The manufacture of india rubber relates to the production of two principal products. One is masticated unvulcanized sheet and thread rubber; the other is unmasticated mixed and cured rubber, otherwise vulcanized rubber. For the purposes of the rubber-stamp maker an intermediate product is required, namely, unmasticated mixed sheet which is uncured. This is really incompletely vulcanized india rubber.

It will be evident from the description to come that it is not advisable for any one without considerable apparatus to attempt to clean and to wash (“to sheet”), to masticate, or to mix india rubber. These operations are best accomplished in the factories. The partially vulcanized (“mixed sheet”) or the pure masticated article are regular articles of commerce. Yet a full insight into the manipulation of india rubber can only be obtained by understanding its treatment from the gum up to the two separate lines of products we have indicated.

A third type of product is coated tissue, such as Mackintosh. This really is a sequence of one of the other two processes and a few words will be said of it in concluding the chapter.

As the caoutchouc is received by the manufacturer it appears an utterly intractable mass. It occurs in lumps of every size, varying in color and odor, and very tough but elastic. In virtue however of the properties already described, its power of cohering when cut, and its softening when heated, it becomes amenable to treatment.

It is to some extent received in such assorted condition as to secure even grades, and then each grade may be washed by itself. It is thrown into water which is in many cases kept at the boiling point by steam-heat and left there for some hours. It absorbs some water and also softens. Some gum is so soft that it will not stand hot water. For such the water is kept cold. The purer gum floats; such pieces as have stones, dirt, iron, etc. in them, perhaps placed there purposely from fraudulent motives, sink and can be picked out for separate treatment.

The lumps are next cut up. A revolving circular knife driven by power is often used, and sometimes an ordinary knife is adopted. At this part of the operation there is frequently need for sorting, as the grades received may have inferior pieces mixed with the good. The cutting is mainly to secure good grading, and to remove concealed impurities. The gum then goes to the washing rollers, called the washer and sheeter. (See cut, p. 37.)

These are heavy corrugated rolls made very short, 9–18 inches in length, to prevent springing. They are grooved or corrugated and have a screw adjustment for regulating their distance apart. They are geared together so as to work in corresponding directions, like a clothes wringer or a rolling mill of any kind. The pieces of gum are fed into the rolls and are drawn between and through them. The friction tends to heat the gum. To prevent this and also to effect the washing, a supply of water, either hot or cold, is kept playing upon the mass. This dissolves out all soluble matter and washes away mechanically the chips, dirt, etc. which may be present. The whole operation is one of main force. The caoutchouc is torn and distended and delivered as a rough perforated sheet. It is passed repeatedly through the machine, the rollers being gradually brought closer together, or else different sets of rolls are used, set to different degrees of fineness. The wash water passes through a screen which catches any small detached fragments of gum.

Other types of machines have been introduced; the above is a representative form.

The rough sheets must now be perfectly dried, as water impairs the final product. This is done in drying rooms by steam heat, generally, at a temperature of about 90° F. (32° C.) The windows, if there are any, are painted to exclude sunlight, which operates to deteriorate raw gum. When absolutely dry the caoutchouc is removed and stacked away for use.

To prepare pure gum for the manufacture of sheet rubber and as a starting point for many other preparations, the india rubber is “masticated” in special apparatus. The machine consists of a fixed cylinder within which is a corrugated roller set eccentrically and rotated by power. The perfectly dry sheets in the masticator are pressed and rolled and ground and produce a mass of even consistency. Here the welding or cohering action again appears in its fullest development. The perfect dryness of the mass enables it to keep reuniting as fast as divided. The action is assisted by the heat generated, which is not inconsiderable. Sometimes the caoutchouc is warmed before introduction, and sometimes the roller is heated by passing steam through it.

The masticating machine the French picturesquely term the wolf (loup) or devil (diable). It is given from sixty to one hundred turns a minute, and a machine large enough to treat fifty pounds of gum in a charge, requires five horse-power to drive it. In it the sheeted gum is ultimately brought to the state of a perfectly homogeneous dark brown translucent mass.

The masticated rubber is peculiarly amenable to mechanical and chemical treatment. It can be shaped by heat and pressure, and it is the most soluble form and is used for making cement and solution, and is moulded into blocks for the manufacture of sheet and thread rubber. In the process neutral body pigments, such as oxide of zinc, or soluble transparent ones, such as alkanine may be introduced; easily decomposed matter cannot be incorporated on account of the heat.

In all these machines special provision is made to prevent any oil from getting into the gum. There is no greater enemy to india rubber than oil or fats of any description. The flanges in the masticator that roll just inside the bearing are for this purpose.

Sheet rubber is made from the blocks of masticated gum by slicing. A machine is used for the purpose which carries a knife which works back and forth in the direction of its length at high speed, making two thousand cuts a minute. The knife is kept wet by a stream of water, and about sixty cuts are made per inch. In many articles made from this sheet the marks of the cuts can be seen as a fine ribbing. The appearance is familiar to many readers.

The sheet is often cut from rectangular blocks, but cylindrical blocks are also used. The latter are rotated in front of the knife edge and a long, continuous sheet can thus be obtained.

The sheet rubber can be cut into threads on webbing and braid. Everyone has noticed that these threads are usually square. The method of preparation accounts for it. Vulcanized sheet is now almost universally used for threads.

Round threads however can be made by forcing softened or partly dissolved gum through a die.

It is from unvulcanized masticated sheet that toy balloons, tobacco pouches, etc., are made. It is the starting point for india rubber bands. For the usual form of the latter article the sheet is cemented into a long tube which is afterwards cut transversely, giving bands of any desired width. To make any of these articles satisfactory vulcanization is imperative. Unvulcanized rubber for many years was used, but it is now completely displaced by the vulcanized product. Sheet rubber is made as above; is vulcanized by some of the absorption processes described in the chapter on vulcanization.

We now come to the second product: regularly mixed and cured rubber. Its starting point is the washed india rubber from the washer and sheeter.

We have seen that the pure gum or caoutchouc is very sensitive to changes of temperature. At the freezing point of water it is hard and rigid, and at the boiling point is like putty in consistency. There are several substances which can be made to combine with the gum and which remove from it this susceptibility to change of temperature. The process of effecting this combination is called vulcanization, and the product is called vulcanized india rubber. Sulphur is the agent most generally employed.

In the factory the normal vulcanization is carried out in two steps, mixing and curing. The washed sheet india rubber which has not been masticated and which must be perfectly dry is the starting point, and the mixing rolls shown in the cuts are the mechanism for carrying out the first step. These are a pair of powerful rollers which are geared so as to work like ordinary rolls, except that one revolves about three times as fast as the other. They are heated by steam, which is introduced inside of them. The sheet is first passed through them a few times to secure its softness, and then the operative begins to sprinkle sulphur upon it as it enters the rolls. This is continued, the rubber passing and repassing until perfect incorporation is secured. About ten per cent. of sulphur is added, and a workman can take care of thirty pounds at a time.

This material is incompletely vulcanized. It is in its present condition very amenable to heat and is ready for any moulding process. Generally it is rolled out or “calendered” into sheets of different thickness from which articles are made in moulds by curing.

These sheets are of especial interest to the reader as they are the material from which most small articles are made, including rubber stamps.

This rolling of the mixed india rubber into sheets of definite thickness is done by special calendering rolls. The product is termed “mixed sheet.”

In the mixing rolls the incorporation of other material is often brought about. Zinc white, lead sulphide, antimony sulphide, chalk, clay, talc, barium sulphate, plaster of paris, zinc sulphide, lead sulphate, white lead, oxides of lead, magnesia, silica, form a list of ordinary mixing ingredients. These lower the cost of the finished material and are often serious adulterants. For some cases the addition if not carried too far is not injurious, or even may be beneficial. A proper admixture renders the gum more easily moulded and treated in the shaping processes.

The next step in the vulcanizing process is the heating of the mass, which step is called “curing.” Up to a temperature in the neighborhood of that of boiling water the mixed rubber can be heated without change except as it is softened. But if the heat is increased it begins to get a little more elastic and less doughy, and eventually becomes “cured” or vulcanized. The temperature for vulcanization is about 284° F. (140° C.). The word “about” is used advisedly, for it is not only a question of heat but of time of exposure. After vulcanizing, including the curing, india rubber cannot be moulded to any great extent. In the manufacturing process, therefore, it is before curing placed in the moulds, heated, shaped by pressure, and by exposure to a higher heat in a steam oven called a vulcanizer, is at once cured.

To prevent adherence to the moulds they are dusted over with ground soapstone, and the rubber itself is often thus coated.

The methods of vulcanization and curing, which may be of special use to the reader, are given in the chapters devoted to that subject (chapter XI.), and in the one devoted to rubber stamps.

Hard rubber, termed ebonite when black, and vulcanite when of other colors, is simply vulcanized rubber containing a large percentage of sulphur added in the mixing process.

The manufacture of coated tissues is effected in several ways. The following is a typical process. A mixture of one part washed and sheeted india rubber with one part zinc white, one fourth part sulphur, and about one third part naptha is mixed into a dough-like mass and is spread upon the cloth by machinery. The latter is simple. It consists of a bare board arranged to move under a scraping bar. The cloth is placed on the board and carried under the bar. The coating mixture is fed on one side of the bar upon the surface of the cloth. As it passes under, a regulated amount, according to the set of the bar, adheres. It is then dried by steam heat and recoated, until ordinarily six coats, each about one one-hundredth of an inch in thickness, have been given. Three coats are given in each direction with intermediate drying. The fabric is then cured by heat in vulcanizers.

Sometimes the sulphur is omitted from the mixture and cold curing, as described later, is adopted. When the goods are made up the seams are secured with rubber cement, a thick solution of masticated gum. Such seams have to be vulcanized.

Sometimes two such fabrics before curing or vulcanization, are placed face to face and allowed to adhere and are then cured or vulcanized.

Enough has been said in this outline of the manufacturer’s treatment of india rubber to show that the first treatment requires machinery. Very little can be done with mortar and pestle, although in making up solution these simple instrumentalities are available. As a starting point for making small articles masticated sheet rubber and mixed sheet rubber are the staple materials. The preceding steps are best accomplished in the factory.