﻿CLAYS 
  OF 
  N^EW 
  YOEK 
  813 
  

  

  cooling 
  of 
  porcelain 
  in 
  a 
  muffle 
  kiln 
  the 
  iron 
  is 
  changed 
  back 
  to 
  

   the 
  ferric 
  condition 
  with 
  its 
  accompanying 
  yellow. 
  

  

  When 
  a 
  kiln 
  full 
  of 
  ware 
  is 
  finished, 
  the 
  material 
  at 
  times 
  has 
  

   to 
  be 
  sorted, 
  as 
  it 
  seldom 
  happens 
  that 
  all 
  the 
  ware 
  drawn 
  from 
  the 
  

   kiln 
  is 
  perfect. 
  The 
  sources 
  of 
  flaws 
  in 
  the 
  burned 
  ware 
  may 
  be 
  

   either 
  faults 
  in 
  the 
  body 
  or 
  bad 
  firing. 
  

  

  In 
  connection 
  with 
  body 
  faults: 
  the 
  more 
  plastic 
  and 
  finer 
  

   grained 
  the 
  clay 
  mixture 
  used, 
  the 
  quicker 
  it 
  shrinks 
  in 
  drying; 
  

   masses 
  which 
  are 
  fat 
  shrink 
  more 
  than 
  those 
  which 
  are 
  rich 
  in 
  

   fluxes, 
  such 
  as 
  feldspar, 
  or 
  are 
  very 
  lean. 
  The 
  size 
  of 
  the 
  quartz 
  

   and 
  feldspar 
  grains 
  is 
  of 
  importance, 
  for, 
  if 
  they 
  are 
  in 
  the 
  form 
  of 
  

   fine 
  powder, 
  they 
  are 
  not 
  very 
  plastic, 
  but 
  if 
  ground 
  extremely 
  fine 
  

   they 
  develop 
  a 
  certain 
  amount 
  of 
  pastiness, 
  and 
  this 
  is 
  accompanied 
  

   by 
  an 
  increased 
  shrinkage. 
  If 
  the 
  clay 
  mixture 
  was 
  not 
  properly 
  

   worked, 
  or 
  was 
  too 
  soft, 
  or 
  the 
  thickness 
  of 
  the 
  molded 
  object 
  is 
  not 
  

   the 
  same 
  throughout, 
  or 
  the 
  mechanically 
  combined 
  water 
  is 
  not 
  

   evenly 
  distributed 
  through 
  the 
  material, 
  the 
  ware 
  is 
  very 
  likely 
  to 
  

   warp 
  in 
  burning. 
  The 
  shrinkage 
  may 
  also 
  be 
  uneven 
  if 
  the 
  

   pressure 
  exerted 
  by 
  the 
  molder 
  is 
  not 
  uniform, 
  and 
  cracks 
  occur 
  

   when 
  the 
  molded 
  piece 
  is 
  stronger 
  on 
  one 
  side 
  than 
  on 
  the 
  other. 
  

   Flaws, 
  such 
  as 
  air 
  bubbles, 
  appear 
  only 
  when 
  the 
  ware 
  is 
  burned. 
  

  

  Firing 
  errors 
  are 
  usually 
  due 
  to 
  too 
  quick 
  heating 
  or 
  cooling. 
  

   If 
  cracks 
  are 
  caused 
  in 
  the 
  early 
  part 
  of 
  the 
  burning, 
  they 
  increase 
  

   as 
  the 
  firing 
  proceeds. 
  Cracks 
  formed 
  in 
  the 
  body 
  as 
  a 
  result 
  of 
  

   too 
  rapid 
  cooling 
  are 
  not 
  generally 
  seen 
  with 
  the 
  naked 
  eye, 
  but 
  the 
  

   ware 
  produces 
  no 
  ring 
  when 
  struck. 
  Another 
  cause 
  of 
  cracking 
  is 
  

   an 
  uneven 
  temperature 
  on 
  the 
  two 
  sides 
  of 
  the 
  object. 
  Over 
  burn- 
  

   ing 
  as 
  well 
  as 
  under 
  burning 
  of 
  porcelain 
  tends 
  to 
  produce 
  fine 
  

   cracks 
  in 
  the 
  body. 
  

  

  The 
  glaze 
  is 
  also 
  a 
  source 
  of 
  much 
  worriment 
  to 
  the 
  manufac- 
  

   turer. 
  It 
  should 
  of 
  course 
  have 
  the 
  same 
  coefficient 
  of 
  expansion 
  as 
  

   the 
  body 
  to 
  which 
  it 
  is 
  applied. 
  If 
  under 
  burned 
  tlie 
  glaze 
  mil 
  not 
  

   appear 
  thoroughly 
  glassy 
  and 
  develops 
  fine 
  cracks, 
  but, 
  if 
  over 
  

   burned, 
  a 
  chemical 
  action 
  is 
  apt 
  to 
  take 
  place 
  between 
  the 
  glaze 
  

  

  