CHEMICAL  STONE- WARE  MANUFACTURE. 
537 
become  thoroughly  dried  ;  after  which,  those  termed  double-glazed 
stone-ware,  such  as  spirit  cans,  druggists'  pots,  &c.,  are  dipped 
in  liquid  glaze,  and  taken  to  the  kiln  to  burn.  This,  as  usually 
constructed,  is  a  large  circular  room,  about  ten  to  fifteen  feet  in 
diameter,  and  fourteen  feet  high.  It  is  lined  with  the  best 
Wortley  fire-bricks,  and  has  at  its  sides  a  number  of  openings 
to  admit  the  heat  from  the  fires.  It  has  also  apertures  in  the 
roof  to  allow  the  escape  of  the  waste  heat  and  smoke.  The 
articles  to  be  burnt  are  placed  in  the  kiln,  piled  up  on  one 
another,  as  is  the  case  with  small  goods,  or,  if  large,  singly  on 
slabs  or  quarries.  The  opening  is  then  built  up  and  carefully 
cemented  over,  when  the  fires  aie  lighted,  and  are  gradually  in- 
creased until  every  article  in  the  interior  becomes  heated  to  an 
intense  white  heat,  which  process  takes,  with  a  fifteen-foot  kiln, 
from  forty  to  fifty  hours,  consuming  ten  tons  of  coal.  If  the 
articles  are  to  be  salt-glazed,  a  quantity  of  very  coarse  salt  is 
thrown  in  through  the  holes  left  in  the  roof  of  the  kiln,  and  into 
each  fire-hole,  shortly  before  the  termination  of  the  firing.  This 
is  at  once  converted  into  vapor  by  the  intense  heat,  and  is  decom- 
posed. The  soda  of  the  salt  unites  with  the  silica  of  the  clay, 
and  forms  a  fusible  glass  or  glaze  upon  the  surface.  So  perfect 
is  this  glazing,  that  articles  thus  protected  have  been  found  to 
resist  for  twenty  years  without  deterioration  the  action  of  the 
nitrous  acid  of  commerce.  After  the  glazing  operation  the  kiln 
is  carefully  stopped,  to  prevent  draughts  or  cold  air,  and  is  then 
allowed  to  cool  for  thirty  to  forty  hours  before  it  is  opened  and 
the  articles  removed. 
For  other  kinds  of  glazing  a  different  process  is  employed,  the 
vessels  being  dipped  into  a  composition  of  materials  that  melt  dur- 
ing the  firing  into  the  kind  of  glaze  required.  It  may  be 
noticed,  that  many  stone-ware  articles  are  of  two  colors — as, 
for  example,  a  brown  top  with  a  bufi'-colored  base ;  the  darker 
top  given  by  the  addition  of  a  proportion  of  manganese  to  the 
glaze. 
Messrs.  Cliflf  have  introduced  several  improvements  into  the 
process  of  firing  in  their  patent  kiln.  For  instance,  the  articles 
are  not  exposed  to  the  direct  action  of  the  fire,  but  are  enclosed 
in  an  inner  kiln,  the  fire  playing  between  the  two,  and  the 
draught  also  traversing  numerous  hollow  pipes  which  are  con- 
