22  Hydrofluoric  Acid  in  Glass  Industry.  {^JXy,^!™' 
but  better  results  are  obtained  from  a  three-dip  process  of  5  seconds 
each,  to  be  increased  from  time  to  time  as  the  acid  weakens. 
Spent  Acid  is  that  acid  which,  after  a  day's  work  of  5/6  hours, 
is  reduced  from  60  per  cent,  to  30  per  cent.,  which  can  be  carried 
over  each  succeeding  day  by  means  of  drawing  off  the  acid  in  a  re- 
serve tank,  avoiding  the  possibility  of  conflict  with  the  mud  which 
deposits  at  the  bottom  of  tank — the  stronger  acid  can  be  added  to 
the  weaker  until  contact  is  obtained;  by  this  method  you  establish 
a  means  of  unceasing  use  without  waste  of  a  single  pound  of  ma- 
terial. 
Soda  Glass. — This  is  inclined  to  be  more  or  less  brittle  and  re- 
quires careful  treatment — to  one  just  starting  in  to  acquire  a  good 
polish  precaution  is  necessary  in  the  beginning  until  the  operation 
completely  satisfies  the  dipper.  We  would,  therefore,  advise  that 
a  mixture  of  2  parts  hydrofluoric  acid  and  1  part  oil  vitriol  be  used 
in  5  separate  dips  as  follows :  5,  8,  10,  12,  14  seconds  each,  and  by 
this  means  one  can  be  governed  by  their  own  judgment  by  decreas- 
ing the  number  of  dips  to  2  of  from  20/25  seconds  each.  Care  must 
be  taken  in  each  instance  to  not  only  allow  of  a  good  drainage,  but 
the  glass  must  be  thoroughly  dry  before  being  placed  in  each  suc- 
cessive dip,  otherwise  it  will  run  and  leave  blistered  and  blurred 
imperfections. 
Polish. — It  is  the  aim  of  every  cutting  shop  to  establish  the 
best  possible  polish :  this  can  be  had  by  treating  the  glass  as  already 
described — but  by  preparing  the  dipping  acid  the  night  preceding 
the  day's  work,  gives  the  acid  an  opportunity  of  mellowing  by 
means  of  proper  diffusion — a  well-provided  hood  covering  the  tank 
is  essential  in  order  to  prevent  the  escape  of  gas. 
The  Pop. — In  the  acid  so  frequently  referred  to  by  the  dipper, 
all  depends  upon  the  manner  of  care  devoted  to  its  proper  keeping. 
•'It  so  frequently  occurs  that  carboys  containing  acid  are  allowed  to 
remain  at  the  freight  station  or  in  the  outer  yard  when  the  tempera- 
ture is  below  freezing — cold  weather  contracts  and  when  the  stopper 
is  removed,  the  pop  is  lacking,  all  due  to  neglect  of  the  operator. 
Acid  should  at  all  times  be  kept  in  a  warm  place,  under  which  con- 
ditions best  possible  service  is  obtained. 
Lead  Fluoride  is  that  deposit  which  sometimes  settles  at  bottom 
of  carboy,  which  when  agitated  drifts  into  the  dipping  tank  and 
adheres  to  the  glass,  causing  much  inconvenience;  care,  therefore, 
