PROCESS  FOR  TINNING  METALS. 
269 
and  the  bath  may  be  kept  saturated  with  tin  by  means  of 
anodes  of  tin,  by  the  action  of  the  galvanic  current.  If  it  be 
observed  that  the  bath  does  not  deposit  sufficient  metal,  a  cer- 
tain quantity  of  chloride  of  tin  may  be  added  to  it ;  this  at  first 
forms  a  white  precipitate,  which,  however,  is  again  dissolved.  A 
bath  of  this  description,  which  had  been  constantly  employed 
for  a  fortnight  in  tinning,  required  no  addition  of  pyrophosphate, 
so  that  it  might  be  expected  that  nothing  of  the  kind  would  be 
necessary  even  for  a  much  longer  time.  This  process  appears  to 
be  the  only  one  proper  for  protecting  zinc  employed  in  roofing, 
in  sugar  moulds  and  kitchen  utensils,  from  oxidation. 
Oast  iron  tinned  in  this  manner  exhibits  a  fine  silver-like  ap- 
pearance.   The  fluid  for  this  purpose  is  prepared  with — 
Distilled  water  or  rain-water  500  litres 
Pyrophosphate  of  soda  6  kilogrms. 
Commercial  tin-salt  1 
Dried  and  fused  tin-salt  li 
According  to  the  strength  of  the  alkaline  reaction  of  pyro- 
phosphate of  soda,  which  is  not  always  of  the  same  composition, 
the  quantities  of  the  fused  and  acid  tin-salt  must  be  varied.  The 
bath  must  be  kept  at  a  temperature  of  108°-186°  F.  The  au- 
thors consider  this  composition  to  be  the  best,  as  its  slight  alka- 
linity precludes  the  disadvantage  attending  the  use  of  an  acid 
bath,  which  is  favorable  to  oxidation,  whilst  it  does  not,  like  the 
strongly  alkaline  baths,  deposit  the  tin  of  a  bluish  color,  nor 
require  much  washing  to  get  rid  of  its  taste. 
At  first  the  authors  employed  a  separate  galvanic  battery,  but 
it  appears  that  this  is  only  necessary  in  coating  zinc  with  tin. 
For  other  metals  it  is  sufficient  to  immerse  these,  previously  well 
cleaned,  in  the  bath,  together  with  some  pieces  of  zinc,  when 
they  will  be  covered  with  a  dull  coating  of  tin  in  the  course  of 
two  or  three  hours.  This  may  be  polished  with  a  wire-brush. 
If  the  coating  of  tin  is  required  to  be  thick,  the  objects  must  be 
immersed  several  times.  The  bath  may  be  used  almost  constantly  ; 
it  is  sufficient,  before  introducing  new  objects,  to  add  300  grms. 
of  pyrophosphate  of  soda  and  the  same  quantity  of  the  tin- 
salt.    The  pieces  of  zinc  are  gradually  dissolved. 
The  bath  employed  in  tinning  zinc  has  the  following  composi- 
tion : — 
