118 



THE INDIA RUBBER WORLD 



[December 1. 1913. 



The Coagulation of Hevea Latex by Smoking. 



THE subject of coagulation is one that has been treated fre- 

 quently and at length in these columns, but it is always an 

 interesting topic and constantly receiving attention from the 

 rubber experts, among whom is M. V. Cayla, who contributes a 

 detailed discussion of this topic to a recent number of the 

 "Journal d'Agriculture Tropicale." There is not much in his 

 contribution that is strikingly new, but quite a little of it seems 

 well worth reproducing. 



One of the principal points he emphasizes is the question : 

 What factors in the process of smoking produce the admittedly 

 superior quality of Para rubber so sought after in plantation 

 rubber and which is generally attributed to the Amazonian 

 method of smoking? 



With respect to the comparative merits of Para and plantation 

 rubber, some English manufacturers, as a result of factory trials, 

 have pronounced "Smoked Sheets" (sheets smoked after coagula- 

 tion) to be as good as the best Para. The author regards these 

 cases, however, as exceptions, referring to the premium which 

 Para rubber commands, as well as to its loss in washing being 

 four to five times greater than that of plantation rubber; a con- 

 siderably higher scale of value being thus established for the Bra- 

 zilian article. 



THE COAGULATION OF HEVEA LATEX BY SMOICING. 



Most of the processes for coagulating Hevea are, in fact, in- 

 spired by the Amazonian smoking method ; in some cases pretty 

 closely, but not entirely realizing it in any instance. Smoking 

 may be regarded as a comple.x action, or rather a group of actions. 

 In dealing with a material so sensitive as rubber to the slightest 

 external influences, the most infinitesimal factor may possess im- 

 portance. 



The numerous effects which are developed in the preparation 

 of "Para Hard Fine" may be divided into three groups : Chem- 

 ical, physical, and mechanical, which are described as follows by 

 M. Cayla : 



CHEMICAL EFFECTS. 



This concerns the quality of the smoke. If my information on 

 this subject is correct, a complete and systematic chemical study 

 of the diiTerent kinds of smoke has not as yet been made; but 

 details in regard to the fuel used will allow us some reflections. 



The fuel consists either of palm nuts, of resinous woods or 

 of green wood. There is, consequently, a certain diversity. I 

 have mentioned these fuels in the order of the preference ex- 

 pressed for them by the seriiigueiros of the lower Amazon, who 

 further show their preference for the nuts of the particular palm 

 tree known as the Atialca, the Maximiliana and the Euterpe, 

 and for the dry woods of the massaranduba and the pao d'arco. 

 We cannot give the botanical origin of these woods, which ap- 

 pear to be quite plentiful; they are hard woods, usually of a 

 yellowish or reddish color and heavily impregnated with resin- 

 ous substances. The seriitgueiro collects his wood ahead and 

 prepares it, splitting it into sticks. But there are regions that 

 contain few or none of the necessary palms or the valuable 

 resinous trees, consequentl}' the seriitgueiro has a struggle to get 

 them, so that there are considerable differences in the fuel used, 

 not only in the various districts of the Amazon basin but even 

 by the same seriitgueiro. Nevertheless, the smoking, properly 

 conducted, always yields excellent results. 



It is necessary, therefore, to determine whether in the smoke 

 of these various fuels there is always to be found one or more 

 essential substances which impart the qualit>% or whether this 

 quality can be imparted by a whole series of substances belong- 

 ing to the same chemical group and which are to be found, one 

 or another, in the products of combustion. 



It will be seen that it is dangerous to attribute to this or that 

 constituent of the smoke the essential part in the coagulation 

 or the determination of quality. It seems certain, however, that, 

 as has often been stated, these fumes, in addition to their coagu- 

 lative action, exercise a preservative effect by impregnating the 

 coagulated gum with various principles, such as creosote, forma- 

 lin, etc. 



PHYSICAL EFFECTS. 

 Coagulation by smoking is a warm coagulation. The tempera- 

 ture of the smoke, obtained by holding the bulb of a thermometer 

 at a distance of several millimeters (a millimeter equals approxi- 

 mately one-twenty-fifth of an inch) from the mass of latex spread 

 on the bolacha (biscuit) is above 72 deg. C. (162 deg. F.) ; I 

 would not be astonished if it reached 80 deg. C. (176 deg. F.). 

 A considerable portion of the heat would be applied to the 

 vaporization of the water contained in the latex. 



MECHANICAL EFFECTS. 



The masses of rubber coagulated by smoking are still far from 

 possessing the properties of the commercial gum. The coagulum 

 is of a light cream color, of medium elasticity and considerable 

 plasticity. On pressing on the surface, the impression of the 

 finger remains. Withdrawn from the action of the smoke, on 

 cooling and expelling the water (and the substances which it 

 contains dissolved or in suspension) the gum assumes a more and 

 more firm consistency and the impressions disappear. Thus a 

 bolacha (a biscuit of rubber), placed on the ground to dry, will 

 not be round but flattened on one side; if it rests on a mat, it will 

 carry the impression of the mat, etc. There occurs, little by 

 little, an interior process that modifies the physical condition of 

 the coagulum ; while its volume is reduced, the water is elimi- 

 nated and the gum turns brown, almost black, on the outside 

 compared with the inside; which is said to be caused, at least 

 in part, through oxidization by the air. This contraction may last 

 for days and weeks. The work that is done by presses on the 

 plantation is therefore executed spontaneously, but in this instance 

 the operation is evidently much slower. 



The arrangement in successive concentric layers, moreover, fa- 

 cilitates this spontaneous working. A layer B, in its contraction, 

 causes pressure on the underlying layer A ; the layer C, 

 which lies over B, presses in turn on the latter. These two ef- 

 forts are combined to act on the layer A ; and so on. To the 

 contraction belonging to each layer is added the pressure re- 

 sulting from the contraction of the superimposed layer. The 

 pressure to which the various layers are exposed is therefore not 

 only slow, it is also progressive. Consequently, the form even 

 of the bolachas, more or less regularly spherical or ellipsoidal, 

 causes this normal pressure to follow a radial direction. And 

 as we may assume that a good operator spreads on the ball suf- 

 ficiently regular layers, each layer is subjected over its entire 

 surface to a pressure distinctly regular in intensity and directed 

 towards the center. 



That which I have just described shows that there are several, 

 differences between the method perfected for pressing the rubber 

 on the plantations and the Amazonian method, which may be 

 characterized as slow, continuous, progressive and regular. 



These observations, incomplete tho they are. enable us to- 

 understand how difficult it is at present to chopse among all the 

 factors we have considered (and possibly still others) those 

 particular factors that play the determining part in producing 

 the high quality imparted by smoking. We shall not be as- 

 tonished, however, to find that the chemical properties of the 

 Amazonian smoking process do not play so vital a part that we 

 cannot use entirely different fuels and still get good results; nor 



