﻿808 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  o 
  

  

  total 
  shrinkage 
  of 
  14^. 
  Incipient 
  vitrification 
  occurred 
  at 
  1700 
  

   r., 
  complete 
  at 
  1860° 
  F., 
  and 
  viscosity 
  above 
  2000° 
  F. 
  

  

  From 
  the 
  analysis 
  made 
  by 
  H. 
  H. 
  Griffon 
  it 
  is 
  seen 
  that 
  the 
  

   clay 
  approaches 
  closely 
  to 
  the 
  formula 
  : 
  

  

  IKO, 
  .7KA, 
  4Si02 
  

   which 
  is 
  similar 
  to 
  that 
  of 
  an 
  alkaline 
  glaze, 
  but 
  with 
  an 
  excess 
  of 
  

   Rg 
  ^s' 
  -^^^ 
  addition 
  of 
  lead 
  increases 
  this 
  excess 
  of 
  bases, 
  and 
  it 
  

   is 
  necessary 
  to 
  add 
  silica 
  also. 
  

  

  For 
  many 
  years 
  the 
  slip 
  has 
  been 
  used 
  as 
  a 
  glaze 
  without 
  the 
  

   addition 
  of 
  any 
  artificial 
  fluxes, 
  for 
  attempts 
  in 
  this 
  direction 
  had 
  

   always 
  been 
  without 
  success. 
  A 
  number 
  of 
  experiments 
  were 
  

   made 
  by 
  Mr 
  Griffen, 
  to 
  determine 
  in 
  what 
  manner 
  it 
  was 
  possible 
  

   to 
  lower 
  the 
  fusibility 
  of 
  the 
  slip 
  clay, 
  and 
  make 
  it 
  run 
  more 
  easily 
  

   without 
  destroying 
  its 
  richness 
  of 
  color. 
  The 
  addition 
  of 
  lead 
  

   alone 
  gave 
  a 
  transparent 
  and 
  greenish 
  colored 
  glaze, 
  which 
  showed 
  

   a 
  tendency 
  to 
  blister; 
  alkalis 
  added 
  alone 
  gave 
  the 
  same 
  result. 
  

   It 
  is, 
  therefore, 
  necessary 
  to 
  add 
  other 
  materials 
  with 
  the 
  lead. 
  

   Good 
  results 
  were 
  obtained 
  by 
  adding 
  iron 
  alone, 
  but 
  the 
  combina- 
  

   tion 
  of 
  chromium, 
  manganese 
  and 
  iron 
  produced 
  the 
  best 
  effect. 
  

   The 
  chromium, 
  Mr 
  Griffen 
  finds, 
  takes 
  from 
  the 
  iron 
  its 
  tendency 
  

   to 
  run 
  into 
  greenish 
  and 
  yellowish 
  tints. 
  The 
  best 
  form 
  in 
  which 
  

   to 
  introduce 
  the 
  chromium 
  is 
  as 
  chromate 
  of 
  lead, 
  this 
  giving 
  the 
  

   finest 
  color 
  effect 
  ; 
  but, 
  as 
  an 
  excess 
  of 
  this 
  sort 
  also 
  has 
  a 
  tendency 
  

   to 
  cause 
  blistering, 
  it 
  is 
  well 
  to 
  add 
  som6 
  of 
  the 
  chromium 
  in 
  the 
  

   form 
  of 
  chromate 
  of 
  iron. 
  

  

  The 
  following 
  recipe 
  is 
  for 
  a 
  moderately 
  low 
  heat 
  glaze, 
  the 
  

   variation 
  being 
  for 
  different 
  conditions. 
  

  

  Albany 
  slip 
  clay 
  63 
  . 
  30 
  to 
  70 
  

  

  White 
  lead 
  25 
  . 
  30 
  to 
  17 
  

  

  Flint 
  6.30 
  to 
  7 
  

  

  Oxid 
  of 
  iron 
  72 
  to 
  .79 
  

  

  Oxid 
  of 
  manganese 
  .56 
  to 
  .61 
  

  

  Chromate 
  of 
  lead 
  1.27 
  to 
  1 
  . 
  40 
  

  

  Chromate 
  of 
  iron 
  67 
  to 
  .73 
  

  

  Oxid 
  of 
  zinc 
  1.88 
  to 
  2.07 
  

  

  1 
  

  

  