The  work  of  the  hardeners'  shop  is  practically  confined  to
the  following  abstractors  of  heat  : —

Ordinary  water  (Hard)  Oil,  Mineral  (Heavy)

Soft  or  rain-water  Oil,  Cotton-seed

Sea-water  Oil,  Linseed  (Gummy)

Salted  water  (Brine)  Oil,  Whale  (Expensive)

Acidulated  water  (Sulphuric)  Oil,  Lard

Spray  of  water  Oil,  Fish  (Offensive  odours)

Mercury  (Rapid)  Oil,     Mixed     (Proprietary  uninflam-

mable)

Compressed  air  For  tungsten  steel,  Light   kerosene

group

Taking  water  first,  as  being  the  most  generally  useful,  it
must  be  understood  that,  whether  this  is  hard  and  limey
or  pure  and  soft,  as  rain-water,  does  make  some  difference,
especially  in  treating  tool  steels.  Case-hardened  steels  are
of  less  importance,  and  they  are  almost  invariably  cooled
out  in  the  ordinary  service  water  of  the  district.  Fine  tool
quenching  is  always  conducted  upon  a  more  restrictive
scale,  and  as  it  is  not  so  necessary  to  keep  a  circulation  in
the  tanks  no  large  supply  is  required.  All  water  used  for
quenching  must  be  kept  clean,  and  free  from  oil  or  grease.
Salted  Water  or  Brine  may  be  made  of  any  density.
But  this  and  sea- water  are  rapid  coolers.  The  heat  conduc-
tivity of  brine  is  greater  than  that  of  plain  water,  so  much
so  that  it  is  seldom  used  upon  steel  liable  to  crack  in
hardening  or  cooling  off.  Brine  may  be  used  for  parts
where  there  is  no  liability  to  fracture  or  deformation  due
to  ultra-rapid  chilling.  The  advantage  gained,  if  any,  is
entirely  in  reference  to  the  harder  skin  secured  upon  the
wrork.
