ADeimber^i9aor2n'}    Fire-Proofing  Treatment  of  Wood.  591 
borax.  Tungstate  of  soda  also  figured  at  an  early  date  in  the  list 
of  fire-proofing  salts  as  well  as  the  salts  of  zinc  and  the  chlorides  of 
the  alkalies  and  calcium  and  magnesium.  Antedating  all  of  these 
however,  going  back  indeed  to  the  records  of  ancient  Greek  and 
Rome,  was  alum,  which  has  always  been  a  favorite  fire-proofing 
material,  used  both  alone  and  in  admixture  with  other  compounds. 
All  of  these  materials  can  under  circumstances  exert  a  notable 
fire-retarding  effect  and  have  served  as  the  basis  of  a  variety  of 
patented  processes  for  the  treatment  of  wood. 
But  we  must  not  lose  sight  of  the  fact  that  the  problem  of  satis- 
factorily impregnating  wood  for  fire-proofing  purposes  is  a  mechanical 
as  well  as  chemical  one,  and  it  will  be  best  to  look  at  the  mechanical 
side  of  it  for  a  few  moments.  The  typical  apparatus,  until  recently 
employed  everywhere  wherewith  to  saturate  lumber  with  fire-proof- 
ing solutions,  was  a  large  cylinder,  running  from  60  inches  diameter 
and  70  to  80  feet  long  to  84  inches  diameter  and  105  feet  long, 
closed  at  one  end,  with  a  movable  head  at  the  other,  swinging  hori- 
zontally or  lifting  vertically  to  open  or  close.  It  was  fastened  when 
closed  by  a  complicated  system  of  radial  bolts  to  the  external  end 
of  the  cylinder.  The  cylinder  itself,  composed  of  steel  plates  riveted 
together,  was  intended  to  be  filled  with  truck  loads  of  lumber,  and 
when  the  entrance  door  was  closed  and  locked  the  wood  was  sub- 
jected, after  some  preliminary  treatment,  to  hydraulic  pressure 
through  the  medium  of  the  treating  solution,  which  envelops  the 
surface  of  each  piece  of  lumber  and  which  the  pressure  was  intended 
to  force  into  it  at  every  point. 
With  cylinders  of  such  enormous  diameters  and  riveted  plates, 
the  pressure  that  can  be  withstood  is  relatively  light  and  as  a  Con- 
sequence the  time  of  saturation  is  necessarily  long. 
The  preliminary  treatment  before  referred  to  is  usually  a  steaming 
of  the  wood  followed  by  application  of  a  vacuum  for  the  purpose  of 
facilitating  the  final  step  of  impregnation.  A  pressure  of  150  pounds 
is  quite  as  much  as  can  be  maintained  as  an  average  in  such  a 
cylinder,  and  to  effect  a  complete  saturation,  even  with  soft  woods 
I  inch  thick,  requires  in  such  a  case  from  36  to  40  hours.  A  core 
saturation  in  heavier  timbers,  such  as  4  x  4  inches  or  6  x  6  inches, 
is  rarely  if  ever  obtained  even  in  soft  woods,  and  never  in  the  hard 
woods. 
A  radical  improvement  upon  this  method  of  working  was  effected 
