﻿634 
  W^EW 
  YORK 
  STATE 
  MUSEUM 
  

  

  of 
  better 
  grades 
  of 
  ware, 
  such 
  as 
  front 
  brick 
  and 
  terra 
  cotta, 
  the 
  

   preparation 
  of 
  the 
  clay 
  is 
  often 
  a 
  matter 
  of 
  the 
  greatest 
  importance, 
  

   in 
  order 
  to 
  provide 
  a 
  mass 
  of 
  material 
  which 
  will 
  be 
  homogeneous 
  

   throughout, 
  and 
  whose 
  physical 
  properties 
  shall 
  not 
  vary. 
  It 
  some- 
  

   times 
  happens 
  that 
  this 
  operation 
  means 
  simply 
  the 
  breaking 
  up 
  of 
  

   the 
  clay 
  thoroughly 
  or 
  the 
  loosening 
  of 
  all 
  the 
  clay 
  particles. 
  The 
  

   greater 
  the 
  care 
  with 
  which 
  these 
  operations 
  are 
  carried 
  on 
  the 
  

   more 
  homogeneous 
  will 
  be 
  the 
  material 
  and 
  the 
  better 
  the 
  grade 
  

   of 
  the 
  wares 
  produced. 
  

  

  Removal 
  of 
  foreign 
  matter 
  

  

  This 
  can 
  be 
  sometimes 
  rendered 
  harmless 
  either 
  by 
  distributing 
  

   it 
  in 
  a 
  finely 
  divided 
  condition 
  through 
  the 
  clay, 
  or 
  by 
  the 
  addition 
  

   of 
  chemicals, 
  or 
  sometimes 
  it 
  may 
  be 
  removed 
  entirely, 
  the 
  method 
  

   employed 
  depending 
  on 
  the 
  character 
  of 
  the 
  clay. 
  

  

  Cleansing' 
  clay. 
  This 
  includes 
  the 
  removal 
  of 
  roots, 
  pyrite, 
  lime 
  

   pebbles, 
  and 
  similar 
  substances. 
  The 
  simplest 
  method 
  is 
  by 
  hand- 
  

   picking, 
  which 
  is 
  slow 
  and 
  incomplete. 
  The 
  custom 
  followed 
  at 
  

   the 
  present 
  day 
  is 
  either 
  to 
  dry 
  the 
  clay 
  and 
  pass 
  it 
  through 
  a 
  sieve 
  

   of 
  the 
  proper 
  mesh, 
  or 
  to 
  treat 
  it 
  to 
  a 
  washing 
  process 
  or 
  even 
  to 
  

   an 
  air 
  separation. 
  

  

  Cleaning 
  dried 
  clay. 
  Most 
  clays 
  are 
  naturally 
  moist, 
  but 
  when 
  

   occurring 
  in 
  the 
  form 
  of 
  shale 
  the 
  percentage 
  of 
  water 
  is 
  usually 
  

   very 
  low; 
  very 
  sandy 
  clays 
  are 
  also 
  apt 
  to 
  run 
  low 
  in 
  moisture. 
  

   With 
  dried 
  clays, 
  the 
  purification 
  can 
  be 
  accomplished 
  by 
  first 
  pul- 
  

   verizing 
  the 
  material, 
  and 
  then 
  allowing 
  the 
  product 
  to 
  fall 
  through 
  

   a 
  strong 
  air 
  current, 
  the 
  effect 
  of 
  this 
  being 
  to 
  separate 
  the 
  particles 
  

   according 
  to 
  their 
  specific 
  gravity, 
  those 
  of 
  clay 
  being 
  carried 
  far- 
  

   thest, 
  while 
  heavier 
  particles, 
  such 
  as 
  pyrite, 
  are 
  dropped 
  first, 
  a 
  

   fairly 
  complete 
  separation 
  taking 
  place. 
  

  

  Wet 
  process 
  of 
  purification. 
  This 
  is 
  done 
  by 
  subjecting 
  the 
  clay 
  

   to 
  a 
  washing 
  process. 
  {See 
  " 
  Preparation," 
  p. 
  799) 
  

  

  Separation 
  of 
  iron 
  particles. 
  In 
  the 
  manufacture 
  of 
  certain 
  

   products, 
  and 
  also 
  certain 
  glazes, 
  it 
  is 
  necessary 
  that 
  the 
  material 
  

  

  