hazard as well as an explosive. Prepare zinc chloride under a ventilation hood, out in the open, or near open- ings to the outside to reduce inhalation of the fumes or the danger of explosion. It is essential that precautions be taken to prevent flames or sparks from coming in contact  with  the  liberated  hydrogen. Another type of corrosive flux in use is known as SOLDERING SALTS. Commercially  prepared  solder- ing salts are normally manufactured in a powder form that is water soluble that allows you to mix only the amount  needed. After a corrosive flux has been used for soldering, you  should  remove  as  much  of  the  flux  residue  as possible from the work. Most corrosive fluxes are water soluble; therefore, washing the work with soap and water and then rinsing thoroughly with clear water usually  removes  the  corrosive  residue.  To  lessen  dam- age, you should ensure the work is cleaned immediately after the soldering. SOLDERING  TECHNIQUES The   two   soldering   methods   most   often   used are  soldering  with  coppers  or  torch  soldering.  The considerations  that  apply  to  these  methods  of  soldering are as follows: 1. Clean all surfaces of oxides, dirt, grease, and other foreign matter. 2. Use the proper flux for the particular job. Some work requires the use of corrosive fluxes, while other work   requires   the   use   of   noncorrosive   fluxes. Remember,  the  melting  point  of  the  flux  must  be BELOW  the  melting  point  of  the  solder  you  are  going to use. 3. Heat the surfaces just enough to melt the solder. Solder does not stick to unheated surfaces; however, you should be very careful not to overheat the solder, the soldering coppers, or the surfaces to be joined. Heating solder above the work temperature increases the rate of oxidation and changes the proportions of tin and lead. 4.  After  making  a  soldered  joint,  you  should remove as much of the corrosive flux as possible. Sweat Soldering Sweat soldering is used when you need to make a joint and not have the solder exposed. You can use this process  on  electrical  and  pipe  connections.  To  make  a sweated  joint,  you  should  clean,  flux,  and  tin  each adjoining  surface.  Hold  the  pieces  firmly  together  and & ,,,  // Figure 6-9.—Soldering a seam. heat the joint with a soldering copper or a torch until the solder melts and joins the pieces together. Remove the source of heat and keep the parts firmly in position until the solder has completely hardened. Cleaning any resi- due from the soldered area completes the job. Seam  Soldering Seam  soldering  involves  running  a  layer  of  solder along the edges of a joint. Solder seam joints on the inside whenever possible. The best method to use for this process is soldering coppers, because they provide better control of heat and cause less distortion. Clean and flux the areas to be soldered. If the seam is not already tacked, grooved, riveted, or otherwise held together, tack the pieces so the work stays in position. Position the piece so the seam does not rest directly on the support. This is necessary to prevent loss of heat to the support. After you have firmly fastened the pieces together, solder the seam. Heat the area by holding the copper against the work. The metal must absorb enough heat from the copper to melt the solder, or the solder will not adhere. Hold the copper so one tapered side of the head is flat against the seam, as shown in figure 6-9. When the solder begins to flow freely into the seam, draw the copper along the seam with a slow, steady motion. Add as much solder as necessary without raising the copper from the work. When the copper becomes cold, you should use the other copper and reheat the first one. Change coppers as often as necessary. Remember, the best soldered seams are made without lifting the copper from the work and Allow  the  joint  to without  retracing  completed  work. cool and the solder to set before 6-6