﻿CLAYS 
  OF 
  NEW 
  YOEK 
  519 
  

  

  If 
  the 
  action 
  of 
  the 
  fire 
  is 
  oxidizing, 
  the 
  presence 
  of 
  ferrous 
  salts 
  

   need 
  not 
  be 
  considered, 
  provided 
  tlie 
  lieat 
  is 
  raised 
  high 
  enough 
  to 
  

   oxidize 
  them. 
  

  

  The 
  rapidity 
  with 
  which 
  the 
  temperature 
  is 
  raised 
  is 
  important, 
  

   for 
  if 
  the 
  heat 
  is 
  raised 
  too 
  quickly 
  the 
  outer 
  portion 
  of 
  the 
  clay 
  

   may 
  shrink 
  and 
  become 
  dense 
  before 
  the 
  air 
  has 
  had 
  time 
  to 
  per- 
  

   meate 
  the 
  clay 
  and 
  oxidize 
  the 
  iron 
  in 
  the 
  center 
  of 
  the 
  body. 
  This 
  

   is 
  the 
  cause 
  of 
  the 
  black 
  cores 
  sometimes 
  seen 
  in 
  bricks 
  whose 
  sur- 
  

   face 
  is 
  red. 
  

  

  The 
  same 
  variety 
  of 
  colors 
  seen 
  in 
  the 
  raw 
  clay 
  may 
  be 
  similarly 
  

   produced 
  in 
  the 
  burned 
  clay, 
  the 
  result 
  being 
  conditioned 
  on 
  the 
  

   relative 
  amounts 
  of 
  ferrous 
  and 
  ferric 
  compounds. 
  Ferrous 
  oxid 
  

   alone 
  produces 
  a 
  green 
  color 
  when 
  burned, 
  while 
  ferric 
  oxid 
  alone 
  

   may 
  give 
  a 
  purple 
  or 
  red, 
  and 
  mixtures 
  of 
  the 
  two 
  may 
  produce 
  

   yellow, 
  cherry 
  red, 
  ^dolet, 
  blue 
  and 
  black.-^ 
  The 
  more 
  intense 
  the 
  

   heat, 
  the 
  deeper 
  the 
  color 
  produced 
  by 
  the 
  iron. 
  At 
  veiy 
  high 
  

   temperatures 
  it 
  is 
  difficult 
  or 
  impossible 
  tO' 
  obtain 
  an 
  oxidizing 
  

   action 
  in 
  the 
  kiln 
  or 
  furnace. 
  

  

  Seger^ 
  found 
  that 
  combinations 
  of 
  ferric 
  oxid 
  with 
  silica 
  pro- 
  

   duced 
  a 
  yellow 
  or 
  red 
  color 
  in 
  the 
  burned 
  clay, 
  while 
  similar 
  com- 
  

   pounds 
  of 
  the 
  ferrous 
  salt 
  showed 
  blue 
  and 
  green. 
  

  

  The 
  black 
  coloration 
  produced 
  by 
  iron 
  oxid 
  in 
  hard 
  firing 
  is 
  

   often 
  to 
  be 
  seen 
  on 
  breaking 
  open 
  the 
  arch 
  bricks 
  of 
  a 
  kiln. 
  The 
  

   surface 
  of 
  such 
  bricks 
  may 
  frequently 
  get 
  black, 
  this 
  being 
  due 
  in 
  

   part 
  to 
  the 
  slagging 
  action 
  of 
  the 
  ashes 
  fro'm 
  the 
  fire 
  which 
  stick 
  to 
  

   them. 
  

  

  The 
  coloration 
  of 
  clays 
  by 
  iron 
  in 
  burning 
  will 
  be 
  farther 
  dis- 
  

   cussed 
  under 
  that 
  head. 
  

  

  The 
  amount 
  of 
  ferric 
  oxid 
  permissible 
  or 
  desirable 
  in 
  a 
  clay 
  de- 
  

   pends 
  on 
  the 
  use 
  to 
  which 
  it 
  is 
  to 
  be 
  put. 
  Kaolins 
  or 
  plastic 
  clays 
  

   to 
  be 
  used 
  in 
  the 
  manufacture 
  of 
  white 
  bodies 
  should 
  contain 
  less 
  

   than 
  Ifo 
  if 
  possible. 
  A 
  greater 
  amount 
  might 
  be 
  present, 
  provided 
  

  

  1 
  Keramik. 
  p. 
  25G. 
  

  

  2 
  NotizUatt. 
  1874, 
  p. 
  16. 
  

  

  