﻿516 
  NEW 
  YOKK 
  STATE 
  MUSEUM 
  

  

  The 
  iron 
  oxids, 
  limonite 
  and 
  hematite, 
  are 
  present 
  in 
  nearly 
  all 
  

   clays. 
  They 
  may 
  be 
  introduced 
  by 
  percolating 
  waters, 
  or 
  result 
  from, 
  

   the 
  decomposition 
  of 
  any 
  of 
  the 
  iron-bearing 
  silicates, 
  such 
  as 
  horn- 
  

   blende, 
  mica 
  or 
  garnet. 
  They 
  are 
  not 
  infrequently 
  distributed 
  

   through 
  the 
  clay 
  in 
  a 
  very 
  finely 
  divided 
  condition, 
  or 
  may 
  form 
  a 
  

   thin 
  film 
  around 
  the 
  other 
  mineral 
  grains. 
  Limonite 
  tends 
  to 
  color 
  

   the 
  unbumed 
  clay 
  brown 
  or 
  yellow, 
  while 
  hematite 
  imparts 
  a 
  red 
  

   color. 
  Ferric 
  carbonate 
  may 
  give 
  gray 
  tints. 
  Mica 
  is 
  found 
  in 
  

   most 
  clays. 
  Hornblende 
  and 
  garnet 
  are 
  probably 
  wanting 
  in 
  a 
  few, 
  

   Pyrite 
  is 
  present 
  in 
  many 
  clays, 
  specially 
  stoneware 
  and 
  fire 
  clays^ 
  

   its 
  yellow, 
  glittering 
  metallic 
  particles 
  being 
  easily 
  recognizable. 
  

   These 
  particles 
  may 
  be 
  either 
  fine 
  grains, 
  or 
  large 
  lumps, 
  the 
  former 
  

   of 
  which 
  have 
  to 
  be 
  separated 
  by 
  washing, 
  the 
  latter 
  by 
  hand- 
  

   picking. 
  Pyrite 
  alters 
  under 
  the 
  influence 
  of 
  weathering 
  or 
  burn- 
  

   ing 
  to 
  sulfate 
  of 
  iron, 
  which 
  is 
  soluble 
  in 
  water 
  and 
  may 
  indirectly 
  

   or 
  directly 
  act 
  as 
  a 
  discoloring 
  agent 
  on 
  clay 
  wares, 
  provided 
  the 
  

   clay 
  is 
  not 
  burned 
  to 
  vitrification. 
  If 
  burned 
  tO' 
  thisi 
  point 
  however 
  

   the 
  pyrite 
  acts 
  as 
  a 
  flux 
  (and 
  according 
  to 
  Wipplinger-^ 
  a 
  very 
  strong 
  

   one) 
  forming 
  little 
  specks, 
  or 
  larger 
  ones, 
  according 
  to^ 
  size 
  of 
  pyrite 
  

   grains, 
  of 
  fused 
  ferrous 
  alumina 
  silicate. 
  In 
  all 
  iron-bearing 
  

   minerals 
  found 
  in 
  clays, 
  the 
  iron 
  exists 
  in 
  one 
  of 
  two 
  conditions, 
  

   YJz., 
  as 
  ferrous 
  or 
  ferric, 
  and 
  the 
  fusibility 
  of 
  any 
  given 
  clay 
  de- 
  

   pends 
  somewhat 
  on 
  this 
  fact, 
  for 
  the 
  reason 
  that 
  ferrous 
  compounds 
  

   lower 
  the 
  fusing 
  point 
  of 
  a 
  clay. 
  In 
  burning 
  any 
  clay 
  the 
  ferrous 
  

   salt 
  will 
  be 
  changed 
  to 
  the 
  ferric 
  condition, 
  provided 
  the 
  fire 
  is 
  

   oxidizing 
  in 
  its 
  action, 
  but 
  if 
  the 
  action 
  is 
  reducing, 
  the 
  iron 
  

   will 
  remain 
  in 
  the 
  ferrous 
  form. 
  The 
  action 
  of 
  weathering 
  

   agents 
  in 
  nature 
  is 
  often 
  sufiicient 
  tO' 
  oxidize 
  the 
  iron 
  in 
  thci 
  clay, 
  

   so 
  that 
  in 
  most 
  clays 
  more 
  ferric 
  than 
  fe'rrous 
  iron 
  will 
  be 
  found. 
  

   Evidence 
  of 
  this 
  change 
  in 
  the 
  condition 
  of 
  the 
  iron 
  can 
  often 
  be 
  

   detected 
  by 
  the 
  red 
  or 
  yellow 
  color 
  of 
  the 
  upper 
  or 
  more 
  porous 
  

   layers 
  of 
  the 
  clay, 
  the 
  lower 
  layers 
  being 
  colored 
  gray. 
  A 
  gray 
  

   color 
  may 
  at 
  times 
  be 
  produced 
  also 
  by 
  the 
  presence 
  of 
  organic 
  

  

  1 
  Keramik. 
  p. 
  26. 
  

  

  