Hardness and toughness are actual drawbacks in the utilisation of rubber which is required for vulcanising. When the output of plantation rubber began to increase and to displace the inferior wild sorts, manufacturers complained of the increased power consumption of their machines. The power was required mainly to "break down" or "mill" the rubber preliminary to the mixing with sulphur and other ingredients. It is obvious that a material such as raw rubber cannot be mixed with powders such as sulphur with a pestle and mortar, or in any simple form of mixing machine. This difficulty was overcome by the earlier experimenters by immersing the rubber in a bath of molten sulphur. The latter was gradually absorbed and "dissolved" in the rubber, and the heat of the bath caused the dissolved sulphur to combine with the rubber to produce vulcanised rubber. The limitations of such a process are apparent. Thus the vulcanised rubber retains the form in which it was originally shaped. Moreover, other ingredients, such as mineral matters, cannot be dissolved or absorbed by the rubber in this manner. The method eventually adopted consisted in "breaking down," "milling," or "masticating" the rubber by passing it continuously between differentially geared steam-heated rollers. By this means a high-grade rubber is converted into a soft, plastic mass, which will "take up" sulphur, mineral matter, and other ingredients as desired. The mixing operation may be carried through on the same roller machine as was used for breaking down the rubber, or separate machines of other designs may be adopted. Details of the process will be found in books dealing with rubber manufacturing.[35] It will suffice here to explain that when rubber is kneaded between two hot rollers moving at different speeds the rubber forms a continuous band around the slower moving roller, and if the distance between the rollers be adjusted the excess of rubber held back by the nip of the rollers will form a "bank" or moving wedge-shaped mass on the top of the nip. This closes the space between the rollers, so that sulphur and powder placed on the rubber pass round towards the nip, and are there driven into the rubber. In this manner it is easy to mix, say, 10 per cent. of sulphur into the rubber without a single particle falling through. In technical mixes where large quantities of powders require to be mixed there is always some caking, and part of the powder falls between the rollers into a tray underneath. This is swept up with a broom and put back on to the rollers, the process being repeated until the whole of the ingredients have been incorporated. [35] For instance, "India-Rubber and its Manufacture," by H. L. Terry.