When inspecting the boiler, you may find cracks or holes in the furnace lining. To make necessary repairs, mix some of the fire clay you used for brick mortar into a thick mixture. Use more mortar than you used for the brick mortar mix. Use a trowel to apply this wash. While   standard   firebrick   generally   is   used   for normal  refractory  work,  plastic  firebrick  is  recom- mended  for  emergency  patches  and  for  building  up furnace openings. Plastic firebrick is unfired firebrick in  a  stiff  plastic  condition.  It  offers  a  particular advantage in that, because of its plastic nature, it can be pounded  into  places  where  otherwise  a  firebrick  of special  shape  would  be  required.  The  fusion  point  of plastic firebrick is practically equal to that of standard firebrick.  Because  of  the  moisture  in  the  plastic material, however, a greater degree of shrinkage takes place.  This  factor  prevents  its  general  use  for sidewalls. It provides an excellent material, though, for repairing brickwork, topping off side and back walls, repairing  and  constructing  the  burner  openings  and, general,  for  any  part  of  the  furnace  not  exposed  to temperatures  in  excess  of  2000°F.  It  is  particularly adapted  for  use  in  place  of  specially  formed  brick  of complicated  shapes. Plastic  firebrick  material,  as  received  from  the factory,   ordinarily   contains   enough   moisture   for working.  Avoid  the  addition  of  water  or  any  foreign material. In laying up, chunks of plastic just as taken from  the  can  should  be  rammed  tightly  into  place (preferably in horizontal layers). In general, the more solidly the section of plastic is rammed up, the better it will be. As  the  next  step,  the  plastic  section  should  be vented  with  3/16-inch  holes.  Ensure  that  the  holes extend clear through the plastic and are not more than 2 inches  apart.  This  positioning  allows  deeper  heat penetration   during   the   baking-out   process.  It  also permits  ready  escape  of  the  steam  formed  from  the moisture in the plastic. Do NOT trowel the surface of a new plastic section. This tends to prevent the escape of steam during baking out. The plastic section should be held in place with as many anchor bolts as would have been provided had standard  firebrick  been  used  instead  of  plastic.  The plastic section should be air-dried. This takes from 48 to 72 hours, depending upon the atmosphere. As soon as  practicable  after  air  drying,  the  furnace  should  be fired  with  a  small  fire  and  gradually  brought  up  to operating temperature to complete baking out. Plastic requires a temperature of about 2900°F to 3000°F for baking  out.  If  small  shrinkage  cracks  open  up,  they should  be  filled  with  fire  clay.  If  large  cracks  occur, they should be filled with plastic. When  used  for  patches,  as  in  the  case  of  brick falling out, the hole should be cleaned out to give at least 4 inches of body thickness to the plastic brick. In building up furnace openings, the use of a metal form is desirable.  However,  it  is  not  absolutely  necessary  if care  is  exercised  in  making  openings  of  the  proper shape and concentric with the atomizer at every point. If  furnace  openings,  as  built,  have  a  smooth  surface, they should be roughened with a stiff wire brush before baking  out. The following ways to maintain newer boilers are recommended. The boiler is normally shipped with a completely  installed  refractory.  This  consists  of  the rear head (fig. 2-19), the inner door, and the furnace liner   (fig.   2-20).   Follow   the   instructions   in   the manufacturer's  manual  for  the  boiler  you  are maintaining.   Where   specific   directions   or requirements are furnished, follow them. Normal   maintenance   requires   little   time   and expense  and  prolongs  the  operating  life  of  the refractory.  Preventive  maintenance  through  periodic inspection   keeps   the   operator   informed   of   the condition  of  the  refractory  and  helps  guard  against unexpected downtime and major repairs. Frequent  wash  coating  of  refractory  surfaces  is recommended.   A   high-temperature   bonding   air-dry type of mortar diluted with water to the consistency of light   cream   is   used   for   this   purpose.   Recoating intervals  vary  with  operating  loads  and  are  best determined by the operator when the heads are opened for inspection. Maintenance  consists  of  occasional  wash  coating of the entire liner. Face all joints or cracks by applying high-temperature bonding mortar with a trowel or use your  fingertips.  This  should  be  done  as  soon  as  the cracks   are   detected.   Should   segments   of   the   liner become  burned  out  or  broken,  replace  the  entire refractory. Any refractory that may break out should be removed as soon as detected, so it will not fuse to the bottom of the furnace and obstruct the burner flame. Remove  the  existing  refractory  and  thoroughly clean that portion of the furnace covered by the liner to remove  all  old  refractory  cement  or  other  foreign material  to  ensure  the  new  liner  seats  firmly  to  the steel.  Inspect  all  furnace  metal  for  soundness.  There may be metal clips welded in the furnace at the extreme end of the liner. These clips were installed to prevent shifting  during  original  shipment  and  serve  no  other 2-15