114 SWEETENED CONDENSED MILK SEALING 



genera! use by the tin smith, are not very satisfactory. Unless 

 they are drawn out and filed down into a fine point, their use is 

 not conducive of neat work, progress is comparatively slow and 

 leakers are often numerous. When gas is available the automatic 

 soldering copper may be used to advantage. In this tool the 

 copper tip, which is long and slender is automatically heated by 

 a current of gas passing through the handle and burning at the 

 copper tip. The handle of the device is connected with the gas 

 and air pipes by means of flexible rubber tubing. No time is 

 lost waiting, for the copper to heat and the flame can be so 

 regulated that the temperature of the copper tip is right and 

 uniform. This is important, because perfect work is impossible 

 unless the coppers have the proper temperature. 



Machine-soldering is now rapidly replacing hand-soldering. 

 The principle of the older types of soldering machines consisted 

 of revolving discs on which the tin cans were placed. The cap 

 was held in place by a vertical rod pressing on it. The solder 

 was applied by hand, the hot soldering copper was held over 

 the groove in the can while the cans revolved. This method had 

 no particular advantage over the hand soldering. There was 

 little, if any, saving of time and the quality of the work was not 

 much, if any, better. 



There are now on the market newer types of soldering ma- 

 chines, most ingeniously constructed and their operation in fac- 

 tories with large outputs economizes labor and time. 



Solder. The solder used for sealing should be of standard 

 composition. In this country, canning establishments are prone 

 to use a very poor quality of solder. It contains from 45 to 55 

 per cent lead. Lead is a poisonous metal; its use in the canning 

 industry should, therefore, be regulated by law. In Germany, 

 the law requires that solder used in tin cans for food products 

 must not contain over 10 per cent of lead. 



Where the sealing is done by hand the solder is most con- 

 veniently used in the form of thin bars or wire. The wire is 

 usually bought already cut up in segments, each segment furnish- 

 ing solder enough to seal one can. In the newer types of sol- 

 dering machines the solder wire is automatically fed from spools. 



