The peculiarities of the different sorts of washed rubber are not easily described, although they are very evident to any experienced person. Washed Para, negrohead, Ceara, Borneo, Rangoon, and Penang, are the most important. Washed Para has the peculiar odour of the raw rubber, and, on drying, takes a dark-brownish colour ; negrohead has a strong, rather offensive smell, and, as met with in commerce, is generally of a blackish colour ; Ceara has a rich light-brownish colour ; Borneo is generally black ; Rangoon and Penang have a reddish tint when washed and dried.
Masticating.—This process is necessary only for the production of sheet-rubber, slabs or blocks; as it does not destroy the strength of the rubber, it frequently forms a preliminary treatment, when extra grinding would destroy the firmness. Pigmented masticated rubber is sometimes required. There are several ways of obtaining this ; (1) by incorporating the pigment roughly with the whole of the rubber, or only a portion of it ; (2) by mixing in the "grinders" or mixing-rolls, and finishing off the batch in the masticator ; (3) by dusting the pigments on the rubber as it is put into the masticator, which may cause a little loss ; (4) a useful plan in many cases is to mix the pigment with a little of the rubber by grinding, and to run it out as a rough sheet, which is cut up, and masticated with the femainder of the rubber. Since the object of the process is not to weaken the rubber, it is evident that the incorporation of the pigments ought not to protract the process more than can he helped. The heat generated in masticating, even when the machine is not heated by steam, renders it almost impossible to incorporate some pigments, as sulphur and red-lead, in this way. Very small proportions of dry pigment can be added at a time in the masticator; consequently, to produce heavily pigmented masticated rubber, an excess of pigment is first ground in by the grinders, and a proportion of this product is passed with pure rubber through the masticator. A great deal of experience is necessary for the production of good cut sheet ; the speed at which the masticator is driven is one of the most important conditions for the production of a block from which can be cut sheets, that will not be curly, and will yet be sufficiently worked not to buckle when heated, nor be rendered soft.
Sometimes, especially in oold weather, the rubber is slightly heated on a steam-chest, before being put into the masticator ; but more frequently the axis of the machine is made hollow, so as to admit of its being warmed by passing steam through. If a masticator has been standing for some time, and become a little rusty, it is best cleaned by passing through a batch of some rough compound containing rsg fibre. When newly set up, a little grinding takes place in the bearings,
and it is important that the metal so detached should not become incorporated with the rubber. This renders it necessary to work the machine for a few days on some unimportant compound.
Fig. 844 shows a front view of the masticator, the case being removed ; and Fig. 845, an end view, representing the excentric position of the fluted roller. The speed at which this roller revolves should not exceed 30-40 rev. a minute for strong masticated sheet rubber ; a greater speed may be used where the object is simply to partially work for grinding purposes. The roller R is cast with a shoulder projecting about in. above the plates at each end, inside the frame, to prevent the lubricant reaching the rubber. The door 0, which is similar to a strong grating, is hinged to the upper part H of the frame, and is secured, when working, by a wedge fitting between the door and a recess formed by it in the lower part I of the frame. The strong toothed-wheel G is keyed to the fluted roller R, and worked off a pinion on the main shaft. The steam inlet is at S. Tho roller, by its excentric working, kneads the rubber, and by the alternate expansion and contraction in the upper and lower parts of the machine, a new surface is being continually presented to the roller. A 12-in. block will require 3-6 hours' working, to yield a good uniform sheet when cut up. The length of the machine is an important matter : the longer the block, the less is the waste in cutting the ends true, and generally the same is required whether the masticator will work up 40 or 240 lb. of rubber. These machines are made of a size to take much larger quantities of rubber ; but there is a limit to the size when required for fine cut sheet : a block of 240-260 lb. cannot well be exceeded for this purpose. The diameter is generally uniform ; the length only is increased when extra quantities are to be worked up. A masticator which will give a block 12-13 in. in diameter, with a 3-in. hole in the centre, is the most practicable ; if this diameter be exceeded, the heat from friction will be extreme, unless the speed is very low. Should the charge be stopped by the block not going round with the axis, it may be forced down with a crowbar, so as to grip the axis, when it will be instantly carried round. When indisrubber is being over masticated, it emits a peculiar odour, as if being roasted, and the block will be full of holes or cavities, and will have a darker colour.