﻿CLAYS 
  OF 
  NEW 
  YORK 
  811 
  

  

  coefficient 
  of 
  expansion 
  may 
  also 
  be 
  diminished 
  if 
  the 
  percentage 
  

   of 
  boracic 
  acid 
  in 
  the 
  clay 
  is 
  increased 
  at 
  the 
  expense 
  of 
  the 
  silica. 
  

   The 
  amount 
  of 
  alumina 
  exerts 
  but 
  little 
  influence 
  on 
  the 
  expansion 
  

   or 
  contraction 
  of 
  the 
  glaze, 
  but 
  a 
  small 
  percentage 
  of 
  alumina 
  prer 
  

   vents 
  glazes 
  which 
  are 
  poor 
  in 
  alkalis 
  from 
  becoming 
  opaque. 
  

  

  The 
  tenacity 
  of 
  adherence 
  of 
  the 
  glaze 
  to 
  the 
  body 
  depends 
  on 
  

   the 
  composition 
  of 
  both 
  and 
  also 
  on 
  the 
  temperature 
  of 
  the 
  kiln. 
  

  

  We 
  can 
  say 
  that 
  the 
  power 
  of 
  the 
  body 
  to 
  carry 
  a 
  glaze 
  without 
  

   causing 
  it 
  to 
  crack 
  is 
  influenced 
  by 
  its 
  rational 
  composition, 
  its 
  

   degree 
  of 
  plasticity, 
  the 
  fineness 
  of 
  the 
  quartz 
  grains 
  which 
  it 
  con- 
  

   tains, 
  and 
  the 
  temperature 
  at 
  which 
  it 
  is 
  burned. 
  

  

  Burning 
  wliite 
  earthenware 
  and 
  china 
  

  

  This 
  is 
  done 
  in 
  saggers, 
  which 
  are 
  oval 
  or 
  cylindric 
  receptacles 
  

   about 
  20 
  inches 
  in 
  diameter, 
  8 
  inches 
  in 
  hight, 
  with 
  a 
  flat 
  bottom. 
  

   The 
  saggers 
  are 
  filled 
  with 
  the 
  pieces 
  of 
  the 
  unburned 
  ware 
  and 
  

   are 
  set 
  one 
  on 
  top 
  of 
  the 
  other, 
  so 
  that 
  the 
  bottom 
  of 
  one 
  forms 
  a 
  

   cover 
  for 
  the 
  one 
  below 
  it, 
  the 
  joint 
  between 
  them 
  being 
  closed 
  by 
  

   means 
  of 
  a 
  strip 
  of 
  soft 
  clay. 
  The 
  use 
  of 
  these 
  saggers 
  is 
  to 
  protect 
  

   the 
  ware 
  from 
  the 
  smoke 
  and 
  gases 
  of 
  the 
  kiln 
  fire, 
  which 
  would 
  

   tend 
  to 
  discolor 
  it. 
  

  

  The 
  requisite 
  of 
  a 
  sagger 
  clay 
  is 
  that 
  it 
  stand 
  slightly 
  more 
  heat 
  

   than 
  the 
  ware 
  placed 
  in 
  it. 
  Saggers 
  are 
  generally 
  .made 
  from 
  a 
  

   plastic, 
  refractory 
  clay, 
  with 
  as 
  great 
  an 
  admixture 
  of 
  grog 
  (ground 
  

   up 
  fire 
  brick 
  or 
  old 
  pottery) 
  as 
  possible, 
  but 
  an 
  excess 
  of 
  the 
  latter 
  

   is 
  deleterious. 
  The 
  color 
  burning 
  properties 
  of 
  a 
  sagger 
  clay 
  are 
  

   of 
  little 
  importance. 
  Saggers 
  are 
  made 
  in 
  various 
  ways, 
  sometimes 
  

   being 
  turned 
  on 
  a 
  wheel, 
  or 
  again 
  being 
  formed 
  in 
  plaster 
  molds, 
  or 
  

   around 
  wooden 
  forms. 
  In 
  Germany 
  metal 
  formB 
  are 
  now 
  mostly 
  

   used, 
  because 
  they 
  permit 
  the 
  working 
  of 
  a 
  stiffer 
  mass, 
  and, 
  the 
  

   clay 
  containing 
  less 
  water, 
  the 
  saggers 
  after 
  molding 
  shrink 
  and 
  

   tear 
  less, 
  while 
  in 
  addition 
  they 
  dry 
  more 
  quickly. 
  The 
  interior 
  of 
  

   the 
  sagger 
  is 
  frequently 
  coated 
  with 
  a 
  slip 
  of 
  kaolin 
  and 
  quartz, 
  in 
  

   order 
  that 
  the 
  ware 
  may 
  not 
  receive 
  any 
  discoloration 
  from 
  this 
  

  

  