698 CAOUTCHOUC 



part of the bottom of tho drum. The drum itself consists of 2 semi-cylinders ; the 

 under one of which is made fast to a strong iron framing, and the upper one is hinged 

 to the under one behind, but bolted to it before, so as to form a cover or lid which 

 maybe opened or laid back at pleasure in order to examine the caoutchouc from time 

 to time, and take it out when fully kneaded. In tho centre of tho lid a funnel is made 

 fast, by which the cuttings and shreds of the India-rubber are introduced, and a 

 stream of water is made to trickle in, for washing away the foul matter often em- 

 bedded in it. The power required to turn the axis of one of these mills as tho drums 

 or boxes are called may bo judged of from the fact, that if it bo only 2 inches in 

 diameter, it is readily twisted asunder, and that it requires to bo 3 inches to withstand 

 every strain produced by the fixed teeth holding tho caoutchouc against the revolving 

 arms. Five pounds constitute a charge of tho material. Mills vary in size, and charges 

 range from 7 to 14 Ibs. 



One of the most remarkable phenomena of the kneading operation, is the prodigious 

 heat disengaged in the alternate condensation and expansion of the caoutchouc. 

 Though the water be cold as it trickles in, it soon becomes boiling hot, and emits 

 copious vapours. When no water is admitted, the temperature rises much higher, so 

 that the elastic lump, though a bad conductor of heat, cannot be safely touched with 

 the hand. As we shall presently find that caoutchouc suffers no considerable or per- 

 manent diminution of its volume by the greatest pressure which can be applied, we 

 must ascribe the heat evolved in the kneading process to the violent intestine move- 

 ments excited throughout all the particles of the elastic mass. 



During the steaming much muddy water runs off through apertures in the bottom 

 of the drum. In the course of half an hour's trituration the various pieces become 

 Agglutinated into a soft, elastic, ovoid ball, of a reddish-brown colour. This ball is 

 now transferred into another similar iron drum, where it is exposed to the pricking and 

 kneading action of 3 sets of chisel points, 5 in each set, that project from the revolving 

 shaft at angles of 120 deg. to each other, and which encounter the resistance occasioned 

 by 5 stationary chisel teeth, standing obliquely upwards from the bottom of tho 

 drum. Here the caoutchouc is kneaded dry along with a little quicklime. It soon 

 gets very hot ; discharges in steam through the punctures, the water and air which it 

 had imbibed in tho preceding washing operation ; becomes in consequence more com- 

 pact ; and in about an hour assumes the dark brown colour of stationers' rubber. 

 During all this time frequent explosions take place, from the expansion and sudden 

 extrication of the imprisoned air and steam. 



Instead of close boxes and tearing teeth or knives, rollers of iron are now employed 

 (1858): their forms are corrugated, cut or indented, the pieces of India-rubber are 

 thrown between, and by heat and pressure are cleansed and incorporated ; streams of 

 water, warm or cold, regulate these operations at will, of course with large rollers of 

 metal exposed to air and streams of water. The temperature is now kept so low that 

 the previous statement may appear exaggerated to those who work with more power 

 but with less velocity in the modern arrangements. Mr. Hancock, however, says, ' the 

 heat it acquires is very surprising ; I have found in cutting a heavy charge open, and 

 closing it upon the bulb of a thermometer, that the temperature reached 280 ; ' this 

 heat was only due to motion of the machine and friction upon the rubber, as the 

 materials and the machine were cold at tho outset of the experiment. 



From the second set of drums the ball is transferred into a third set, whose revolv- 

 ing shaft being furnished both with flat pressing bars, and parallel sharp chisels, per- 

 pendicular to it, exercises the twofold operation of pricking and kneading the mass, 

 so as to condense the caoutchouc into a homogeneous solid. Seven of the finished 

 balls, weighing, as above stated, 5 Ibs. each, are then introduced into a much larger 

 iron drum of similar construction, but of greater strength, whose shaft is studded 

 all round with a formidable array of blunt chisels. Here the separate balls become 

 perfectly incorporated into one mass, free from honeycomb cells or pores, and there- 

 fore fit for being squeezed into a rectangular or cylindrical form in a suitable cast-iron 

 mould, by the action of a screw-press. When condensed to tho utmost in this box, 

 tho lid is secured in its place by screw bolts, and the mould is set aside for several 

 days. It is a curious fact, that Mr. Sievier tried to use this as a moulding force, by 

 tho hydraulic press, without effect, as tho cake of caoutchouc after being so condensed, 

 showed much more resiliancy than after the compressing action of the screw. The 

 cake-form generally preferred for the recomposed, ground, or milled caoutchouc, is a 

 rectangular mass, about 18 inches long, 9 inches broad, and 5 inches thick. 



This is sliced into cakes for the stationer, and into sheets for making tapes and 

 threads of caoutchouc, by an ingenious self-acting machine, in which a straight steel 

 blade, with its edge slanting downwards, is made to vibrate very rapidly to and fro in 

 a horizontal plane ; while tho cake of caoutchouc clamped or embraced at each side 

 between two strong iron bars, is slowly advanced against the blade by screw-work, 



