109 



removed in the serum by the machining of fresh coagulum has 

 not yet been ascertained, but is probable. In support of this we 

 have the fact that Fine Hard Para which is comparatively slow 

 curing, contains all the serum constituents and should be rapidly 

 curing, but the vulcanization is retarded by the smoking process. 



Dealing briefly with the factors involved in the variability 

 of the first type of rubber in which the vulcanization is accelerated 

 by the amount of some protein degradation product formed during 

 the maturing of the coagulum by natural biological processes, the 

 following factors accelerate rate of cure : 



(1) High moisture content up to a certain limit ; 



(2) Large amount of serum ; 



(3) Thickness of coagulum up to a certain limit, which 



afEects the rate of drying ; 



(4) Slow drying ; 



(5) Concentration of latex ; 



(6) Action of certain chemicals, especially the alkalis, soda 



and potash and their carbonates. 

 The following factors retard the rate of vulcanization : 



(1) Low moisture content in the prepared coagulum ; 



(2) Removal of serum ; 



(3) Rapid drying ; 



(4) Thinness of coagulum which produces rapid drying and 



is also responsible for the x'emoval of serum by 

 machining ; 



(5) Presence of antiseptics, especially formalin, also boi'acic 



acid and tannic acid ; 



(6) Certain coagulants, e.g., the mineral acids especially 



hydrochloric, sulphuric and hydrofluoric acids and 

 certain salts especially the alums ; 



(7) Excess of coagulant — the efEect of excess of weak organic 



acids, such as acetic, formic, citric, tartaric and oxalic 

 is not very marked, while excess of the mineral acids 

 has a marked retarding effect ; 



(8) Smoking ; 



(9) Artificial methods of drying — not very great in the case 



of thin crepe, which is the only type of rubber subjected 

 to artificial methods of drying ; 



(10) Dilution of latex with water ; 



(11) Action of water as a solvent. 



With i"egard to the second factor responsible for variability in 

 i-ate of cure the influence of various factors in retarding the rate of 

 cure has not yet been investigated, but it is probable that smoking, 

 mineral acid coagulants and alum and other antiseptics and chemicals 

 will retard vulcanization. 



