﻿CLAYS 
  OF 
  NEW 
  YOEK 
  547 
  

  

  It 
  sometimes 
  happens 
  that 
  the 
  clay, 
  instead 
  of 
  shrinking 
  dur- 
  

   ing 
  the 
  burning, 
  appears 
  to 
  expand, 
  and 
  this 
  is 
  specially 
  the 
  case 
  

   in 
  very 
  quartzose 
  clays, 
  for 
  the 
  quartz 
  has 
  the 
  property 
  of 
  expanding 
  

   at 
  high 
  temperatures. 
  If 
  the 
  clay 
  contains 
  a. 
  large 
  amount 
  of 
  

   quartz, 
  the 
  expansion 
  of 
  the 
  latter 
  will 
  not 
  only 
  tend 
  to 
  decrease 
  

   the 
  shrinkage 
  but 
  may 
  even 
  co'unteract 
  it 
  and 
  cause 
  the 
  clay 
  to 
  

   expand. 
  This 
  may 
  sometimes 
  account 
  for 
  the 
  presence 
  of 
  cracks 
  in 
  

   the 
  burned 
  ware. 
  

  

  As 
  the 
  addition 
  of 
  quartz 
  to 
  diminish 
  shrinkage 
  also 
  tends 
  to 
  

   decrease 
  the 
  tensile 
  strength 
  of 
  the 
  clay, 
  there 
  will 
  be 
  a 
  certain 
  

   limit 
  in 
  each 
  case 
  beyond 
  which 
  the 
  addition 
  of 
  quartz 
  must 
  not 
  

   proceed, 
  otherwise 
  the 
  clay 
  will 
  not 
  hold 
  together 
  in 
  molding 
  or 
  

   drying. 
  

  

  Organic 
  matter 
  and 
  combined 
  water 
  tend 
  tO' 
  increase 
  the 
  shrink- 
  

   age 
  in 
  burning, 
  but 
  lime 
  has 
  the 
  opposite 
  tendency, 
  some 
  calcareous 
  

   clays 
  even 
  appearing 
  to 
  swell. 
  

  

  Clays 
  containing 
  a 
  large 
  amount 
  of 
  feldspar 
  will, 
  instead 
  of 
  

   showing 
  a 
  steady 
  shrinkage 
  up 
  to 
  the 
  temperature 
  of 
  complete 
  

   vitrification 
  or 
  sintering, 
  often 
  exhibit 
  a 
  temporary 
  increase 
  of 
  

   volume 
  when 
  the 
  fusing 
  point 
  of 
  the 
  feldspar 
  (about 
  2300° 
  F. 
  for 
  

   orthoclase) 
  is 
  reached. 
  

  

  The 
  shrinkage 
  of 
  most 
  clays 
  in 
  burning 
  does 
  not 
  proceed 
  regu- 
  

   larly 
  and 
  steadily 
  to 
  the 
  temperature 
  of 
  vitrification, 
  for 
  some 
  reach 
  

   their 
  maximum 
  density 
  at 
  a 
  comparatively 
  low 
  temperature, 
  far 
  

   below 
  that 
  at 
  which 
  they 
  vitrify. 
  

  

  Between 
  the 
  points 
  at 
  which 
  the 
  moisture 
  has 
  ceased 
  coming 
  

   oft' 
  and 
  that 
  at 
  which 
  the 
  combined 
  water 
  begins 
  to 
  escape, 
  the 
  

   clay 
  shrinks 
  little 
  or 
  none 
  at 
  all; 
  consequently 
  the 
  heat 
  can 
  be 
  

   raised 
  rapidly 
  in 
  this 
  interval, 
  but 
  above 
  and 
  below 
  these 
  two 
  

   points 
  it 
  must 
  proceed 
  slowly 
  to 
  prevent 
  cracking 
  or 
  warping 
  of 
  

   the 
  ware. 
  

  

  Method 
  of 
  counteracting 
  shrinkage 
  

  

  As 
  many 
  clays 
  shrink 
  to 
  such 
  an 
  extent 
  in 
  drying 
  that 
  they 
  

   crack, 
  it 
  is 
  often 
  found 
  necessary 
  to 
  add 
  materials 
  that 
  will 
  prevent 
  

  

  