536  CHEMICAL  STONE-WARE  MANUFACTURE. 
combination  with  a  certain  amount  of  the  Devon  and  Dorset  clay 
for  the  larger  vessels. 
The  articles  intended  for  the  use  of  chemical  manufacturers 
require  great  care  in  making  and  burning,  both  as  to  the  proper 
proportions  of  the  materials  employed  and  their  due  admixture. 
These  vessels  have  frequently  to  resist  the  action  of  the  strong- 
est acids,  and  that  at  a  high  temperature,  consequently  it  is 
absolutely  necessary  that  they  should  admit  of  a  certain  amount 
of  expansion  and  contraction  without  breaking.  This  quality  is 
obtained  by  the  mixtures  of  complex  materials,  that  of  each 
manufacturer  varying  from  the  other. 
The  clays  employed  at  these  potteries  are  sent  from  the  pits 
in  cubical  tesses,  weighing  usually  about  thirty-five  pounds  each. 
After  having  been  well  dried  these  are  reduced  to  powder  in  a 
crushing  mill.  The  due  admixture  requisite  to  form  the  articles 
required  is  then  thrown  into  a  pug  mill,  which  is  an  upright 
cylinder,  about  six  feet  deep  by  two  in  diameter,  having  a  per- 
pendicular shaft  running  through  it.  This  shaft,  which  is  caused 
to  rotate  by  steam  power,  has  a  number  of  blades  set  on  it,  form- 
ing a  kind  of  screw.  These  arms  or  blades  so  work  amongst  the 
powdered  clays,  (fcc,  &c.,  as  to  mix  them  intimately  with  the 
water  admitted,  at  the  discretion  of  the  workman,  into  the 
cylinder  through  a  pipe,  and,  by  their  continued  action,  to  force 
it  out  of  the  bottom  in  a  plastic  state  of  such  uniform  consistency, 
and  so  free  from  bubbles  of  air,  that  it  is  fit  for  the  thrower, 
who  fashions  it  into  the  required  form  on  the  potter's  wheel. 
This  is  a  kind  of  lathe,  formed  of  a  shaft,  having  a  vertical  in- 
stead of  ihe  usual  horizontal  position.  On  the  upper  extremity 
of  the  upright  shaft  of  this  lathe  is  fixed  a  small  circular  disk, 
which  revolves  with  it.  On  this  disk  the  workman  places  the 
lump  of  prepared  clay,  and,  by  pressure  with  his  hands,  aided 
by  a  few  simple  tools,  fashions  this  yielding  material  into  any 
required  shape  with  a  truly  marvellous  degree  of  dexterity  and 
rapidity.  It  is  with  the  aid  of  this  simple  contrivance  that  al- 
most all  circular  articles  are  made,  from  penny  ink-bottles, 
which  are  sold  at  2s.  6d.  per  gross,  to  the  enormous  vessels 
manufactured  by  Messrs.  Clifi",  some  of  which  are  capable  of 
holding  800  gallons. 
The  vessels  having  been  made,  are  placed  in  the  drying-room  to 
