-"^m 



Heating. 163 



finish or lustre, as well as for other reasons, it was found neces- 

 sary to continue to use them for vulcanizing shoes, clothing, 

 and other fabrics, after the steam-heaters were adopted, and 

 preferred for many other articles. The difficulty of distributing 

 dry heat evenly throughout an oven, is one that all who have 

 had experience in baking of any kind are familiar with. The 

 same thing may be said of the heating of dwellings. The diffi- 

 culty is much increased at the high temperature necessary for 

 vulcanizing. 



This obstacle, however, proved to be trifling compared with 

 the losses and discouragements that at first attended the manu- 

 facture in consequence of the fabrics being blistered in heating ; 

 but as the causes of this effect have been particularly stated at 

 the close of the chapter, it is not necessary to repeat them in 

 this place. 



The expedients that are resorted to for heating different 

 fabrics, and also for giving the articles their respective shapes, 

 are very various ; this is unavoidable, as all the articles, unless 

 it be those that are made of the very thin fabrics, retain after- 

 wards the shape that is given them when vulcanized. The 

 gum softens in the oven, so that, when in sheets before it is 

 vulcanized unsupported by being spread on cloths, it will not 

 bear its own weight ; for this reason also it becomes necessary 

 to support many articles, such as shoes and garments, on lasts 

 and forms. 



These, however, were of no avail in the vulcanizing of hollow 

 ware, such as balls, &c. ; and this was not attempted until 1848, 

 when the writer invented the process of heating them by the 

 expansion of air contained in the articles placed in moulds. 

 Further reference is made to this invention in a subsequent 

 article on moulding. To attain all that appeared desirable to 

 render the heating of every description of goods practicable, 

 there was yet needed some means of heating gum-elastic 

 veneers and small articles made of the hard compounds, such 

 as buttons, covered buckles, &c., the former of which could 

 not be treated like other sheets of gum ; and the expense of a 



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