THE PREPARATION OF RUBBER 71 



(2) Dilution of the Latex ivith Water and allowing it to 

 Stand. — This method can be conveniently used for the 

 preparation of Ceara rubber if the latex flows sufficiently 

 freely to be collected in bulk. The latex is simply diluted 

 with water and allowed to stand for twenty-four hours 

 or so, until coagulation occurs. 



In the case of some latices dilution with water often 

 retards coagulation indefinitely. 



(3) Dilution of the Latex with Water and " Creaming^ — 

 This process is specially applicable to Castilloa latex, 

 which creams very readil3\ The latex is diluted with 

 water and is allowed to stand until the " cream," con- 

 sisting of the rubber globules, rises to the surface. The 

 liquid is run off as completely as possible from the cream, 

 which is then washed by shaking it up with afresh quantity 

 of water and allowed to separate as before. This treat- 

 ment may be repeated several times if desired. The 

 cream is afterwards converted into solid rubber by 

 pressure, by centrifugalising, or by allowing it to dry 

 on wire or calico frames. 



(4) Centrifugal Methods. — In some cases the latex can 

 be quicldy and conveniently coagulated by spinning it 

 in a centrifugal machine. This method is chiefly applic- 

 able to those latices which cream readily, such as Castilloa 

 latex. Special forms of centrifugal machines have been 

 introduced for treating rubber latex. 



(5) Action of Heat. — A large number of latices are 

 coagulated by the application of heat, some requiring 

 to be only gently warmed whilst others have to be boiled 

 before the change occurs. Whenever heat is employed 

 to brmg about coagulation, great care should be taken to 

 avoid overheating the rubber, as if this occurs the product 

 becomes permanently sticky and depreciated in value. If 

 direct heat be used it is advisable to dilute the latex 

 with water, especially if it is at all thick, and to keep 

 it well stirred during the heating ; the rubber may be 

 removed from the liquid in small portions as it separates, 

 and the heating continued until the milldness of the 

 liquid has disappeared. Other methods of preventing 

 overheating which are sometimes applicable are (1) to 

 immerse the vessel contaming the latex in boiling water ; 

 (2) to pour the undiluted latex slowly into a large quantity 

 of boiling water ; or (3) to pass steam into the latex. 



