 
  clay 
  becomes 
  more 
  refractory. 
  

   This 
  is 
  the 
  case 
  in 
  Belgium, 
  where 
  the 
  clay 
  used 
  as 
  chamotte 
  is 
  

   burned 
  tv»dce. 
  (Bischof. 
  2d 
  ed. 
  p. 
  265) 
  

  

  In 
  the 
  case 
  that 
  cast 
  off 
  wares 
  are 
  used, 
  it 
  is 
  necessary 
  to 
  see 
  

   that 
  the 
  pieces 
  have 
  not 
  become 
  impure 
  by 
  any 
  slagging 
  action 
  

   that 
  may 
  have 
  taken 
  place 
  in 
  the 
  kiln. 
  Powdered 
  fire 
  bricks 
  are 
  

   sometimes 
  used 
  as 
  chamotte. 
  

  

  Graphite 
  and 
  coke. 
  These 
  are 
  materials 
  that 
  are 
  often 
  added 
  

   to 
  clay 
  to 
  increase 
  the 
  refractoriness, 
  but 
  they 
  also 
  serve 
  the 
  pur- 
  

   pose 
  of 
  imparting 
  to 
  the 
  ware 
  a 
  greater 
  heat 
  conductivity 
  and 
  

   making 
  it 
  more 
  resistant 
  to 
  changes 
  of 
  temperature. 
  It 
  is 
  this 
  

   last 
  property 
  that 
  makes 
  a. 
  mixture 
  of 
  clay 
  and 
  graphite 
  specially 
  

   adapted 
  to 
  the 
  manufacture 
  of 
  crucibles. 
  The 
  graphite 
  should 
  con- 
  

   tain 
  90 
  fo 
  or 
  more 
  of 
  carbon. 
  It 
  should 
  also' 
  be 
  intimately 
  mixed 
  

   with 
  the 
  clay. 
  

  

  Coke 
  is 
  sometimes 
  used 
  instead 
  of 
  graphite, 
  but 
  is 
  less 
  refrac- 
  

   tory, 
  and 
  works 
  best 
  in 
  materials 
  which 
  in 
  use 
  are 
  not 
  in 
  contact 
  

   with 
  the 
  air. 
  

  

  Sawdust. 
  This 
  also 
  acts 
  as 
  a 
  diluent 
  but, 
  unlike 
  the 
  others, 
  bums 
  

   when 
  subjected 
  to 
  a 
  high 
  temperaiture, 
  and 
  leaves 
  a 
  ca^dty 
  behind. 
  

   It 
  is 
  therefore 
  necessary 
  that 
  the 
  particles 
  should 
  not 
  only 
  be 
  small, 
  

   but 
  of 
  even 
  size. 
  Sawdust 
  and 
  similar 
  substances 
  leave 
  more 
  or 
  less 
  

   ash 
  behind, 
  whose 
  mineral 
  constituents 
  may 
  act 
  as 
  fluxes. 
  As, 
  

   owing 
  to 
  the 
  formation 
  of 
  these 
  pores, 
  the 
  clay 
  may 
  be 
  somewhat 
  

   weakened 
  or 
  loosened, 
  it 
  is 
  necessary 
  that 
  only 
  very 
  plastic 
  clays 
  or 
  

   those 
  of 
  high 
  tensile 
  strength 
  should 
  be 
  used. 
  

  

  Fusibility 
  

   Change 
  on 
  heating. 
  In 
  the 
  heating 
  of 
  a 
  clay, 
  or 
  subjecting 
  it 
  to 
  

   a 
  gradually 
  rising 
  temperature, 
  it 
  not 
  only 
  shrinks 
  but 
  begins 
  to 
  

   harden. 
  If 
  raised 
  only 
  to 
  a 
  temperature 
  suflicient 
  to 
  drive 
  off 
  all 
  

   moisture, 
  the 
  clay 
  will 
  still 
  be 
  soft 
  enough 
  to 
  permit 
  its 
  being 
  

   scratched 
  with 
  the 
  finger 
  nail. 
  If 
  the 
  temperature 
  is 
  raised 
  still 
  

  

  