For other purposes the rubber is swollen in a solvent, such as coal-tar naphtha, and subsequently masticated; the soft dough is then shaped or spread on cloth, and vulcanised after allowing the solvent to evaporate. Here, again, the properties of the raw rubber are of immense importance. Thus, the more plastic the dough, the less solvent required, and the less there is to drive off before vulcanising. The plasticity of the dough will depend on the plasticity of the raw rubber, and so forth. It is evident that the physical properties of the raw rubber are of great importance. They directly affect the manufacturing operations up to the vulcanising stage, and indirectly affect the results obtained on vulcanising. The actual vulcanising consists of heating the mass of mixed rubber for a definite time and at a definite temperature, each "heat" being chosen to suit the particular mixture. These data are arrived at empirically--that is, by trying a number of "heats" and choosing that which appears the most suitable. The suitability will depend on the nature of the article, the service to which it is to be put, and the time it is intended to last. All vulcanised rubber goods, whatever the process, have a limited life or period during which they can be relied on to give useful service. After a time, vulcanised rubber tends to harden, cracks appear on the surface when the article is bent or stretched, and eventually the rubber becomes rotten and "perished." This tendency varies with the quality of the original raw rubber and the conditions of vulcanising. Before plantation rubber was available, the manufacturers were dependent on inferior wild grades for a great part of their output, and, consequently, the goods made from these inferior rubbers never showed very good mechanical properties and soon deteriorated. The severest critics of plantation rubber have admitted the advantages to the manufacturers of the replacement of the lower wild grades by plantation rubber.[36] But even the best grades give a vulcanised product which rapidly deteriorates if the vulcanisation is carried too far. This results from too long heating, or too high a temperature, and the product is termed "overvulcanised" or "overcured."[37] The appearance of the product is deceptive, as the physical properties are remarkably good if the overvulcanising is not more than 50 to 100 per cent. in excess of the normal cure. Only in the case of very much overvulcanised rubber do we obtain a product which is brittle from the beginning. [36] See Williams, "The Rubber Industry," 1914, p. 284. It must also be remembered that the inferior wild grades were derived from latices often containing a large proportion of "resinous" matter, and which could not yield a really high grade of vulcanised rubber whatever the care and skill employed in preparation.