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162 GOODYEAR ON GUM-ELASTIC. 



HEATING. 



Heat is the great agent, when sulphur is present, in pro- 

 ducing the change wrought in the gum, called vulcanizing. 

 It is only by the use of these two agents, sulphur and heat, that 

 any substance has yet been produced, possessing the properties 

 described, as belonging to metallic or vulcanized gum-elastic. 



The ovens or heaters that are made use of in the manufacto- 

 ries, are heated either by steam or hot air let into them upon 

 the goods. The choice of method depends upon circumstan- 

 ces. Hot air ovens are commonly used by the shoe manufac- 

 turers, because a higher finish or lustre is thereby obtained on 

 the goods. Some articles made of colored coated fabrics are dis- 

 colored by the steam, to which the steam heat is not therefore 

 applied ; but for car and other springs, for masses of gum, for 

 drapery, stayed compound, and numerous other articles and 

 fabrics, the steam heat is preferred. 



The time occupied in heating the goods or vulcanizing them, 

 is commonly from four to six hours, during which time the 

 heat, in the hot-air, or steam oven, in which the goods have 

 pi'eviously been placed, is gradually raised to a heat varying 

 from 250 to 300° of Fahrenheit, usually to about 265 or 270°. 

 The different degrees of heat which are applied, depends upon 

 the thickness of the gum, and the greater or less proportion of 

 lead or other substances that are mixed with it. While the heat 

 is being applied in the oven or heater, the gum becomes soft, ap- 

 proaching very nearly a melted state, at about the heat of 248°, 

 after which the gum is hardened, as the heat is increased, until 

 the change in it is complete. 



The successful application of heat, so as to obtain a uniform 

 result, in vulcanizing the goods without blistering them, was a 

 labor attended with extreme difficulty. Dry ovens with heated 

 air were first used ; and for the purpose of obtaining the desired 



